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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
INTRODUCTION:
Biomechanics is defined as the application of the principles of
mechanical engineering in the living organism.An understanding of the
biological response to mechanical stimuli is of paramount importance
for promoting long term success of removable partial
dentures.Mechanical forces exerted on removable partial dentures
during functional mandibular movements should be properly directed to
the supporting tissues to elicit the most favourable response. It may
also be considered as the study of the problem of distributing the
energy generated by the muscles of mastication so that it will be
expressed at the occlusal surface with the maximum efficiency
consistent with the minimum damage to the supporting structures.
The primary consideration in partial denture construction is to
distribute the forces on the occlusal surfaces with the minimum
damage to the supporting tissues.Partial dentures are subjected to
many forces,such as chewing(vertical and lateral), lifting( sticky foods),
and actions of the tongue,lips and cheeks.
The manner in which alveolar bone surrounding the natural teeth
responds to force differs markedly from that of the residual bone
remaining after the extraction of the teeth. Fundamental to
understanding partial denture design is a solid grasp to simple
mechanical principles.It is necessary to understand the essential physics
involved in the working of the prosthesis.
Designing a removable partial denture which optimally satisfies the
prosthodontics requirement of support, function and esthetics is a
daunting challenge.When poorly designed without taking into
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
consideration of the biomechanical principles involved would make the
removable partial denture as a tooth extractor especially in a distal
extension removable partial denture. All removable partial dentures
direct mechanical forces to bone which is the ultimate supporting
tissue.The mucosa of the residual ridge transmits compressive forces
through the submucosa to the underlying bone without changing the
nature of the forces frequently resulting in pressure induced resorption.
The natural teeth are attached to the bone by means of a periodontal
ligament which converts much of the masticatory compressive forces to
tensional forces favourably stimulating alveolar bone. In the oral cavity
one would find a number of sources of stress generation, the human
body is built in such a manner that it learns to adapt to any stressful
situation. However when we try to create an artificial replacement of
that natural component which is lost, we are at loss in making it fully
functional and adaptable.
Designing of partial denture necessitates a proper planning for the
form and extent of dental prosthesis and studying of all the factors
involved. The prosthesis must be designed following the most
favourable biomechanical principles, as the proper design helps in
reducing the harmful effects on the supporting structures. The optimal
goal is to provide useful, functional removable partial denture
prostheses by striving to understand how to maximize every opportunity
for providing and maintaining a stable prosthesis.
Because removable partial dentures are not rigidly attached to teeth,
the control of potential movement under functional load is critical to
providing the best chance for stability and patient accommodation. The
consequence of prosthesis movement under load is an application of
2
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
stress to the teeth and tissue that are contacting the prosthesis.It is
important that the stress not exceed the level of physiologic tolerance,
which is a range of mechanical stimulus that a system can resist without
disruption or traumatic consequences.
In the terminology of engineering mechanics, the prosthesis induces
stress in the tissue equal to the force applied across the area of contact
with the teeth and/or tissue. This same stress acts to produce strain in
the supporting tissue, which results in load displacement in the teeth and
tissue. The understanding of how these mechanical phenomena act
within a biological environment that is unique to each patient can be
discussed in terms of biomechanics.
It is important for clinicians providing removable partial denture service
to understand the possible movements in response to function and to be
able to logically design the component parts of the removable partial
denture to help control these movements. The following biomechanical
considerations provide a background regarding principles of the move-
ment potential associated with removable partial dentures, and the
subsequent chapters explain various factors associated with removable
partial denture and how they are used to control the resultant move-
ments of the prostheses.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
REVIEW OF THE LITERATURE
Goodmann.J et al21(1963) stated that the design of a free-end partial
denture restoration required a careful balance between the requirements
of retention and the stresses that the retainers would exert on abutment
teeth He founded a simple solution to reduce stress to abutment teeth
through the use of Balance of force principle.
Augsburger.R26 (1963)postulated that mathematical equations could be
used to outline quantitative values in the design of removable partial
dentures and numerical values were used to simulate forces imposed
upon abutment teeth by retention and support components of the
denture. He concluded that this system of analysis could be applied to
designs of removable partial dentures but the factors of the patient’s
attitude toward cosmetics and functional comfort must be considered.
Maxfield et al37(1979)measured the forces applied to abutment teeth by
removable partial dentures computed by applying an extension of the
Pythagorean theorem,they found that the transmitted forces vary when
different removable partial denture designs were used.They also
suggested that improving adaptation of the extension bases to the
residual ridge was an excellent means for providing maximum support,
increasing patient comfort, and decreasing forces to abutment teeth.
Cecconi.T.B et al32 (1975) had performed an invitro study using several
types of rests to determine which type of rests transmits forces to
abutment teeth in the most favourable manner. He concluded that the
rests with gingival seats at maximum depth in abutment teeth
significantly decreased abutment tooth movement and bilateral loading
4
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
of a removable partial denture caused significantly less abutment tooth
movement than unilateral loading.
Knowles E.L16(1958) reviewed the engineering principles associated
with removable partial denture and he proposed that the primary
biomechanical principles to be considered were support, bracing, and
retention.
Seong-kyum kin et al74 (2007) had conducted an invitro study to
investigate the biomechanical effects of mandibular Removable partial
denture with various prosthetic designs under unilateral loading using
strain gauge analysis.They concluded that splinting of two isolated
abutments by bridge reduced the peri-abutment strain in comparison
with unsplinted abutments under unilateral loading.
Asher L.M52(1992) had proposed biomechanical consideration for the
use of the rotational path removable partial denture for a patient with a
tooth-bounded ridge on one side and a distal extension ridge on the
opposite side.He concluded that by including a rotational path rigid
retentive element in a design that accommodated rotational movement
in function,exceptional stability was achieved with minimal stress to the
abutments.
Rachman Ardan76(2008) conducted an in vitro study about the
masticatory force on the fulcrum point of first class lever on the lower
jaw distal free end denture using two dimensional static model in
sagital direction.He concluded that the main problem of distal free end
Removable Partial Denture is lateral displacement. He also stated that
the first class lever retainer design on distal free end RPD generated
5
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
leverage to the abutment tooth and created cumulative pressure in main
fulcrum point.
Devan M M4 (1952) had stated that mouth needing a bilateral partial
denture was in a state of mutilation.He suggested for the preservation
of the partial denture foundation the horizontal forces falling on the
saddles and transverse forces falling on the abutments should be
reduced.He also suggested the all-out use of every available tooth and
tissue bearing for preservation of partial denture foundation.
Arthur.R.C1(1951)proposed that the amount of force imposed upon the
denture may be reduced by maintaining the sharpness of tooth cusps
and by decreasing the size of the food table.
Kwin Chi Luk59(1979) had demonstrated the design of a unilateral
rotational path removable partial denture to restore a single
edentulous space with a tilted mandibular molar.He suggested that
the stability and retention of the denture were controlled anteriorly
by the buccal retentive clasp and lingual guide plate of the
conventional direct retainer, and posteriorly by the rigid retainer
with its buccally and lingually extended proximal plates.
Theodore Berg51(1992) had compared photo elastically the stress
distribution characteristics of maxillary bilateral distal-extension
removable partial dentures retained by light and heavy ERA
extracoronal attachments.He also compared the pattern of stress
distribution in photoelastic model with one prosthesis included
supporting rests and the other had no rests.He concluded that there
was significant difference in stress distribution.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
MC Cracken et al15(1958) discussed the design, location, and the
purpose of the various components of the partial denture and pointed
out that some of the violations of sound biologic and mechanical
principles were committed.
Lawrence W.A11(1956) had analysed the lateral force transmitted to
denture base location and clasp design.He constructed an experimental
model to simulate the type of tooth movement found in the
mouth. He also constructed partial dentures of various designs and
evaluated the torques and rotational patterns of removable partial
denture.
MC Cleod.S N3(1982) had shown that with a rotating retainer an
axis of rotation exists about the fulcrum line on either side of the
dental arch. He also concluded that lack of alignment of the
rotational axis on either side of the arch produces torque on the
abutments when the prosthesis was in function.
Ceconi T.B29(1971) conducted an in vitro study and determined the
effect of two types of partial dentures movement, stress breakers on
abutment tooth movement and ridge displacement. He measured
these movements when the stress breakers were both active and not
active.
MC Cleod34(1977) had shown that with a rotating retainer an axis
of rotation existed about the fulcrum line on either side of the
dental arch. He concluded that lack of alignment of the rotational
axis on either side of the arch produced torque on the abutments
when the prosthesis was in function. He also altered the retainers and
7
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
accomodated the lateral movements associated with the rotational axis
and compensated the torque.
Lammie8(1954) stated forces acting on RPD always be resolved into
three components, a purely vertical force, uncomplicated by a
simultaneously acting horizontal component on a lower free-end
saddle.He explained the forces acting on bilateral free end saddle and
the treatment options for bilateral free end saddles.
McCracken7(1953) stated that two distinctly different types of
partial dentures exist,tooth borne and tooth tissue borne.He further
stated that the advantage of this method of classification was there
exists a definite relationship between each other.
Arthur R.C1(1953) had planned partial denture with special reference to
stress distribution based on the physiologic rest position of the
mandible. He stated that there were two major factors involved in
controlling the forces of mastication.They were the reduction of
the amount of force imparted to the denture during mastication
and, the wide distribution of the forces to the tissues.
Arthur J.Kroll20 (1963) had demonstrated clasp design on an extension
based removable partial denture.He considered that the factors of stress
controlled when there was minimal tooth coverage and gingival
coverage. He introduced the RPI clasp that minimised tooth coverage
and reduced stress on the abutment tooth.
Ceconi T.B29(1971) had studied the effects of the sagittal inclination
of the residual ridges.He compared bilateral vs unilateral loading on
abutment tooth movement and also load vs non load side movements
of the abutment teeth.He found out that the angulation of the
8
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
residual ridge in a sagittal plane altered the direction and
magnitude of abutment tooth movement.
Chester Perry10(1956) stated few basic rules and few basic requirements
such as support,retention,stability and esthetics must be met if the
restoration had to function adequately with comfort.
Kratochovil20(1963) demonstrated the effects of rest placement using a
training aid. He found that as the denture base was followed
posteriorly, the arc of movement became nearly perpendicular to the
surface of the mucosa
ZBen Ur61 (1999) had discussed the factors affecting denture design
related to the position of the abutment teeth, the symmetry design,the
cross sectional shape of the residual ridges and the treatment of
complications of the edentulous distal extensions.
Aviv.I48(1989) had proposed that an axis of rotation was created through
most distally placed occlusal rests when distal-extension removable
partial denture was loaded.He further stated that if the residual ridges
were of unequal lengths, the axis of rotation may not be
perpendicular to the residual ridges.
Beckley W.R27(1969) had considered the correct distribution of stress
between the abutment teeth and the denture base had been a point
of contention among the three schools of thought advocating
broken stress, functional bases, and wide distribution of stress. He
proposed a technique that took advantage of beneficial aspects of each
method.
9
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Weinberg28(1971) had analysed the lateral force in relation to denture
base location and clasp design. He constructed an experimental model
to simulate the type of tooth movement found in the mouth.He
constructed partial dentures of various designs on the model and
tested torque and rotation patterns.
Ben-Ur et al61(1999) performed rigidity tests on maxillary major
connector with different designs and mandibular major connectors of
the lingual bar type with different cross sectional shape and
thickness.They concluded that the most rigid maxillary major
connector was anterior-posterior palatal bar and the most flexible was
the U-shaped palatal bar.
Nathan K.C. Luk49(1990) had analysed mathematically occlusal rest
design for cast partial dentures.He concluded that a decrease in occlusal
width had increased the bending stress and required thicker rest for
compensation.He concluded by his mathematical analysis that the
traditional spoon-shaped occlusal rest seat dimensions had complied
with the mechanical requirements for non-precious cast metal occlusal
rests in RPD.
Akaltan et al72(2005) had evaluated the effects of two distal extension
removable partial denture designs on tooth stabilisation and periodontal
health. They concluded that RPD treatment did not damage remaining
teeth and periodontal tissue if the dentures were carefully planned, the
prostheses and oral hygiene were checked at regular recall
appointments.
Miura et al46(1992) had examined the effect of direct retainer and major
connector designs on RPD dynamics under simulated loading.They
10
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
found out that rigid direct retainers and rigid major connectors
decreased both RPD movement,abutment tooth displacement under
loading. Their results provided the basic data that supported the
importance of rigidity of the RPD components to control denture
dynamics.
RoyMacgregor41(1983) reviewed the literature on planning the
support of bounded saddles of removable partial dentures or fixed
bridges. He also used three-dimensional photoelastic analysis to
examine stresses on removable partial dentures of different designs.
Neil D J17(1958) discussed the problems associated with lower free end
removable partial dentures,the problems associated with not restoring
the dentition, the tissue damage associated with wearing of removable
partial denture,the problems associated with denture design and
technical consideration in fabricating lower free end removable partial
denture.
Beckerd L.S44(1988) had analysed the influence of saddle classification
on removable partial denture.He classified distal extension saddle
situation based on support it derives into class-1(tooth borne),Class-2
(mucosa borne),class -3(problem type have inadequate abutments to
support the saddle and probably also inadequate mucosa support).
Kelly K.E et al9(1953) explained physiologic approach to partial denture
design.He advocated several methods to reduce lateral stresses by
means of rigid lingual and palatal bars so these stresses would be
distributed over as many abutments as practicable and by using stress
breakers.
11
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Potter B.R24(1967) stated that lateral stresses applied to a natural
tooth are so destructive to the supporting structures, whereas
stresses in the direction of the long axis of the tooth were well
tolerated. He proposed that the design of removable partial dentures
must minimize lateral stresses by distributing these stresses over
as many teeth and as much supporting tissue as possible, and
occlusion that provide damaging stresses should be avoided.
Avant W.E28(1971) discussed retention and fulcrum lines in planning
for removable partial dentures.He described a primary retention line
and compared with a primary fulcrum line.
Bickley W.R27 (1969) discussed a method for constructing removable
partial denture, incorporating broad distribution of stress,the
principles of broken stress and functional bases.His technique
minimised lateral stresses by keeping all the forces in a vertical
direction and by allowing rotation without torquing of the teeth.
Mensor C.M25(1967) suggested the rationale behind resilient hinge
action stress breaker. He started with the known motion differences
between anchor tooth and the free-end denture base.He also made
an attempt to differentiate the entire movement complex of
mastication into individual components.
Davis M.M et al3(1952) had explained the design and force distribution
in removable partial denture.They suggested that movement of a
removable partial denture in function was rotary in that the
movement takes place in three planes.They also added that the
“instantaneous center of rotation” theory could be meaningfully
12
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
applied to partially edentulous situations even though the theory
was based on movement in one plane.
Hindels W.G3(1952) discussed the load distribution in extension saddle
removable partial denture.He insisted that the method used to make
impressions of the supporting and retaining anatomic structures of
the mouth was of basic importance for obtaining optimum
distribution of the masticatory load in the construction of
removable partial dentures.
Blatterfein et al44(1988) evaluated loading forces on mandibular distal
extension prosthesis.They classified the concepts of design into 4 basic
categories the flexible denture base design,the floating denture base
design,the mucofunctional concept,the enodosseous implant concept.
Hirschritt et al14(1957) differentiated tooth and tissue bearing areas of
the mouth and simplified the design of each individual partial denture. He
said that a tooth borne unit, with its splinting and supporting ability,
protected the teeth against overstressing.
Deboer.J43(1988) reviewed the position of rests on occlusal surfaces of
abutment teeth for distal extension removable partial denture and
proposed rational alternatives to improve denture stability and
prognosis by the selection of sites for occlusal rests in distal
extension removable partial dentures.
McCartney38(1980) analysed motion vector of an abutment for a distal
extension removable partial denture.He determined intraorally the
effect of various rest placements and clasp designs of a
mandibular bilateral distal extension removable partial denture. He
13
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
also found that the magnitude and direction of abutment tooth
movement under vertical loading of the denture base.
Boero.E et al30(1972) explained considerations in the design of
removable prosthetic devices with no posterior abutments. He explained
that the basis of good restorative dentistry consists of establishing
an equilibrium of forces so stresses were conducive to develop a
physiologic continuum rather than pathosis.
Marie.K.M21(1963) compared the average measurements of forces
required to dislodge two kinds of circumferential clasps in
different amounts of undercuts,one with a half-round retentive arm
and the other with a round retentive arm under tensile load.His
findings indicated the use of cast round clasps were advantageous
in clinical fit and reduction of transmitted forces to the abutment.
Steefel V18(1962) explained the importance of diagnosis and functions of
removable partial dentures.He proposed the objectives of removable
partial denture design as bilateral distribution of stresses, the various
types of retainers (direct and indirect), cosmetic effects, and
function. He explained certain methods to achieve these objectives.
Kaires K.A12(1956) studied the effect of partial denture design on
force distribution. He fabricated a mandibular model and tested partial
denture to determine the effect of various denture designs on the
distribution of stress.
Frechette R.A2(1951) analysed lower distal extension removable partial
denture. He determined the magnitude of forces imparted to
abutment teeth when known loads were applied to a denture.
14
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Wills D J et al36(1979) performed experiments on macaque monkeys.
He compared the support provided for base plates resting on
groups of teeth, palatal mucosa and a combination of both.
Ben Ur et al61(1999) proposed that retentive clasp components could be
created to minimize torquing forces on abutment teeth incorporated in
the support and retention of bilateral distal extension removable partial
dentures.
15
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
EVOLUTION OF REMOVABLE PARTIAL DENTURE
Early Concepts- The band, the clasp, and sectional construction(before 1950)
The early concepts of RPD design were primarily developed by
dentists who recorded the techniques that were successful in their
practices.The first recorded description of an RPD was by Heister in
1711 when he reported carving a block of bone to fit the mouth(Fig-
1).Fauchard,' who is considered by many to be the father of modern
dentistry, described the construction of a lower RPD in 1728 using two
carved blocks of ivory joined together by metal labial and lingual
connectors. (Fig-1).
The first mention of a maxillary RPD using a palatal connector was
by Balkwell in 1880.Retentive clasps were first discussed by Mouton in
1746. In 1810,Gardette described the use of the wrought band clasp.(Fig-
2)The bands completely encircled the tooth and often extended into the
gingival sulcus. The destruction of the marginal gingiva and the tooth due
to constant vertical movement of the prosthesis led tothe first description
of an occlusal rest in 1817.
In 1817, Delabarre" referred to "hooks" (clasps) and the use of
"little spurs" (occlusal rests) to prevent irritation around the abutment
teeth.In 1899, Bonwill recorded his techniques for clasping abutments
16
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
with individually contoured gold circumferential clasps that were then
soldered to "the plate" (major connector). Bonwill also advocated the use
of "lugs" (rest seats) so that the prosthesis would be supported by the
abutments.(Fig-3)
In 1913, Roach presented a wrought wire circumferential clasp as
an improvement over the wide wrought band clasp. The first mention of a
bar clasp or "infra bulge" clasp was by Henrichsen in 1914,, but the bar
clasp did not gain popularity until Roach"' promoted this concept in 1930.
'I'he concept of rotational factors, which the early writers called
"balance," was first described by Balkwell" in 1880. Prothero" is credited
with coining the term "fulcrum line." William Taggart(Fig-4) proposed
the lost-wax casting technique for dentistry in 1907(Fig-5). This principle
was applied to RPDs by Norman B. Nesbet.(Fig-6)
In 1916,he described the technique for casting clasp assemblies for
RPDs. His refinement of the alloy and prosthetic tooth attachment
allowed the successful creation of short spanned unilateral RPDs. Nesbett
described the “inlay fit” of the clasp assemblies attained afterassembling
the separately cast components on a plaster cast.
Chayes had developed a parallelometer(Fig-7) in 1920 to help
guarantee parallel alignment both clinically and in the laboratory.The first
commercially available instrument developed specifically for use in
surveying models of teeth was designed by Weinstein and Roach in 1921.
The leap into full-arch, one-piece RPD castings was officially
made by Akers(Fig-8) when he published this technique in 1925.
Although descriptions of line tracings on the teeth occur prior to this
time, the term “height of contour”is credited to Edward
17
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Kennedy.Willis(1935)was among the first to describe in writing the
technique for dental model surveying and blocking out undesirable
undercuts. He was the first to use the term “path of insertion” for RPDs in
relation to a chosen plane and described tripoding. Roach, who was the
first to describe reciprocation, was aware that most retentive clasps were
actively exerting force on the abutment teeth.
During the 1930s and 1940s, there was persistent disagreement as
to how to approach the two dissimilar tissues encountered with the distal
extension RPD-teeth and the mucosa covering the residual ridgeThe
discussion centered around how to equalize forces placed on the hard,
relatively immovable, abutment teeth and the soft, relatively movable,
edentulous tissue areas
According to Steiffel, the prominent clinicians of the time could be
placed into the following three groups:
(1) those advocating some sort of stress-breakers between the
abutments and the major connector
(2) those advocating broad stress distribution to multiple abutments
and the edentulous area and
(3) those advocating physiological or functional basing
Steffel placed himself into “the broad stress distribution” group but
conceded that all three methods could be successful if properly executed.
He rejected the common practice of constructing a distal extension RPDs
from a single impression.
18
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Before 1950, RPD concepts were mostly developed by a small
group of authors who presented their theories and techniques based
primarily on empirical observation.
INVESTIGATIVE YEARS 1950 TO 1970
It was in 1950s that some of these clinical debates were resolved in
an evidence based approach.During these years several longitudinal
studies have performed that showed extensive pathological changes in
the periodontium and increased caries activity for patients who wore
RPDs. These studies gave credence to the then prevailing attitude of the
profession, as well as of the public, that RPDs were detrimental to the
existing dentition and were considered an interim appliance on the
pathway to complete dentures those days.
It was pointed out that in the 1950s, the partial denture concepts in
Europe were vastly different from the accepted concepts in North
America. In Europe, the RPDs tended to provide a flimsy design with
wrought wire clasping and, usually, no rest seats. In North America, the
partial denture design tended to include rigid major connectors, cast
clasps, and rest seats.
In 1956,Kaires showed that the lingual bar of a lower RPD should
be rigid to distribute forces across the arch. Also, an increase in residual
ridge coverage reduced forces to abutment teeth. In 1956, Frechette
showed that multiple occlusal rests helped to distribute forces to more
abutments and, thus, reduced forces to the terminal abutments.
Holmes and Leupold both showed that distal extension partial
dentures constructed on one-piece casts exhibit more movement of bases
than those constructed using an altered cast procedure. The original
19
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
altered cast technique was first presented to the profession by Applegate.
Advantages of the altered cast procedure have more recently been
confirmed by Vahidi and by Leupold et al.
During the 1960s and early 1970s, two influential clinicians
increased the popularity of the bar clasp concept started by Henrichsen
and Roach many years before. Kratochvil promoted the use of the I-bar
clasp with a mesial occlusal rest as a means of reducing the force on a
clasped abutment when dealing with distal extension RPDs. Krol
modified Kratochvil’s concept with his mesial rest proximal platc-I bar
(RPI) design.(F ig-9)
Some of the problems encountered included insufficient
vestibular depth, soft tissue undercut below the abutment tooth, and lack
of “I-bar usable” undercuts. As a result of these limitations for the I-bar
system, there evolved a modification that combined the I-bar and
circumferential clasp designs. This clasp design is called the mesial rest-
proximal plate-Akers clasp (RPA) and was developed by Krol and
Eliason.
The mesial rest and proximal plate are identical to the RPI system,
but the buccal retentive arm becomes a circumferential or Akers clasp
engaging a mesial undercut. The superior border of the rigid portion of
the Akers clasp should contact the tooth on the survey line.Nelson et al
suggested using a cast round clasp rather than the conventional half round
design to form the retentive Akers clasp.
RESEARCH IN EARNEST- 1970 TO PRESENT
During the 1970s, there began to appear a large number of studies
beginning with in vitro research. Cecconi et al showed that force to the
20
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
abutment teeth was transmitted via the rest seats, and that this force was
the same with or without retentive clasps.
Robinson showed that forces to abutment teeth with distal
extension RPDs are minimized with a mesial rest (as opposed to a distal
rest) and that a wrought wire retentive clasp has the same force on the
abutment as an I-bar design when used with a mesial rest. He also
demonstrated that no clasp is passive, as had been deemed essential by
nearly all theoretical concepts proposed in the past.
Nally showed that a mesial rest created the least amount of
abutment movement and that abutment movement increased with the
removal of indirect retainers. Browning et al confirmed the value of the
mesial rest with either the I-bar or the wrought wire clasp design. Frank
and Nicholl showed that indirect retainers have little to do with retention
of a distal extension RPD; rather, it is the guide planes that create
retention in conjunction with clasping. They showed that indirect
retainers do help with force distribution and, thus, are a beneficial
component in RPD design. An earlier study by Fisher and Jaslows
supports the findings of Frank and Nicholls.
Photoelastic studies provided a new laboratory research tool for
evaluating RPD design. Kratochvil and Caputo showed that an RPD
framework that had been properly adjusted to fit the abutments created
less force to the abutments than a framework that had not been adjusted.
Thompson et al reported the most favorable force to abutments came
with a mesial rest and either a wrought wire or an I-bar retentive clasp.
Pezzoli et al confirmed the value of mesial rests, indirect retainers, and
multiple rest seats on force distribution.
21
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Clayton and Jaslow measured the movement of the clasps on the
corresponding abutments. Browning et al showed that the clasp moves
more than the corresponding abutment. The major reasons for using
wrought wire clasps are that the wire is more flexible than a cast clasp
and that wire can flex in three dimensions. The fallacy in Clayton and
Jaslous study is that movement of the clasp does not necessarily translate
into movement of the abutment, and, thus, comparisons of the force
placed on the corresponding abutment by measuring the movement of the
clasp is invalid.
This study has been widely misquoted as justification for using an
I-bar instead of the more flexible wrought wire clasp. Clayton and
Jaslow's study does confirm that there is no such thing as a passive clasp.
From the increased interest in scientifically evaluating the design
concepts of the past, there began to emerge the following sound basic
principles for RPD design:
1. Major connectors should be rigid.
2. Multiple rest seats appear to distribute forces favorably.
3. Mesial rests appear to provide some advantage when used
with distal extension RPDs.
4. Parallel guide planes are beneficial for retention and stability
of a prosthesis.
5. The I-bar or the wrought wire retentive clasp, in combination
with a mesial rest, may be a superior design for the distal
extension RPD
22
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
6. The altered cast procedure reducrs movement of the distal
extension RPD at least initially.
PERIODONTAL AWARENESS
Clinical research began to gain momentum as periodontal
awareness increased. More valid and reliable concepts for RPD design
evolved that relied less on empirical observation. . In 1966, Rudd and
O'Leary did a brief longitudinal study in which they reported that, when
proper guide planes were established on periodontally treated patients,
mobility to abutment teeth remained the same or improved.
In 1977, Schwalm et al reported the results of a 2-year
investigation in which acceptable RPD design principles were used and
initial plaque control instructions and basic periodontal therapy were
instituted, but there was no periodic recall. Bergman and Ericson reported
that in a 3-year cross-sectional study, they found no adverse periodontal
results associated with the wearing of RPDs.
UNCONVENTIONAL DESIGNS
Swing lock design
The swinglock design was first introduced to the dental profession
by Simmons in 1963(Fig-10). Simmons took advantage of the casting
properties of the chrome cobalt metals to devise a hinge and lock system
that allowed for a retentive labial bar that can he opcncd and closed by
the patient. This radical technology alloys for successful use of
periodontally compromised abutment teeth, as well as situations in which
critical abutments are missing.
23
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Bolender and Becker have suggested certain specific indications
for the swinglock design including: periodontal compromised abutments,
missing key abutments,abutment mobility, limited economics, and
maxillofacial prosthesis.They recommend the use of multiple rest seats
and suggest placing the hinge and clasp of the labial retentive bar atleat
one tooth distal to the terminal abutments.
Antos, Renner, and Foerth prefer no rest seats and place the hinge and
clasp of the labial retentive arm next to the terminal abutments.
The dual path or rotational path design
The dual path (or rotational path) RPD(Fig-11)concept is relatively
new, having been introduced by king and Graver in 1978. Initially, the
dual path design arose out of the need for an RPD that would be esthetic
when anterior pontics are present primarily, the desire to eliminate
anterior clasping. This technique uses proximal undercuts adjacent to the
edentulous spaces for retention without clasps. The “first path” of
insertion of the framework is into these proximal undercuts.
As soon as the framework has gained access to the desired
undercuts, it is rotated into the “second path” of insertion to complete
seating the prosthesis.Initially, the dual path design was limited to tooth
borne situations in which anterior teeth were missing.
The swinglock and dual path concepts are good examples of design
modifications that have evolved because of a need to solve special
problems.
24
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Return to Precision:Sectional construction revisited
Following the introduction of Akers’ one-piece casting technique,
several authors maintained that sectional construction was preferred due
to the superior fit obtained. Several modern methods of sectional
construction have been discussed. More recently, improvements in laser-
welding technology have allowed predictable unification of metal
components.
Cecconi et al described a component approach in which individual
parts are fabricated and joined on the definitive cast by means of
autopolymerized acrylic resin or laser welding.Brudvik et al showed that
this technique reduced distortion of large castings, the cumulative effect
of which is optimum control of the framework fit.
Cecconi advocated the advantages of sectional casting as being
Eliminating the need for time-consuming trial
placement of the framework.
fabrication of tooth- and tissue-supported elements
can be done separately.
dissimilar materials may be used. In component
RPDs, cobalt–chromium or nickel–chromium alloys
may be used for rigid major connectors, and gold
alloysmay be used for clasp assemblies where
improved accuracy and flexibility may be required
Similarly, acrylic resin denture base and acrylic resin
teeth may be combined with metal or porcelain where
necessary.
25
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Key turning points in RPD philosophy included Bonwill’s band-
free RPD design, Akers’ one-piece casting technique, and the
ramifications of the one-piece technique’s application
BIOMECHANICAL CLASSIFICATION OF REMOVABLE PARTIAL DENTURE(Based on the nature of the supporting tissues- Occlusal forces are
transmitted to the teeth used as RPD abutments)
A. TOOTH BORNE (Tooth supported /Dentoalveolar supported)(Fig-12)
1. Abutment teeth border all edentulous areas where tooth
replacement is planned.
2. Functional forces are transmitted through abutment teeth to bone.
B. TOOTH - MUCOSA BORNE (Tooth and Mucosa supported, Dento-
alveolar and muco-osseous supported or extension base)(Fig-13)
1. Exhibits one or more edentulous areas which are not bordered by
abutment teeth (extension base RPDs).
2. Forces are transmitted through abutment and mucosa to bone.
3. The majority of these are distal extension RPDs.
4. This category may apply to tooth bordered situations when
excessive abutment tooth mobility is present or when long span
tooth bordered edentulous areas are present precluding primarily
26
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
tooth support.
C. MUCOSA BORNE. (Muco-osseous supported)(Fig-14)1. Regardless of the natural teeth present, support is derived entirely
from the mucoosseous segment.
2. This category includes prostheses fabricated from hard or
combinations of resilient and hard denture base materials such as
stayplates which function as interim or transitional prostheses.
3. These prostheses usually do not contain a metal framework and
usually should not be considered definitive treatment.
MECHANICAL PRINCIPLES OF REMOVABLE
PARTIAL DENTURE
Definition:Dental Biomechanics is defined as the relationship
between the biologic behavior of oral structures and the physical influence of a dental restoration.
Bio------pertaining to living systems(eg:inflammation, Caries,resorption.. ..etc)
Mechanical----related to forces and its application to objects(Eg:looseness of teeth,bone resorpt ion . .e tc)
Mechanics may be classified into two general categories: Simple & complex.
Complex machines are combination of many simple machines.There are six simple machines(Fig-15)(Fig-16)(Fig-17)
1. Lever
2. Inclined plane
3. Wedge
27
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
4. Screw
5. Wheel
6. Axle and pulley
A removable partial denture in the mouth can perform actions of two
simple machines,LEVER & INCLINED PLANE
LEVER :
The lever is a simple rigid bar supported at some point along it is
length.It can be used to move objects by application of force(weight),
much less than weight of object being moved.
Types of lever:
Classification is based on location of fulcrum (support), load(resistance), and direction of effort (force).
Note:
The load is the weight or force to be acted upon.
The effort is the weight or force required to cause the action.
The fulcrum is the pivot about which these forces act.
In a perfect system which is static:
The effort × the distance from the fulcrum = the load ×the distance from the fulcrum.
There are three fundamental levers around which the whole removable
partial denture revolve.But, the first fundamental facts are
1. A lever system works at mechanical advantage when the effort is
less than the load.
28
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Mechanical advantage =Effort arm /Resistance arm
The length of fulcrum to resistance is called resistance arm,
while the length of lever from fulcrum to the point of application of
force is called effort arm.
2. A lever system works at a mechanical disadvantage when the
effort is greater than the load.
3. To be in balance(equilibrium) the forces on either side of the
fulcrum should be equal. That is the effort multiplied by its
distance from the fulcrum is equal to the load multiplied by its
distance from the fulcrum.
4. Whenever the effort arm is longer than the resistance arm the
mechanical advantage favors the effort arm,proportionately to the
difference in length of the two arms.In other words when the effort
arm is twice the length of the resistance arm a 25lb weight on the
effort arm will balance a 50 lb weight at the end of the resistance
arm.The opposite is also true and helps in cross arch stabilization.
The fundamental levers are
1. The first class lever
2. The second class lever
3. The third class lever
THE FIRST CLASS LEVER:(Fig-18)
The fulcrum (F) is in center of the bar, resistance (R) is at one end
and the force (E) is at opposite end (called cantilever).
Cantilever: It is a beam supported only at one end, when force is
29
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
directed against unsupported end of beam cantilever can act as first class
lever.
Archimides said that “Give me a lever long enough, I can lift the whole
world”
CLINICAL APPLICATION OF CLASS-I LEVER
A cast circumferential direct retainer engages the
mesiobuccal undercut and is supported by the disto-
occlusal rest.If it is rigidly attached to the abutment
tooth, this could be considered a cantilever design, and
detrimental first class lever force may be imparted to the abutment if
tissue support under the extension base allow excessive vertical
movement toward the residual ridge.Every effort should be made to avoid
lever of Ist class as it causes more damage to the supporting structures.
THE SECOND-CLASS LEVER:(Fig-19)
The fulcrum at one end, the force at opposite end & the resistance
in center. This type is seen as indirect retention in removable partial
denture.Works at a mechanical advantage cannot work at a mechanical
disadvantage as the load is always near the load.
CLINICAL APPLICATION OF CLASS-II LEVER
Typical examples for clinical application of class-II lever in
removable partial denture is seen in indirect retention in removable
partial denture and Equipose removable partial denture.In equipoise
restoration the occlusal rest (F) located mesially, while the retentive tip
(R) positioned distally, and the saddle(E) located distal to the retentive
tip i.e.the (Resistance) located in between the (Fulcrum) & (Effort).
THE THIRD CLASS LEVER:(Fig-20)
30
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The fulcrum at one end, the resistance at opposite end and the
force in the center.This type is not encountered in removable partial
denture.Eg: tweezers.
INCLINED PLANEInclined plane is nothing but two inclined surfaces in close
alignment to one another. The direct retainers and the minor connectors
slide along the guide plane of the teeth and can act as inclined planes if
not prepared correctly.
When a force is applied against an inclined plane it may
produce two actions:
Deflection of the object, which is applying the force
(Denture).
Movement of the inclined plane itself (tooth).These results
should be prevented to avoid damage to the abutment teeth.
31
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
POSSIBLE MOVEMENTS OF REMOVABLE PARTIAL
DENTURE(Fig-21)
Three fundamental planes and three axis as related to the human head.
I.SAGITAL PLANE
The first plane is a sagittal plane. Movement in this plane occurs
relative to a medio-lateral axis that is perpendicular to the sagittal plane.
One movement is rotation about an axis through the most posterior
abutments. This axis may pass through occlusal rests or any other rigid
portion of a direct retainer assembly located occlusally or incisally to the
height of contour of the primary abutments This axis, known as the
fulcrum line, is the center of rotation as the distal extension base moves
toward the supporting tissue when an occlusal load is applied.
The axis of rotation may shift toward more anteriorly placed
components, occlusal or incisal to the height of contour of the abutment,
as the base moves away from the supporting tissue when vertical
dislodging forces act on the partial denture. These dislodging forces result
from the vertical pull of food between opposing tooth surfaces, the effects
of moving border tissue, and the forces of gravity against a maxillary
32
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
partial denture. If it is presumed that the direct retainers are functional
and that the supportive anterior components remain seated, rotation rather
than total displacement should occur.
Vertical tissue ward movement of the denture base is resisted by
the tissue of the residual ridge in proportion to the supporting quality of
that tissue, the accuracy of the fit of the denture base, and the total
amount of occlusal load applied. Movement of the base in the opposite
direction is resisted by the action of the retentive clasp arms on terminal
abutments and the action of stabilizing minor connectors in conjunction
with seated, vertical support elements of the framework anterior to the
terminal abutments acting as indirect retainers. Indirect retainers should
be placed as far as possible from the distal extension base, affording the
best possible leverage against lifting of the distal extension base
II.HORIZONTAL PLANE
The second is the horizontal plane. Movement in this plane occurs
around a vertical axis that is perpendicular to the horizontal plane. The
movement is rotation about a longitudinal axis as the distal extension
base moves in a rotary direction about the residual ridge.This movement
is resisted primarily by the rigidity of the major and minor connectors and
their ability to resist torque. If the connectors are not rigid, or if a stress-
breaker exists between the distal extension base and the major connector,
this rotation about a longitudinal axis applies undue stress to the sides of
the supporting ridge or causes horizontal shifting of the denture base.
III.FRONTAL PLANE
The final plane is a frontal plane. Movement in this plane occurs
relative to an antero-posterior axis running perpendicular to the frontal
33
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
plane. The movement is rotation about an imaginary vertical axis located
near the center of the dental arch.This movement occurs under function
because diagonal and horizontal occlusal forces are brought to bear on the
partial denture.
It is resisted by stabilizing components, such as reciprocal clasp
arms and minor connectors that are in contact with vertical tooth surfaces.
Such stabilizing components are essential to any partial denture design,
regardless of the manner of support and the type of direct retention
employed. Stabilizing components on one side of the arch act to stabilize
the partial denture against horizontal forces applied from the opposite
side. It is obvious that rigid connectors must be used to make this effect
possible.
34
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
FORCES ACTING ON REMOVABLE PARTIAL DENTURE
Removable partial dentures (RPD) have to be in a state of
equilibrium, i.e., a state in which opposing forces or influences are
balanced. Keeping in mind Devan's statement ‘to preserve what
remains,’’ forces should be given major consideration while designing a
partial denture, to ensure the dynamics of these appliances without
deleterious effects to the supporting structure.
The Supporting structures for removable partial are structurally
adapted to receive and absorb forces within their physiological tolerance.
The ability of these structures to tolerate forces is largely dependent upon
the magnitude, the duration and the direction of these forces in addition to
the frequency of force application.
The magnitude of forces acting on partial dentures depends on age
and sex of the patient, the power of the muscles of mastication and the
type of opposing occlusion.Natural teeth are better able to tolerate
vertical directing forces acting on them. This is because more periodontal
35
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
fibers are activated to resist the application of vertical forces. On the
other hand, lateral forces are potentially destructive to both teeth and
bone. Lateral forces should be minimized in order to be within the
physiologic tolerance of the supporting structures.
Removable partial dentures are subjected to a composite of forces
arising from three principal fulcrums. One fulcrum is on the horizontal
plane that extends through two principal abutments, one on each side of
the dental arch, and generally is termed the principal fulcrum line.This
fulcrum controls the rotational movement of the denture in the sagittal
plane ( i e, denture movement toward or away from the supporting
ridge). Rotational movement around this fulcrum line is the greatest in
magnitude,but is not necessarily the most damaging.
The resultant force on the abutment teeth is usually mesioapical or
distoapical, with the greatest vector in the apical direction the fibers of
the periodontal ligaments are arranged so that axially aligned forces are
resisted 17 times greater than the non-axial loads. Therefore, horizontal or
lateral forces are of much less magnitude and can be more destructive to
the hard and soft tissues of the periodontium.
A second fulcrum line lies in the sagittal plane and extends through
the occlusal rest on the terminal abutment and along the crest of the
residual ridge on one side of the arch.In a Class I situation, there would
be two such lines, one on each side of the arch.
This fulcrum line controls the rotational movements of the denture
in the frontal plane (ie, a rocking movement over the crest of the ridge).
This movement is easier to control than the first and usually not as great
in magnitude. The resultant forces are more nearly horizontal and are not
36
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
well resisted by the oral structures. Therefore, these forces can be
moderately damaging and should be given thorough consideration in the
design process.
The third fulcrum is located in the vicinity of the midline, just
lingual to the anterior teeth.This fulcrum line is oriented vertically and
controls rotational movement in the horizontal plane (ie,the flat, arcuate
movements of the prosthesis). Due to its orientation, the force resulting
from this movement is almost entirely horizontal. Consequently,these
forces can be extremely damaging and should receive significant
attention during the design process.
Every effort must be made to control or minimize the rotational
movements related to these three principles
37
TYPES OF FORCES ACTING ON RPD
I.Vertical forces(Fig-22)
a.) Tissue-ward movements b.) Tissue-away movements
II.Horizontal forces:(Fig-23)
a.) Lateral movements b.) Antero-posterior movements.
III.Rotational forces:(Fig-24)
They are due to the variation in compressibility of supporting structures,
absence of distal abutment at one end or more ends of denture bases, and /or
absence of occlusal rests or clasps at any end of the bases.
a.)Rotation of the anterior and posterior extension denture base around coronal (transverse) fulcrum axis:
i.)Rotation of the denture base towards the ridge around the fulcrum axis
(joining the two main occlusal rests)
ii.) Rotation of the denture base away from the ridge around the fulcrum axis
(joining the retentive tips of the clasps.)
b.)Rotation of all bases around a longitudinal axis parallel to the crest of the residual ridge (Buccolingual or labiolingual).
c.)Rotation about an imaginary perpendicular axis, this axis either near the center of the dental arch in class I, or is the long axis of abutment tooth in class II partial denture.
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
I.Vertical forces
a) Tissue-ward movements.
Tissue-ward forces are,“Vertical forces acting in gingival direction
tending to move the denture towards the tissues”.They occur during
mastication, swallowing and aimless tooth contact. Biting forces falling
on artificial teeth are transmitted to the soft tissues and bone underlying
the denture base.
The partial denture should be designed to resist this movement by
providing adequate supporting components. This function of the partial
denture is called “Support”.
b.)Tissue-away movements
Tissue-away dislodging forces are, "Vertical forces acting in an
occlusal direction tending to displace and lift the denture from its
position”.Tissue-away forces occur due to the action of muscles acting
along the periphery of the denture, gravity acting on upper dentures or by
sticky foodadhering to the artificial teeth or to the denture base.
The partial denture should be designed to resist this movement by
providing adequate “Retention”.
II.Horizontal forces
a.)Lateral movements
Lateral forces are “Horizontal forces developed when the mandible
moves from side to side during function while the teeth are in
contact”.Lateral movements have a destructive effect on teeth leading to
tilting, breakdown of the periodontal ligament and looseness of abutment
teeth. The application of lateral forces causes areas of compression of the
38
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
periodontal membrane, which leads to bone resorption. Hence lateral
forces play a major role in bone resorption.
Partial dentures should be designed to prevent the deleterious
effects of lateral forces by using stabilizing or bracing components.
The magnitude of lateral forces could also be minimized by:
1.Reducing cusp angles of artificial teeth.
2. Providing balanced occlusal contacts free of lateral interference
The removable partial denture being anchored to both sides of one
arch and joined by a rigid major connector can provide cross arch
stabilization to forces acting in bucco-lingual direction.
b.)Antero-posterior movements
Antero-posterior forces are "Horizontal forces which occur during
forward and backward movement of the mandible while the teeth are in
contact". This may result in movement of the denture.There is natural
tendency for the upper denture to move forward and for the lower denture
to move backward.
Forward movement of the upper denture could be resisted by:
Anterior natural teeth.
Palatal slope.
Maxillary tuberosity.
The natural teeth bounding the edentulous space.
The backward movement of the lower denture could be resisted by:
The slope of the retromolar pad.
39
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The natural teeth bounding the saddle area.
Proximal plates.
III.Rotational forces
Rotational forces are “Forces acting on the partial denture either in
vertical or horizontal direction causing rotation (torque) of the denture
base around an axis.In tooth supported removable partial dentures, the
abutment teeth on both sides of the edentulous area provide adequate
support and resistance to rotational forces through supporting rests and
clasps placed on them.In distal extension partial denture when vertical
forces are applied the difference in displaceability of the supporting
structures often results in rotation of the partial denture around a fulcrum
axis and application of torque on abutment teeth.
Rotational movements must be counteracted in the partial denture
design to minimize their destructive effect on both,teeth and the residual
ridge.Rotational forces acting on distal extension partial denture may
result in three possible rotational movements these are
i.)Rotation of the denture base around the fulcrum axis (Torque).
ii.)Rotation about a longitudinal axis formed by the crest of the
residual ridge (Tipping movement).
iii.)Rotation about an imaginary perpendicular axis near the center
of the dental arch (Fish tail movement).
a.)Rotation of the anterior and posterior extension denture base
around coronal (transverse) fulcrum axis:
Movement of the component parts of the denture lying on the
opposite side of the fulcrum axis occur in a direction opposite to that of
40
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
the applied force. This leads to rotation of the denture:The fulcrum axis is
an “imaginary line passing through teeth and component parts of the
partial denture around which the distal extension partial denture rotates
when a vertical force is applied”.More than one fulcrum lines may be
identified for the same removable partial denture depending on the
direction and location for force application.
i.)Rotation of the denture base towards the ridge around the fulcrum axis
This movement results from occlusal stresses occurring during
mastication and occlusion of teeth. The free extension denture base
moves tissue-ward while other components on the opposite side of the
fulcrum line moves away from the tissues.This result in rotation of the
denture about a diagonal supportive fulcrum line joining two occlusal
rests on the most posterior abutments on either side of the dental arch.
Tissue ward movement of the base could be limited by supporting
structures, which are:
Supportive form of the residual ridge,
Accurate and properly extended bases.
Artificial teeth set on the anterior two third of the base
Flexible clasps are preferred over rigid clasping to reduce stresses
and torque applied on abutments. If the clasps are rigid, the abutments
tend to rotate distally during tissue ward movement of the denture base
resulting in periodontal breakdown and looseness of teeth.
ii.) Rotation of the denture base away from the ridge.
41
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
This movement occurs due to the pulling effect of forces applied
by sticky food, gravity on upper dentures and the elastic rebound of soft
tissues covering the edentulous areas.
Tissue-away rotation of denture base is counteracted by:
Indirect Retainers: which are the components of partial denture
located on the side of the fulcrum axis opposite to the distal
extension base.
The retentive tip of the clasp arm.
Adequate coverage and extension of the base (direct indirect
retention)
Effect of gravity on mandibular bases.
b.)Rotation of all bases around a longitudinal axis parallel to the
crest of the residual ridge
This rotation occurs due to application of vertical forces on one
side of the arch only. It causes twisting of the denture base.
This movement is counteracted by:
Cross arch stabilization (The action of clasps on the opposite side
of the arch).
Broad base coverage.
Proper placement of artificial teeth (teeth on the ridge or
lingualized occlusion).
Narrow teeth bucco-lingually
The effect of rigid major connectors
c.)Rotation about an imaginary perpendicular axis, this axis either
near the center of the dental arch
42
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Application of horizontal or off-vertical force results in rotation
around an imaginary vertical axis located either about the axis of
abutment in class II or near the center of the dental arch, lingual to
anterior teeth in class I.
It results due to the application of masticatory forces falling on distal
extension bases causing buccolingual movementof the base. This rotation
is called fishtail movement.
This movement is counteracted by :
Providing adequate bracing components in the partial
denture.
A rigid major connector.
Broad base coverage.
Balanced contact between upper and lower teeth.
Forces occuring through a removable restoration can be widely distributed,
directed, and minimized by the selection, the design, and the location of components of
removable partial dentures and by developing a harmonious occlusion.
FORCE CAUSE OF THE FORCE
COUNTERAC--TION OF FORCE
FUNCTION
I.Vertical
Forces
a.)Tissue ward
movements
Functional
movements during
mastication,
swallowing and
occlusion of upper
and lower teeth
-Rests placed on
abutments in
bound saddles
-Rests & proper
coverage in free
end saddles
-Maxillary
Support
43
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
connectors
b.)Tissue
away
movements
Pulling effect of
sticky food provides
gravity on upper
dentures and excess
muscle forces acting
on the periphery of
the denture
-Retainers
-Adhesion &
cohesion
between denture
base & tissues
Retention
II.Horizontal
forces
a.)Lateral
Forces
Side to side
movements of the
mandible while teeth
are in contact.
-Rigid bracing
clasp arms.
-Major
connectors.
-Balanced
occlusion.
-Maximum
extension of
the flanges
Bracing
b.)Antero-
posterior
forces
Forward and
backward
movement of
mandible
while teeth are in
contact
-Abutments
adjacent to
the denture.
-Guiding planes.
Stabilization
III.Rotational
Forces
a.Rotation of
the denture
Functional
movements
.
Supporting rests
and properly
Supporting
rests.
44
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
base towards
the ridge
around the
fulcrum axis
while teeth are in
occlusion
adapted bases -Properly
adapted bases
b.Rotation of
the denture
base aways
from the ridge
around the
fulcrum axis
-Sticky foods gravity
on upper
dentures,elastic
rebound of tissues
under the base
-Indirect
retainers
-Direct retainers
Indirect
retention
45
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
PHILOSOPHYOF PARTIAL DENTURE DESIGN
There are four design concepts, which can be used to distribute the
force evenly along the tissues and supporting tooth structure. They are :
Conventional rigid design.
Stress equalization.
Physiologic basing.
Broad stress distribution.
Physiologic basing(Fig-26)
The philosophy of design agrees in part with the first school about
the relative lack of movement of the abutment teeth in an apical direction
but denies the necessity of using stress directors to equalize the disparity
of vertical movement between the tooth and the mucosa. The belief is that
the equalization can best and most simply be accomplished by some form
of physiologic basing, or lining, of the denture base.(Fig-26)
The physiologic basing is produced either by displacing or
depressing the ridge mucosa during the impression making procedure or
by relining the denture base after it has been fabricated. The reason for
displacing the mucosa during the impression procedure is to record the
soft tissue in its functioning, not anatomic, form. If the tissues are
recorded in their functional state, the denture base, formed over the
displaced tissue, will be better able to withstand the force that is
generated.
46
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
It is obvious that in such situation, the artificial teeth will be
positioned above the plane of occlusion when the denture is in mouth and
not in function. To permit vertical movement of partial denture from
the rest position to the functioning position, the direct retainers or
retentive clasps must be designed with minimal retention and the number
of direct retainer must be limited. The occlusal rest and direct retainers
will also be slightly unseated at rest. They will be completely seated only
when the mucosa beneath the denture base is displaced to its functional
form.
ADVANTAGES
a) The intermittent base movement has a physiologically stimulating
effect on the underlying bone and soft tissue , which reduces the
frequency of relining or rebasing the prosthesis (there will be less
bone loss )
b) Simplicity of design and constructive because of the minimal
retention requirements results in a light weight prosthesis needing
minimal maintenance and repair
c) An additional advantage is gained by the minimal direct retention
used. The looseness of the clasp (combination clasp with wrought
wire retentive arms) on the abutment tooth reduces the functional
forces transmitted the abutment tooth. Hence the abutment teeth
are preserved for longer time duration.
DISADVANTAGES
1. The denture is not stabilized against lateral forces
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
2. The residual ridge receives the greater proportion of forces that
are transmitted by the denture, hence more chances of bone
resorption
3. The load of stabilizing and supporting the denture is limited to a
few teeth instead of being shared by a number of teeth as in other
design philosophies
4. There will always be slight premature contacts between the
opposing teeth and the denture teeth when the mouth is closed.
This is an uncomfortable situation to many patients and may
result a sense of insecurity
5. It is a difficult to produce effective indirect retention because of
the vertical movement of the denture and the minimum retention
of the direct retainer.
Broad Stress Distribution(Fig-25)
According to this philosophy of design, the occlusal load acting on
the denture should be distributed over a wider soft tissue area and
maximum number of teeth. This is achieved by increasing the number of
direct retainers, indirect retainers, and rests and by increasing the area of
the denture base.(Fig-25)
Advantages
This design with multiple clasps acts as a form of removable
splinting.
It increases the health of the abutment teeth (due to splinting
action).
Easier to construct and economical.
Disadvantages
Less comfortable.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Difficult to maintain adequate oral hygiene
Conventional Rigid Design
The denture is designed with rigid component which act like a raft
foundation to evenly distribute the forces on the supporting tissues. This
design is used in all general cases. The flexible component of these
dentures is their retentive terminal.
Advantages
Easy to construct and economical.
Equal distribution of stress between the abutment and the residual
ridge.
Reduced need for relining as the ridge and abutment share the load.
Indirect retainers prevent rotational movement and also stabilize
the denture
during horizontal movements.
Less susceptible to distortion.
Disadvantages
Increased torquing forces on the abutment teeth.
Rigid continuous clasping may damage the abutment teeth.
Dovetail intracoronal retainers cannot be used in these cases as
tipping forces from the denture base will be directly transmitted to
the abutment teeth.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Tapered wrought wire retentive arm (combination clasp) cannot be
used, as it is difficult to construct.
Relining is difficult and inappropriate relining leads to damage of
the abutment teeth.
Stress Equalization or Stress Breaker or Stress Directing Concept(Fig-27)
A stress breaker is defined as, “A device which relieves the abutment teeth of all or part of the occlusal forces" - GPT.
A stress director is a device that allows movement between the
denture base and the direct retainer which may be intracoronal or
extracoronal. Dentures with a stress breaker are also called as Broken
stress partial dentures or Articulated prostheses.(Fig-27).
We know that the soft tissues are more compressible than the
abutment teeth. In a tooth tissue supported partial denture, when an
occlusal load is applied, the denture tends to rock due to the difference in
the compressibility of the abutment teeth and the soft tissue As the tissues
are more compressible, the amount of stress acting on the abutments is
increased. This can produce harmful effects on the abutment teeth.In
order to protect the abutment from such conditions, stress breakers are
incorporated into a denture.
There are two types of stress breakers:
Type I
Here a movable joint is placed between the direct retainer and
denture base. This joint may either be a hinge or a ball and socket or a
50
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
sleeve and cylinder. Adding these stress breakers to the junction of the
direct retainer and the denture base, allows the denture base to move
independently.This decreases the amount of force acting on the abutment.
The combined resiliency of the periodontal ligament and the stress
director will be equal to the resiliency of the oral mucosa overlying the
ridge.
Examples for hinges include DALBO, CRISMANI, ASC 52
attachments.
Type II
It has a flexible connection between the direct retainer and the
denture base. It can be a wrought wire connector, divided or split major
connector or a movable joint between two major connectors.In a split
major connector, the major connector is split by an incomplete cut
parallel to the occlusal surface of the teeth into two units namely the
upper unit (more near to the tooth) and the lower unit. The denture base is
connected to the lower unit and the rests and direct retainers are
connected to the upper unit.
Advantages
The alveolar support of the abutment teeth is preserved as the
stresses acting on the abutment teeth are reduced.
The stress on the residual ridge and the abutment teeth are
balanced.
Weak abutment teeth are well splinted even during the
movement of the denture base.
Abutment teeth are not damaged even if relining is not done
appropriately (after the denture wears out).
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Minimal requirement of direct retention.
Movement of the denture base produces a massaging effect on
the soft tissues.
This avoids the frequent need for relining and rebasing
Disadvantages of stress breakers
1. The broken stress denture is usually more difficult to fabricate and
therefore more expensive
2. Many stress breakers designs are not well stabilized against
horizontal forces.
3. The effectiveness of indirect retainers and cross arch stabilization
is reduced or eliminated altogether.
4. The more complicated the prosthesis, the less the patient may
tolerate it.
5. Spaces between components are sometimes opened up in function,
thus trapping of food and occasionally the tissue of the mouth
leading to injury and periodontal problems.
6. Flexible connectors may be bent and distorted by careless
handling.
7. Repair and maintenance of any stress breaker is difficult, costly
and frequently required.
8. If relining is not done whenever needed, it may result into
excessive resorption of residual ridge.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
SUPPORT MECHANISM IN REMOVABLE PARTIAL DENTURE
Support is derived from bone, for it is to the bone that all forces are
ultimately transmitted, either via the mucosa and periosteum or via
the teeth and periodontal ligament. The mucosa is an inappropriate
tissue to resist occlusal forces, as any complete denture wearer will
attest to. In a partially edentulous situation, using mucosa only invites
iatrogenic damage.Therefore,vertical support must always be provided by
some of the remaining teeth for all removable partial dentures.
CHARACTERISTICS OF SUPPORT BEARING AREAS
The forces directed to the supporting tissues will be partially
absorbed and partially transmitted to adjacent tissues. The percentage of
force absorbed or transmitted will vary depending upon which tissue is
involved. Bone is the tissue which ultimately absorbs the greatest amount
of the force applied to both the muco-osseous and dento-alveolar
segments.
DENTO-ALVEOLAR SUPPORT
A. TEETH.(Fig-28)
Teeth should be
1.Structurally sound and
2.Anatomically favourable.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
1. Structurally sound.
Functional forces are transmitted by a partial denture to the tissues
with which it is in contact.If a denture is supported primarily by the
natural teeth most of the forces will be transmitted to the alveolar
bone through the fibres of the periodontal ligament.
Tooth structure:Structurally sound vital teeth are capable of
withstanding normal functional forces.Excessive forces applied to the
tooth may result in adverse effects such as
i. Tooth fracture.
ii.Tooth movement.
iii. Pulpal irritation.( Pulpal hyperemia or irreversible pulpitis.
Structurally compromised teeth may fail in response to normal
functional forces.Few examples are
i.Teeth with large intracoronal restorations.
ii.Endodontically treated teeth.
Endodontically treated teeth are structurally weak due to dessication of
dentin leading to loss of its organic content which ultimately makes the
dentin brittle.This brittle dentin when subjected to occlusal forces may
fracture and loss of teeth structure.
2.Anatomically favorable.
a. Root surface area.
b. Root morphology.
c. Presence of multiple roots.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
d. Presence of divergent roots
e. Crown to root ratio.
f. Axial Inclination
B.PERIODONTIUM(Fig-29)
These includes gingiva, crevicular epithelium, junctional
epithelium, connective tissue attachment, cementum, periodontal
ligament and alveolar bone. Healthy periodontium permits force
absorption without damaging effects.Excessive forces may increase the
width of the periodontal ligament and result in increased tooth mobility.
Health periodontium should be
1.Anatomically favorable.
a. Normal epithelial and connective tissue attachment.
b.Adequate zone of attached gingiva.
2.Absence of periodontal disease
Plaque induced inflammation may compromise the
periodontium. It can lead to apical migration of the crevicular
epithelial attachment (functional epithelium) and destruction of the
fibroblasts and connective tissue of the connective tissue attachment.
In the presence of inflammation normal functional forces may
accelerate the rate of periodontal attachment loss.
The presence of plaque induced periodontal disease is
associated with a loss of bone height. Moderate forces may
accelerate the
55
BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
disease process resulting in further bone loss, less bone support, and
increasedmobility of the teeth.
A healthy periodontium should have
a.Gingival indices within normal limits.
b.Absence of mobility or hyper mobility.
C.ALVEOLAR BONE.
Residual ridge support(Fig-30)
As has been said, the entire root surface area ot an arch ot teeth is
about 45 cm2 . It is as well to consider this in terms of the area
remaining when the teeth arc lost. It has been calculated that the
entire denture bearing area when all teeth are lost is about 20 cm for
the maxilla, and about 12 cm2 for the mandible. Hence in the
partially edentulous situation, ii will always be preferable, just from this
consideration alone, to use the teeth for support. It residual ridge support
is to be used as well, then it follows that full use should be
made of a fully extended denture base.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The residual alveolar ridge, though, has forces transmitted to it by
the overlying mucoperiostium, and this too will resist forces in a
manner which will depend on its morphology. There is a wide
variety of thickness and type of ridge mucosa, with some areas being
almost seven times thicker than others.There are three main
histological types of mucosa.Buccal mucosa is partially keratinised and
has underlying elastic tissue; mucosa of the floor of the mouth is similar
but non-keratinised. Both these types are not firmly attached to the
underlying bone, in contrast to the third type, the attached ridge mucosa,
which is usually keratinised and much more able to withstand loads.
FORM OF RESIDUAL RIDGE
The residua ridge itself is also uneven in shape, and this will affect
not only resistance to loading forces, but also resistance to laterally
directed forces,inother words, the stability of the denture base overlying
the ridge.In A, a flat ridge will provide good support but poor
stability. The varying thickness of the mucosa and the sharp and
often spongy ridge in B provides poor support. In C, neither good
support nor stability are present, because of the flabby and
displaceable tissue over the ridge.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The ridge often becomes sharp and uneven because of the uneven
resorption of bone following tooth extraction.This depends on many
factors, such as the nature and health of the alveolar bone prior to
extraction of the teeth, and the manner of resorption of the smooth
conical bone, which varies from individual to individual. Also
varying, is the type and position of the muscle attachments, which
may form sharp and pointed ridges.
With age. some of the tendinous attachments may become calcified,
and with increasing resorption, all attachment sites can become
relatively prominent (this is common at the genial and mental
tubercles and along the mylohyoid ridge).
PRESSURE-TENSION THEORY:
Bone tends to resorb in response to compressive force and to be
stimulated by tensional force. In order to preserve remaining alveolar
bone,it is important that functional forces be transmitted to bone
primarily as tension rather than pressure whenever possible.
In tooth borne situations the majority of functional forces are
transmitted as tension to bone through proper rest design and rest seat
preparation.In tooth mucosa borne situations some of the vertical seating
forces are transmitted as tension to the bone through the rests.
Horizontal forces are transmitted as a combination of compressive and
tensional forces to the alveolar bone(e.g.those forces directed through
bracing clasps,proximal plates and minor connectors contacting proximal
tooth surfaces and guiding planes).Vertical displacing forces are
transmitted to the bone as both compressive and tensional forces
(e.g.sticky foods or retentive clasps engaging undercuts).
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
BONE INDEX
The response of bone to pressure varies in terms of the rate of
resorption depending on genetic,nutritional, hormonal and biochemical
and other intrinsic factors. The bone index is determined by analyzing the
previous response of bone to force. The bone index of the alveolar bone
surrounding natural teeth may differ from that of the bone comprising the
residual ridges.
CORTICAL VS. CANCELLOUS BONE
The residual ridge crest is comprised mainly of cancellous bone
and is less resistant to resorption. The facial and lingual inclines of the
residual ridges are comprised of cortical bone and are more resistant to
remodelling. The rate of cancellous bone resorption has been described as
being approximately three times that of cortical bone. Excessive forces
may increase the rate of bone resorption.
Moderate forces may result in accelerated bone resorption when
intrinsic factors, local abnormalities or systemic disorders compromise
the bone index of the individual.
MUCO-OSSEOUS SUPPORTA.MUCOSA.
Keratinized and Firmly bound.
B.SUBMUCOSA.
1. Normal sub mucosa serves as an "hydraulic cushion".
2. Firmly bound and dense.
C.BONE.
1. Cortical bone.
2. Favorable bone index.
3. Presence of muscle attachments which direct tension to bone (or the
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
equivalent in terms of resistance to pressure induced resorption).
DISPLACEMENT CHARACTERISTICS OF PERIODONTIUM AND RESIDUAL RIDGE MUCOSA
The previous discussion of the types and sites of support available
for a partial prosthesis leads to a consideration of the different
characteristics that the support mechanisms may have. A denture that
uses both tooth and residual ridge support is dento-mucosally supported,
and because the nature of the tissues varies, one must be aware of the
loading characteristics of each type When load is applied to a material
there are basically three ways in which the material can react, depending
on its nature,as shown in figure where D is the displacement, and T-time.
A purely elastic material will be displaced immediately, and then
immediately recover to its original form or position on removal of the
force applied it obeys Hooke's law). When a viscous substance, such as
oil, is subjected to load,it will gradually be displaced to reach a resting
state, and will not recover on removal of the force (it behaves as a
Newtonian element).
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
A combination of these two effects occur in viscoelastic materials
which behave as a voigt model.Application of the load will cause a
relatively free phase of displacement or distortion, the rate of which will
lessen until equilibrium is reached. Removal of the load will reveal a
relatively fast recovery phase followed by a prolonged and gradual return
to the original state. Hence the response depends on the rate of loading as
well as the magnitude and duration of the load.
Teeth and mucosa behave as viscoelastic materials, but with quite
different characteristics. When a tooth is loaded,there is an initial rapid
displacement as a result of movement of tissue fluids and cell distortion,
followed by a stiffer more gradual response as the periodontal fibres are
loaded.When the applied force is removed, the tooth recovers to its
original position rapidly,within a minute or two.
The response of oral mucosa,however,is much more akin to a
classical visco-elastic response, and depends far more on the magnitude
and the duration of the loads applied. This has been tested by applying
different loads to an acrylic plate placed on palatal mucosa.For static
loading, when the load was applied suddenly there was an instantaneous
elastic displacement, and as the load was maintained constantly for 10
minutes a further gradual displacement (creep) occurred.
On sudden removal of the load there was an instantaneous elastic
recovery, followed by a viscoelastic recovery that can last upto four
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
hours( the heavier the load, the longer the recovery).For dyanamic
loading an increase in the loading rate reduces the amount of mucosa
displacement at 4 Newtons per second the plate was displaced 500µ and
at 100N/sec it was displaced only 375µm.
More importantly, under functional conditions in the mouth,
loading varies with each chew, and the effects of simulating this have
also been studied. With successive chews, there is a progressive
displacement, but also a progressive failure to recover, so that an
equilibrium at a displaced position relative to the starting position is
reached.
These displacement characteristics of mucosa can be explained by
considering the structure of mucosa itself its thickness and fluid flow
characteristics when depressed will cause the variety of responses,
together with the general physiological tissue characteristics of the host.
This latter becomes an important consideration when age is taken into
account.
It has been shown that there is a decrease in mucosa thickness with
age, and a significant difference in recovery characteristics ,the tissues in
elderly people take many hours to recover from the effect of moderate
mechanical force, whereas the tissues of a 25 year old,for example,
require much less lime to recover from the same force. In figure the
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
mucosa of'young patients (age range 15-25 years recovered faster and
further than that of older patients (age range 72-86 year).
The choice of support
It should be apparent from the above discussion that oral mucosa
presents a much more varied and greater response to loading than the
periodontium.
The diagram illustrates the displacement of tooth-borne and
mucosa-borne plates when a load was applied and maintained for 30
seconds.The tooth-borne plate displaced the least, with a load of 1N as
shown by the line A. The mucosa-borne plate at the same load displaced
further, as shown by the line. B. When this plate was given a load of 4N
the greatest displacement was measured, as the lineC.
Under certain conditions, mucosa can be displaced up to twenty
times that of the periodontium, and this can create many clinical
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
complications especially with dento-mucosally supported dentures.
The practical application of this problem will be dealt with under a
number of different sections following, but it is essential to understand
the biomechanical nature of the problem. It should be obvious now that it
would be preferable to use the teeth for support at all times, and to avoid
any loads on the mucosa at all but there are occasions when the mucosa
must also be used for support, and when it is, there must be some
compensation made for the difference in displacement characteristics of
the mucosa and the teeth.Otherwise, at every bite, the denture will move
in a manner such that not only will the patient will be unable to control
the movement,but the movement may cause iatrogenic damage to the
teeth or tissue.
The most obvious situation where the residual ridge must be used
is when there are no teeth distal to the gap the dento-mucosally (usually
called dento-gingivally.) supported denture. The residual ridge must be
used to support that part of the denture carrying the missing teeth.Less
obvious, are the situations where the state of the teeth and periodontium
are such that they could not carry the extra loads of a prosthesis without
exceeding their physiological tolerance to do so.In this case, once again
iatrogenic damage to the teeth and their supporting tissues may occur.
Over the years, a variety of clinicians have offered suggestions for
classifications of dentures based on support. For any classification to be
useful, it should be:
Consistent
Unambiguous
Generally accepted
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
No one classification is ideal, and perhaps the most useful is that
outlined, in one form or another, by Beckett (1953), Craddock (1956) and
Osborne and Lammie (1974)
Class I: Denture supported by mucosa and underlying bone
Class II: Denture supported by teeth.
Class III: Denture supported by a combination of mucosa and tooth-borne means.
We consider that this classification should now be extended to include a
further type, namely:
Class IV: Denture supported by implants.
It must be realized that this classification is not ranked in order of
precedence but could perhaps be considered in order of complexity of
planning. For this reason, the support options will be discussed in the
above order.
Class I Dentures (deriving their support from mucosa and underlying
bone)
Wills (1978) clarified some misconceptions on the displacement
and deformation properties of oral mucosa with their research on
primates. They determined that the effects of loading mucosa over a long
period were to compress it by up to 45% of its original thickness and,
further, that its recovery was visco-elastic in nature. The time required for
recovery from the displacing forces has also been found to increase with
age. What this clearly means, however, is that prostheses which derive
their support from mucosa and the underlying bone will inevitably do two
things:
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Displace mucosa
Result in further loss of alveolar bone (this is perhaps of greater importance).
From the above, it is clear that in mandibular dentures especially,
mucosa-borne partial prostheses ought to be considered as a last resort, or
possibly as a transitional phase to complete dentures.More latitude exists
in the maxilla, however, where the hard palate affords additional support,
but this is often abused.
Class II Dentures (deriving their support from teeth)
Tooth-supported prostheses gain their support from the teeth via
the supreme qualitative and quantitative support agent, namely the
periodontal membrane. Pressure down the long axis of the tooth imparts
tension in the periodontal membrane, which in turn helps to maintain
alveolar bone. Clearly this is the most desirable form of support and
should be used whenever practical. It has traditionally been taught that
dentures may gain tooth support from incisal rests, occlusal rests or
cingulum rests.
The statements in the foregoing paragraph indicate that,
theoretically, support derived from teeth is more desirable than any other
single form of support, and this is a scientifically established fact.
However, on occasion the clinician has a need to be empirical and to
prescribe what is most appropriate for the patient. For example, a patient
who has been treated for chronic periodontal disease may have lost
considerable bony support,and a cast metal framework utilizing occlusal
rests and cast cobalt chromium clasp assemblies may impart
inappropriate forces on a tooth.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Class III Dentures (deriving their support from a combination of mucosa
and tooth borne means)
In tooth tissue supported RPD attention must be given to both
abutment and edentulous ridge. For the abutment teeth these
consideration are periodontal health, crown and root morphology, C/R
ratio, bone index area, location of abutment in the ridge, and opposing
dentition. For edematous ridge these consideration are the quality of the
ridge, the extent of the ridge covered by denture base, the type and
accuracy of impression technique, and the partial denture design.
The greatest difficulty occur in transition area where tooth support
ends and mucosa support begins ,when functional occlusal load is applied
to denture base,an axis of rotation is created ,the denture tend to rotate
about its most distal abutment inducing heavy torisonal stresses on the
abutment teeth and possible traumatization of the ridge.
The degree and direction of the denture base movement are greatly
influenced by the quality of the supporting residual ridge, the design of
RPD and the extent of the forces exerted on the denture during
function.When RPD with both anterior and posterior denture bases
present a stress problems, since the length of the ridge area extends
anterior and posterior to the fulcrum clasping areas produces a double
acting lever problem for the abutment teeth.
It is perhaps no coincidence that clinicians and patients alike have
embraced the shortened dental arch philosophy. The option to do nothing
or to use a fixed prosthesis to replace one dental unit(e.g. by cantilevering
one unit from the terminal abutment) is seen as being less problematic
than providing a removable prosthesis to replace several missing teeth.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
From the clinician’s viewpoint this is because of the very real and
problematic differences between the two supporting elements,and from
the patient’s perspective because of intensive tissue
coverage.Extrapolating the results of Wills and Manderson (1977) and
Wills et al. (1980), the clear fact emerges that, long after abutment teeth
have returned to their resting positions (after mastication, for example),
the mucosa will remain displaced; this displacement is of the order of 20
times that of the teeth even on the basis of a maximally covered saddle. It
will be self-evident to state that mucosa under minimally covered saddles
will be displaced even more than under maximally covered saddles.This
support differential is thus problematic,and the inherent tendency for a
prosthesis to demonstrate rocking (instability) has resulted in
philosophies of clasping which were based on homeostatic principles of
stress-breaking, whereas others were based on more biological
principles(Kratochvil, 1963; Krol, 1973).
Class-IV Dentures(Tissue Implant supported removable partial
denture)
The design and maintenance of bilateral and unilateral distal
extension partial dentures (Kennedy Class I and II) present challenges for
clinicians,as these dentures require support from the teeth, the mucosa
and the underlying residual alveolar ridges. In particular, the distal
extension removable partial denture is subjected to vertical, horizontal
and torsional forces due to the different resiliencies of alveolar mucosa
and periodontal ligament of abutment teeth that may have adverse effects
during functional and para-functional activities.
To prevent displacement of the denture, precision attachments or
conventional clasps have been widely used. However, the rotational
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
tendency of the RPD after long-term use cannot be eliminated
completely, regardless of design and fit of the denture. To overcome this
clinical challenge, single implants may be placed bilaterally at the distal
extension of the denture base to minimize the potential for dislodgement
of the denture.
The chief goal of placing an implant under the posterior-most
molar of the distal extension denture base is to stabilize the RPD in a
vertical direction. Distal implants effectively convert a Kennedy Class I
or II denture to a Kennedy Class III denture. Therefore, a tooth- and
implant-supported RPD is cheaper (because fewer implants are needed)
and more stable, and may therefore be a better option for patients with
limited financial resources than an implant-supported fixed partial
denture.
IMPLANT CORRECTED REMOVABLE PARTIAL DENTURES
The classification will always begin with the phrase "Implant-
Corrected Kennedy (class)," followed by the description of the
classification. It can be abbreviated as follows:
(i) ICK I, for Kennedy class I situations,
(ii) ICK II, for Kennedy class II situations,
(iii) ICK III, for Kennedy class III situations, and
(iv) ICK IV, for Kennedy class IV situations.
ICK I, for Kennedy class I situations
The Kennedy Class I partially edentulous arch has bilateral distal
extensions. The functional load is transmitted to the teeth and the soft
tissue. Implant location depends primarily on the dimensions of the
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
residual ridge and the biomechanical considerations of the RPD design.
Two distally positioned implants in the area of the second molars would
effectively trans-form the Kennedy Class I configuration to a more
favorable Kennedy Class III.
Theoretically, the implants should be located as distally as possible
to provide maximal support and stability. This is of special importance in
the mandible because of the significant displacement of the denture base
that is not supported by the major connector. The implants might be used
for support only using healing caps or for retention with resilient
attachments connected to the implants. A low-profile attachment is
preferred to decrease the off-load forces to the implants.
Note:Endodontically treated abutments would be specifically
beneficial when used for support only without direct retainers applying
unfavorable lateral displacing forces.
Drawbacks:
An inadequate posterior ridge dimension could restrict implant
placement to a more anterior location.
The Implant therapy is versatile
In the future, the patient might select to restore the edentulous
ridges with fixed implant-supported restorations(in such case the
implants should be located more medially, adjacent to the existing
abutments, to allow future prosthodontic use)
ICK II, for Kennedy class II situations
The Kennedy Class II partially edentulous arch has a unilateral
distal extension. An ISRPD should be usedwhen the tooth loss is
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
extensive.Otherwise,just as when only the molars are missing, the patient
might not use the prosthesis. When the patient has no functional problem,
a shortened dental arch concept,with no prosthesis, should be considered
Placing a single implant in the posterior regionwould modify the
Kennedy Class II configuration to a Kennedy Class III and increase the
stability and reten-tion of the prosthesis. The same considerations dis-
cussed for the Kennedy Class I tissue ISRPD also apply.
ICK III, for Kennedy class III situations
The Kennedy Class III partially edentulous arch has edentulous
space bounded by teeth. Therefore, implants should be used when the
edentulous space is long, the abutments are compromised, and when
thepatient objects to the appearance of the clasps.The implants should be
placed adjacent to the abutments.
ICK IV, for Kennedy class IV situations
The Kennedy Class IV partially edentulous arch hasa single,
anterior edentulous space that crosses the midline and is bounded by the
remaining teeth. The implants should be placed as medially as possible
tothe abutments to provide optimal support. The labial flange of the
prosthesis might serve to restore the lip support in these ISRPDs. The use
of implants inKennedy Class IV partially edentulous patients renders the
use of retentive clasps and elaborated dual-path RPD designs
unnecessary.
Clinical guidelines for Implant Supported removable partial denture
1. Place implants in area of second molars in distalextension patients.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
2. Place implants adjacent to distal abutment in case future fixed
restoration is an option, distal abutments are poor, or patient is
concerned about unesthetic clasp showing.
3. Place implants medially in Kennedy Class IV arch.
4. Use short or narrow-body implants if necessary.
5. Use resilient attachments on the implants.
6. Design a simple RPD; use rest seats and guiding plates similar to
conventional RPD.
7. Use rigid major connector design for maxillary arch.
8. Minimize mandibular lingual flange if difficult for patient to adjust
9. Incorporate retentive elements to denture base under functional
load.
10. Schedule patient for checkups and maintenance appointments
Advantages
Improved esthetics by the elimination of visible clasp assemblies.
Ability to change fulcrums in the arch providing more favorable
biomechanics.
Minimizing rotational and lateral forces on direct and indirect
abutment teeth.
Controlled additional vertical support especially significant in
partially edentulous patients with distal extensions.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Provide additional retention and stability to the prosthesis by
incorporating an attachment mechanism.
Simplify prosthesis design and base extension.
Highly predictable treatment.
Easy to maintain depending on prosthesis design and attachment
system.
Minimize excessive pressure and trauma to soft tissues and
supporting ridge with alteration of the biomechanical forces.
Disadvantages of using implants in removable partial prosthodontics.
Additional costs for treatment.
Additional surgical procedures.
Extended treatment time.
Involve careful treatment planning and interdisciplinary approach
More technique sensitive than a conventional RPD.
Additional maintenance over time depending on prosthesis design
and attachment systems used.
Manual dexterity can be challenged in certain patient populations,
eg., rheumatoid arthritis, limited mobility.
Increased costs to overall treatment.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
FACTORS INFLUENCING MAGNITUDE OF STRESSES
1) Length of the edentulous span
The longer the edentulous span is, the longer the denture
base will be, and the greater the leverage force transmitted to the
abutment teeth will be. For each distal extension base, the fulcrum
is located at or near the occlusal rest on the most posterior
abutment tooth. During function, a load is applied to the artificial
teeth, and the length of the lever arm (i.e,denture base) determines
how much force the associated abutments must withstand.
Therefore, the practitioner must always be aware of the
forces that are generated as a result of removable partial denture
design. Although other factors such as the thickness of the mucosa
and the total area of the residual ridge may affect clinical
outcomes, the length of the edentulous span remains a factor that
warrants particular attention.
When treatment is being planned, every effort should be
made to retain an abutment posterior to the edentulous
space.Preserving a posterior tooth to serve as vertical support, even
as an overdenture abutment, results in improved patient service.
Similarly, the placement of an endosseous dental implant can result
in an equally valuable service.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
2) Quality of support of Ridge
The form of the residual ridge can play a large part in
distributing forces generated by the function of the partial denture.
Large, well-formed ridges are capable of withstanding greater
loads than are small, thin, or knife-edged ridges. Broad ridges with
parallel sides permit the use of denture bases with longer vertical
surfaces. These surfaces help stabilize the removable partial
denture against lateral forces.
The thickness and health of the mucoperiosteuma so
influence the loads transferred to abutment teeth.A healthy
mucoperiosteum approximately 1mm in thickness is capable of
bearing a greater functional load than is thin, atrophic mucosa.
Soft,flabby, displaceable tissue contributes little to the vertical
support of the denture base. This type of tissue allows excessive
movement of the denture base and permits forces transmitted to the
associated structures.
3) Occlusal relationship of the remaining teeth and orientation
of the occlusal plane
Many patients exhibit deflective occlusal contacts that
generate horizontal force vectors. These vectors can be magnified
by removable partial dentures and can be transmitted to the
abutments and residual ridges. To prevent the transmission of
destructive forces, the practitioner must be fully aware of occlusal
conditions and of the mechanics of partial denture movement.The
opposing occlusion can play an important role in determining the
load generated during closure.Some individuals with natural teeth
can exert closing forces of 300 pounds per square inch.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
In contrast, many denture wearers may not be able to exceed
30 pounds per square inch. Therefore, a removable partial denture
that opposes an intact dentition may be subjected to much greater
loading than a removable partial denture opposed by a complete
denture. The area of the denture base against which the occlusal
load is applied also influences the amount of load that is transferred
to the abutment teeth and the residual ridge. If an extension base is
loaded adjacent to the neighboring abutment, there will be minimal
movement of the denture base. As loading moves far away from
the abutment, movement of the denture base will be greater.
Ideally, the occlusal load should be applied in the center of
the denture-bearing area, both anteroposteriorly and faciolingually.
In most mouths, the second premolar and first molar regions
represent the best areas for the application of the masticatory loads.
Artificial teeth should be arranged so that the bulk of the
masticatory forces are applied in these areas.
4) Qualities of clasp
A flexible retentive clasp arm decreases the stress that
will be transmitted to the abutment tooth.
A wrought wire clasp is more flexible than a vertically
projection clasp, hence, it decreases the forces acting on the
abutment tooth and increases the forces transferred to the
edentulous ridge. It provides less resistance to more
destructive horizontal stresses.
5) Clasp design
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
A clasp that is designed to be passive when it is completely
seated on the abutment tooth will exert less load on the tooth than
will one that is not passive. As a result, the fit of a removable
partial denture framework must be carefully refined to ensure that
the prosthesis is completely seated. Only when the framework is
completely seated will the retentive clasp arms be passive. If a
clasp's retentive tip is designed and constructed to lie in a 0.010-
inch undercut, but the framework is not completely seated, the
retentive tip will not be passive. Instead, it will exert a continuous
load on the abutment.
Refinement of the framework's fit is best accomplished by
uniformly coating the tooth-contacting surfaces of the framework
with a disclosing wax.As the framework is seated, wax is
displaced. A tooth to metal binding will show through the wax.
These areas are adjusted until the framework is completely seated
and the clasp arms become passive.
A clasp should be designed so that during insertion or
removal of the prosthesis, the reciprocal arm con-tacts the tooth
before the retentive tip passes over the greatest bulge of the
abutment. This will stabilize or neutralize the load to which the
abutment is subjected as the retentive tip passes over the greatest
bulge of the tooth.
6) Length of clasp
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The flexibility of clasp depends on its length. Doubling the
length increases the flexibility five times. This decreases the
stress on the abutment tooth using a curved rather than a
straight clasp on an abutment tooth will aid to increase clasp
length.
7) Material used in clasp construction
A clasp constructed of chrome alloy will normally exert
greater stress on the abutment teeth, than a gold clasp because
of its greater rigidity. To compensate for this property, clasp
arms of chrome alloys are constructed with a smaller diameter
than a gold clasp.
8) Surface characteristics of abutment
The surface of a gold crown or restoration offers more frictional
resistance to clasp arm movement than does the enamel surface of a
tooth. Therefore, greater stress is exerted on a tooth restored with gold
than on a tooth with intact enamel.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
STRESSES INDUCED BY THE REMOVABLE PARTIAL
DENTURE
The service expectancy of a removable partial denture will be
proportional to the degree of control which is exercised over the stresses
induced by it. This is such an important factor especially in the success of
the extension-base type of prosthesis that it should be emphasized by
analyzing each stress and suggesting clinical and constructional
procedures for bringing about its most effective control. Functional stress
stimuli, within certain limits, are necessary for maintenance of the
supporting structures. Beyond an optimal amount, which may vary to a
considerable degree, stress may become an irritant, however, and may
actually cause retrogressive changes to begin.
In the case of the partial denture, one sure method of avoiding
overload is by the reduction of functional stress loads to a minimum
which is consistent with a conservative restoration of function. In fact, the
total stress load should be well below the estimated tolerance of each
patient. This provides a safety factor to accommodate a variation in the
amount of stress which the structures may tolerate at different periods.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Such variations are seen between one individual and another, where
extreme differences may be noted, but there are also variations from one
period to another in the life of the same individual.
PRESERVING THE ORAL STRUCTURES
Certain basic precautionary measures are indicated to assure that an
oral rehabilitation program will be kept within the tolerance limits of the
prosthodontic patient. The restoration of masticatory function is desired,
but the degree of restoration must be adjusted to the individual's ability to
sustain such increased workloads on the supportive structures.
In addition to limiting the beginning functional load given to the
individual, one must also provide a margin of safety to accommodate for
the depression periods of reduced tolerance limits. Even for the young
patient an occasional subnormal period may be expected. For older
patients there is the added certainty of a slowing up in physical processes
as they approach senescence.
These low tolerance periods and slumps in metabolic function may
come on very gradually and without the patient's recognition. One of the
important reasons for scheduled, periodic rechecks, as a part of partial
denture maintenance service, is to detect evidence of any stress overload
and to correct for this if possible. It is strongly urged that prosthodontists
concentrate less on the idea of restoring full masticatory function for the
partially edentulous patient, and that they exhibit more concern about
maintaining the oral structures which still remain.
INDUCED STRESSES TO BE RECOGNIZED
This is very important to determine how each component part may
assist in the reduction or elimination of induced stresses. For further
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
emphasis of this important matter of stress control, each of the following
potential overloads should be analyzed,its effects upon the supportive
structures noted, and measures for its control outlined. To have a better
overall picture of this problem, however, the induced stresses first should
be enumerated.
The principal ones are:
1. Stresses resulting from an inaccurate appliance;
2. Stresses caused by an interference to appliance insertion and
removal;
3. Stresses which may cause impingement of the gingival
structures adjacent to the remaining teeth;
4. Stresses which develop as a result of the use of a sloping tooth
surface for the support of an abutment occlusal rest;
5. Stresses resulting in impingement by a major connector;
6. Stresses which torque or twist the abutment of an extension-
base prosthesis;
7. Stresses which cause the proximal or lateral tilting of an
abutment.
1.STRESS RESULTING FROM APPLIANCE INACCURACY
When a removable partial denture (of any type) is either oversize
or too small, there will be a continuous pressure on all teeth and other
structures with which it makes contact. The direction of the pressure will
be variable and dependent upon which unit of the prosthesis is
transmitting the contact effect.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The first result of this stress will be orthodontic in nature. If severe,
it may induce hyperemia and discomfort. Usually, the tooth so effected
will respond to the pressure, as in intentional orthodontic therapy, and
will alter its position enough to release the pressure.
As a result of the induced movement, a relation of malocclusion
will usually be produced as a second effect of the inaccuracy of appliance
fit. This has quite serious potentialities unless it is soon rectified. Unre-
lieved occlusal prematurities of this type can result in periodontal
disturbances, not only about the tooth moved but also about those in
adjacent and/or occlusal contact. Such pressures are capable of causing
compression areas in the periodontal membranes of the affected teeth and
may easily lead to destruction of the enveloping alveolar bone.
Ramfjord has said, "Traumatic occlusion may result when
pressure contacts force a tooth into a position having an occlusal relation
which in turn rocks the tooth into another position when functional or
bruximatic stress is applied."
A third effect of appliance distortion may be noted in the cast bases
of inaccurate extension-base partial dentures. Impingement of sub-basal
structures sometimes occurs in the mucosal pad over the mandibular
ridge. It may occur bilaterally and apparently results from a slight
"rebound" of the horseshoe-shaped casting when the sprues are cut.
Mills refers to distortion of this nature in a very valuable study of
volume change as a result of various factors concerned with the making
of large bilateral removable partial denture castings. An inaccuracy as a
result of volumetric change during the congealing of cast metal would be
most noticeable near the free ends of the long castings, such as in a Class
I mandibular appliance.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
This type of mucosal pad impingement would not be encountered
in a partial denture of resin base construction. The resin base having been
related to the cast metal framework after the casting process had been
finished, there would be compensation for the former distortion as far as
the ridge relationship of the base is concerned. This is another advantage
of the resin base.
SOME COMMON CAUSES OF APPLIANCE INACCURACIES
In order to prevent the damage which may arise from a distorted
appliance, the various contributing factors should be appraised. Most of
these can be completely avoided, and all can be reduced to discrepancies
which are quite within the average range of tissue tolerance.
a.FAULTY IMPRESSION
A faulty impression is the first cause of inaccuracy to be
eliminated. The discrepancy may be in either the impression of the
prepared dental arch or the hydrocolloid duplication impression of the
master cast. Partial displacement of the impression from the tray is more
frequent than is suspected. When this accident is apparent (usually at one
heel of the lower tray), there is the temptation to rely upon what seems to
be an accurate replacement by the repositioning of an otherwise perfect
impression back into contact with the tray.
Since the slightest failure to reseat it perfectly will result in a gross
error in a casting the size of an average dental arch, the risk in making
such an attempt is evident. The value of properly locking the impression
material in the tray is apparent when the eventual cost of this error is
taken into account. This distortion could have been avoided by proper
placement of the material in the tray. It could be corrected in a very short
time by retaking the impression.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
A second source of damage to the impression of the dental arch is
from overstraining the material by forcing the impression from a deep
undercut area in a direction which will result in the greatest strain. First
release the surface tension seal at the periphery in the area of least
undercut (let air under the impression). Then remove the impression as
quickly as possible, allowing the direction of the snap removal to take the
line of least resistance. This method will allow less chance of
permanently deforming the elastic impression material.
Improper care of the hydrocolloid impression is a common reason
for the inaccuracy of the resulting appliance. A volumetric change (either
shrinkage or expansion) is only one discrepancy to be guarded against.
Injury to the surface of the resulting master cast by the contact of the
setting stone against hydrocolloid is another danger which also must be
avoided In this connection, an advantage of rubber-base material
(mercaptan) is superior surface of the stone cast which can be obtained.
b.ERRORS IN DUPLICATION
In duplication, the hydrocolloid will have been diluted and, hence,
is more easily abraded. All undercuts which are not to be used should
have been blocked out (filled) to lessen the strain of removing the master
cast. A further safety feature in this connection is to use a duplicating
flask or ring which permits complete displacement of the impression from
it before removal of the master cast is attempted .Mills found that one of
the several factors affecting the degree of accuracy of prosthetic castings
was the use of a non conducting ring of the duplicating flask. When such
a flask is used,gelation of the hydrocolloid will take place from the
bottom upward. This avoids the risk of internal shrinkage voids.
Greater accuracy of the casting resulted, according to the Mills report.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
c.CASTING INACCURACIES.
Incorrect proportions of water to investment can be the source of
casting inaccuracies at two points in the development of a partial denture.
First, the master cast may be affected in this way. The master cast must
be an exact replica of the dental arch. The proper portion of water to in-
vestment should be used to produce a stone which will have minimal
expansion.
The amount of water to investment for the refractory cast also may be
incorrect so that there will be an insufficiency of the necessary setting
expansion of this cast. It takes the combined expansion which is obtained
in three ways to negate the contraction of molten metal as it congeals to
the desired form of the casting:
1. The setting expansion of the refractory material of the proportions
found to give the maximum expansion while still having a
consistency which permits proper handling;
2. The hygroscopic expansion achieved by having water contact the
refractory material as soon as the impression has been filled, and
before this investment begins to set;
3. The thermal expansion of the refractory investment when it is
heated to eliminate the wax pattern (1300° F. for three hours).
Improper W-P ratio is one of the most critical of the several possible
causes of appliance undersize or oversize. Even when every precaution is
taken, it is barely possible to control this factor. It must be admitted that
very often the large removable partial denture casting is not perfect in this
respect the minimal goal that is accepable is one that keeps this error
within the range of tissue tolerance.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Larger, heavier removable partial denture castings, especially those
for maxillary cases, present a need for maximum control of metal
shrinkage. An effective way to accomplish this, is by altering the water-
powder ratio. For instance, from the recommended ratio of 28 cc. of
water to 100 grms of powder, for the average bulk of Ticonium casting,
this may be varied to amounts of water ranging as low as 25/100. A lower
ratio of "Vestic," the more recent refractory investment recommended for
Ticonium castings, has given excellent clinical fits.
Surface abrasion of the casts used in making a partial denture may
easily be responsible for a larger error in appliance.The effect of
hydrocolloid on the surface of casts made of gypsum products is that of a
retarder. If a soft, chalky surface is present on the master cast it will
certainly be reduced by abrasion and the casting will be that much
undersized. To offset this retarding action of the hydrocolloids it is
possible to employ a "hardening" solution (2 per cent potassium sulfate)
as a wash into which the impression is immersed for a few minutes before
it is to be filled.
The surface of the refractory cast may be more easily abraded because
it is much softer than the improved stone of which master casts are most
often made. A potassium sulfate solution may be used again to give a
harder surface to the refractory cast. To further lessen the chances of
abrading the refractory cast, at the time of removing the hydrocolloid, it is
suggested that the removal of the cast be delayed for about 30 minutes
after the allotted setting time. During this additional period the hydro-
colloid is intentionally dehydrated by removing the duplicating
impression from the flask and exposing it to air (a stream of compressed
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
air also may be directed on the cast and exposed hydrocolloid). As the
latter loses water it dries the surface of the cast by absorption.
Removal of the duplicating material can be done with less abrasion by
breaking it away do not attempt to withdraw the refractory cast. If a V-
shaped piece is removed from the hydrocolloid impression in the palatal
and lingual areas, those portions of the impression can be removed with
less rubbing of the lingual surfaces of the abutment teeth. It is also an aid
to use compressed air as a means of loosening these pieces. The re-
fractory cast should not be rubbed or brushed and it should be handled
with great care while it is drying and during the placement of the wax
pattern. If the refractory cast has been reduced, the casting will fit neither
the master cast nor the patient's dental arch.
Proper thermal expansion of the casting mold will be listed here for
emphasis,although it was mentioned in connection with the W-P ratio. It
is usual to maintain a temperature of 1300° F. for three hours to insure an
adequate temperature in the mold center. One important precaution is that
the oven Pyrometer be tested frequently enough to insure its complete ac-
curacy. It occasionally happens that the temperature may be lower than
the pyrometer shows. This condition will give an undersized casting.
Distortion of the cast units during heat treatment or soldering
operations has been reported in only a small percentage of cases
tested.This should be regarded as a possible cause of discrepancy of
appliance fit, particularly if the casting had shown a correct relationship
when tested on the master cast, and then, after the reheating of the
casting, it no longer seemed to fit as well as before.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Even only two or three out of 100 appliances would seem too great a
percentage of failures from this cause. These failures can be avoided by
the use of alloys whose physical properties are acceptably near the ideal
and do not require heat treatment, and by use of a method of construction
which does not entail soldering.
Excessive polishing is responsible for many misfits among partial
denture castings. The reduction of the tooth surface of an occlusal rest
can result in the lowering of the appliance. When some other unit (such as
the suprabulge sector of a clasp) is resting on an occlusally inclined
surface, it then will exert a lateral or proximal pressure on the abutment,
if the appliance is allowed to settle as it would by the reduction of the
undersurface of an occlusal rest. Actually, the appliance no longer fits. It
should be a rule, therefore, that the tooth surfaces of a casting should be
burnished and buffed lightly never ground.
2.STRESS FROM INTERFERENCE TO APPLIANCE
INSERTION
This stress, unlike that from an appliance inaccuracy, is an
intermittent disturbance of tooth alignment. It occurs when a contacting
rigid area of a removable prosthesis passes over a surface bulge of the
abutment tooth. This interference to appliance insertion or removal results
from the failure in mouth preparation to establish parallelism between the
tooth surfaces with which the prosthesis is to make contact. Sometimes it
is not possible or desirable to achieve a parallel relationship of the total
area on which an interference is found, in which case some of the
undercut will remain and must be eliminated at the stage of final survey.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
If the degree of interference is slight, the type of tooth disarrangement
which it causes is of brief duration only.
A second source of this kind of stress arises from the movement of
a retentive clasp out of the infrabulge area and over the abutment height
of contour. In this action, pressure develops from the temporary distortion
of the clasp arm. Ideal clasp design provides a reciprocal support to
counteract the force generated by the retentive clasp.
The advantage of having this reciprocal terminal placed on a
surface which had been made parallel to the path of appliance movement
was emphasized. When this is done, the retentive stress is neutralized
throughout the total period of its generation. Even when unreciprocated,
this stress (retention which is generated by the retentive terminal) is also
very briefly only the time that would elapse in the movement of the
retentive clasp terminal on the infrabulge incline to and over the crest of
abutment contour.
While these two stresses are not to be desired, and can, with proper
preparation of the abutments, be entirely eliminated, the potential damage
which they may cause is undoubtedly much less than the stress generated
by an appliance of inaccurate adaptation.
In the first place, the stress from interference to appliance
placement is brief and not continuous for the entire time that the
prosthesis is in position. As compared to the above mentioned constant
pressure (from a distorted or an inaccurate appliance), it certainly would
be unlikely to cause any increase of trauma, and usually it would be much
less.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Of greater significance, however, is the fact that during the period
of this stress application, the teeth cannot be in occlusal
contact.Therefore, the added trauma arising from occlusal imbalance,
which accompanied the stress caused by the inaccurate appliance, is com-
pletely avoided in the case of stress arising from interference.
After the mouth preparation changes have given parallelism, there
still will be need for the elimination of slight interferences in most
instances. When the master cast is completed, the degree of improvement
may be accurately measured by another study of the cast on the surveyor.
Almost always there will be need to block out remaining undercuts of
minor extent. When this is done,it is possible to make a refractory cast
which will be almost entirely free of interference.
Following the above precautions, any remaining interference
should be very slight. If care is taken in studying the relationship of the
casting to the master cast, these points of interference can be detected
before damage to the cast surface has occurred. Relief of the appliance
can be made to remove the final degree of interference. This method
should be used only as a last resort.
If relief by grinding the appliance is used excessively, the metal
structure may be weakened to an objectionable degree. A more serious
objection, however, is the development of space for the retention of
debris and stagnant saliva between the appliance and the tooth surface.
This is especially hazardous when the prosthesis is resting on an enamel
surface of a caries-susceptible patient.
3.GINGIVAL IMPINGEMENT BY THE REMOVABLE
APPLIANCE
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The gingivae are most susceptible to injury by any pressure
induced by a removable prosthesis. Even minor contacts seem to promote
an unfavorable reaction in these areas. Inflammation in the areas of
contacts made by the units which must cross the gingivae is soon
followed by edema. As these structures become distended, the pressure
increases and a vicious circle of retrogressive change is established.
The end result is a resorptive loss of the adjacent alveolar process
with a pocket formation. Loosening of the abutment follows, and as the
bone level is lowered, the tilting and twisting stresses on the abutment
become more and more an overload. If the abutment tilts, the
impingement of the periodontium in areas of compression will closely
follow.
It frequently is easier to prevent this unfortunate sequeala than to
reduce the condition after it has become well established. There may be
need to give the structures a rest period with the appliance removed from
the mouth for all except the periods of meals. At the time of first seeing
the patient who has this situation, a careful examination subgingivally
should be made to check on the possible presence of subgingival calculus.
Such deposits are at times the cause of this irritation because, as the
gingivae are pressed away "from the cervical area by the accumulating
mass, they are pressed against the overpassing unit of the prosthesis.
After the root surface is freed of deposits and has been polished, a
short rest period then follows. It is best to defer any reduction of the
prosthesis until the patient is again seen, when the amount of adjustment
(if any is needed) can be determined more exactly. Not infrequently, the
edges of the metal base or the connectors are found to be too sharp, or
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
angular. This angularity alone is quite sufficient to initiate the process of
inflammation.
On the side of prevention there are certain definite precautionary
measures to observe. Most important is to be sure of proper occlusal rest
preparations. Without adequate occlusal rest stops, it is useless to expect
the gingiva to escape impingement in these crossing areas. Some have
suggested the use of clasp retainers without occlusal rests. If such
unsupported clasps are under even the slightest tension (as when distor-
tion might have occurred), there will be a cervical pressure generated
enough to produce gingival impingement of increasing severity.
For added emphasis, it seems well to urge again that soft alloys not
be used for a restoration in which to prepare an occlusal rest seat. Silicate
and resin should not be used in this way, and an amalgam filling which is
in situ should not be used if it seems soft or poorly condensed. Getting
the proper rest support still is not enough,it must be protected. The tooth
surface of an occlusal rest must not be reduced in the process of polishing
the prosthesis. Any reduction of the rests will allow the appliance to
move toward the subbasal structures to impinge, first of all, the gingival
crossings.
At the time of construction a slight relief should be made at each
gingival crossing. Particular care should be given to the matter of
rounding the edges of the prosthesis which are adjacent to or which cross
the gingivae. Each time the partial denture patient is seen for maintenance
inspections, the gingival crossings should be checked again for evidence
of over-contact.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
It is quite possible that such a condition may develop even after
years without such trouble. This might come from occlusal rest wear,
from intrusion of the abutment teeth apically, or as a result of subgingival
calculus deposits. The consequence of gingival irritation warrants that
every safeguard be utilized to avoid its beginning. Its cure is not always
easy.
4.STRESS FROM OCCLUSAL RESTS PLACED ON INCLINES
The frequent necessity of using a cuspid tooth for abutment service
makes the problem of effecting a safe transfer of partial denture occlusal
loads to one that is constantly with the prosthodontist. The lingual anat-
omy of the valuable cuspid abutment is frequently steeply inclined. In
fact, some mandibular cuspids present almost a vertical lingual surface.
To apply rests on such surfaces would produce very unfavorable leverage
on the abutments, resulting in areas of impaction in the periodontal mem-
brane. An abutment support cannot accept this destructive overload, even
when the host is capable of normal bone maintenance under increased
stress loads.
A second unfortunate sequela of applying a partial denture loading
on an inclined surface is the possibility that the appliance will slip as the
occlusal load is applied. Appliance movement of this kind can easily
induce the gingival irritation which was discussed in the preceding
paragraphs.
While the most serious situation pertaining to the problem of the
inclined support relates to the use of a cuspid abutment, bicuspids and
molars (especially those with single or fused roots) are also subject to
similar damage unless the rest recess is favorably formed.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
In some mandibular extension base partial dentures, the placement
of a distal occlusal rest on a surface which slopes cervically toward the
edentulous area may result in repeated impingement of the subbasal pad
at the retromolar periphery of the base. This is produced as the prosthesis
slips posteriorly on the inclined surface of the abutment.
Preventing stress which would be caused by locating an occlusal
rest on cervically sloping abutment surfaces can be attained only by
considerable clinical effort. The operator must come to evaluate this extra
expenditure of time and exertion as being an excellent investment in
longevity for his service, and the patient must be sufficiently aware of its
potential value to accede to the considerable additional cost. There is al-
ways the temptation on the part of both to take an easier shortcut. After
seeing the tragic loss of fine abutment teeth from this type of stress, the
prosthodontist of long experience can attest to the merit of proper mouth
preparation.
Specific measures to be taken in the direction of avoiding damage
from this source can be accomplished at the time of preparing the mouth
for partial denture service. The first, and by far the most frequent, is the
making of an adequate occlusal rest recess in bicuspids or molar
abutments. Of primary significance in stress control is that the floor of the
prepared recess must slope from the abutment margin toward its center.
This form creates an angle which is less than 90 degrees between the rest
floor and the vertical minor connector. Then, under stress loads, an
abutment is held firmly against the vertical guiding plane of the minor
connector, thus preventing lateral pressures which would cause
periodontal impingement.
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In the situation of the cuspid abutment,the form of this tooth will
seldom be such as to permit the placing of an adequate occlusal rest. The
reshaping of a cuspid tooth can be done best by the placement of a three-
quarter veneer crown restoration, in which a groove is placed on the
lingual surface just above a raised cingulum.
Occasionally, for some good reason, reconstruction of the cuspid
may be impossible. Than the labio-incisal (embrasure-hook) unit has been
used instead of the raised-cingulum restoration. Another substitute
measure may be suggested for the posterior tooth where an ideal rest
recess cannot be executed for some reason. This is the use of a secondary
(auxiliary) occlusal rest to compensate for any pressure in the mesial
direction which would be generated by the use of the rest on a distal
incline.
As noted, however, this reciprocal action of the auxiliary occlusal
rest is operative only as long as the mesial rest remains perfectly seated.
Should there be a resorptive loss in the sub-basal structures which would
permit rotation of an extension base prosthesis at its cross-arch fulcrum
line, the compensatory action of the mesio-occlusal rest would be
nullified. Thus, again, the best procedure, as in the case of the rebuilding
of the cuspid, is to place a restoration which would permit the proper
occlusal rest recess on the distal portion of the occlusal surface.
5.STRESSES THAT A MAJOR CONNECTOR MAY CAUSE
There are three different ways in which a major connector may
produce impingement of the structures over which it passes. If it is not
rigid, workloads may cause it to flex. When these loads are such as to
cause an extension base to move lingually, the non rigid connectors
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(particularly the lingual bar type) may be forced to flex toward the
subbasal structures. At the weakest point in its anterior arc between the
right and left abutments, the flexible bar will, because of these flexures,
repeatedly press against the mucosal covering.
Localized inflammation, followed by edema, increases this
pressure and soon the underlying bone is involved. The lesion is not
usually very painful and may escape the notice of both patient and dentist
unless the area is carefully examined. If allowed to continue, this type of
impingement may eventually produce a perforation of the mucosal pad.
The small hole is quite smooth and well defined. Through this aperture
one may probe the bone, which may be denuded with the periosteum
detached in an area much larger than the tiny opening. Not infrequently a
sequestrum may be exfoliated, and occasionally the lingual cortical plate
is entirely lost in this area.
A second type of major connector impingement may follow a
lateral shifting of the appliance. This, too is more commonly seen in the
mandibular prosthesis. It usually accompanies the property of flexibility
of the lingual bar, but in this condition, the bar has a tendency to become
straightened (its arc reduces). The result is that the connector moves
laterally to impinge the area lingual to one or the other, but not usually
both sides of the arch. This movement is quite frequently associated with
an occlusal imbalance in which the prosthesis tends to move toward the
side being impinged.
Here, again pressure contacts lead to an inflammatory process, and
this trauma may produce edema to increase the pressure and thus
establish a vicious circle. In less time than one may realize, bone
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destruction may ensue, with definite pocket formation on that side of the
abutment. Mobility of the abutment increases and its loss may be the end
result of the unfavorable sequelae of this impingement.
A third major-connector traumatization may be seen, but with less
frequency than either of those referred to above. This condition is a
generalized contact pressure which results from a change in the relation-
ship of the connector to the underlying structures, when the tooth-borne
portion of the partial denture settles or depresses. While this does not
happen often, it is a situation that can be the result of several conditions,
most of which fortunately can be prevented. This is another problem
which is much easier to prevent than to correct.
Actually, when impingement is found throughout a major
connector, the disturbance is so painful that it will be necessary to remove
the partial denture at once. This is usually seen more frequently in
connection with the mandibular partial denture. There is no reason why
the causes of appliance settlement cannot occur in the maxillary arch.
A probable reason that it is not so frequently associated with the
upper partial denture is that the anterior major connector (palatal bar) is
usually much broader than a lingual bar. As a result, any impingement
would be more widely spread and therefore less likely to exceed tissue
tolerance. It is a clinically observed fact, that the structures of the anterior
palatal area are much less likely to be irritated than those of the lower
arch.
a.Trauma from flexing
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This type of impingement most often occurs with the single lingual
bar,and it is always seen in the case of a bar which is too flexible. During
heavy occlusal impacts, the arc between the right and left abutments
alters in such a manner that the bar springs against the mucosal pad. This
will occur in the area of its greatest flexibility, or at its most acute
curvature. Since length is associated with flexibility, the longer connector
will be most prone to show this defect. One problem that is always
encountered in the design and construction of a lower Class II prosthesis
is that of overcoming the tendency for the long connector to flex.
This was a constant difficulty when the use of the wrought lingual
bar was common practice. It also is usual to find occlusal imbalance
accompanying this situation. The type of prematurity or cuspal
interference for which one should be most watchful is that which would
carry the lower extension base appliance in a horizontal direction.
Prevention of flexure impingements:
1. Use a cast connector employ a less flexible alloy.
2. Increase the bulk, when the connector is long .
3. Alter the form (use a half-pear form instead of half-round or flat).
4. Some alloys of gold that are rigid or may be made rigid by heat
treatment.
5. Add a secondary lingual bar across the cingula of the lower
anterior teeth.
6. Use a linguoplate connector, which will be more rigid because of
being in two planes and somewhat corrugated in form.
7. Widen the anterior palatal bar to include two planes of the palatal
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surface add bulk between the rugal crests.
8. When the palatal arch is high and the rugae are prominently
developed to provide a corrugated undersurface then it is
unnecessary to also use a posterior palatal bar.
When the palate is low and flat (with a less well developed system of
rugae), it is necessary to use both posterior and anterior palatal
connectors. A principal reason that the assembled partial denture,
utilizing a wrought connector, has proven less than satisfactory is because
it is too flexible; this is especially true of the lingual bar. By casting this
bar, it was possible to change the form and vary the bulk to remove this
objection. The selection of a less flexible alloy is possible, when it is to
be cast. Such choice is much more limited in the ready-formed bars.
Since it is not easy to draw the less ductile alloys, they are avoided in the
manufacture of the ready-made wrought bar.
b.Trauma from lateral appliance movement
A lateral shift of the partial denture may tend to occur in certain
conditions, with the result that there is a pinching of the tissue beneath
the major connector. Such movement would be encouraged by the use of
weak tooth support, especially when this condition is accompanied by the
use of a flat ridge from which a base could not gain much resistance to
lateral stresses.
If considerable occlusal disharmony is added to these conditions,
there is a probable chance that an area of thin, unyielding tissue might be
pinched between the base and the surface of the bone. If this trauma
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continues, the chronic irritation may result in bone necrosis. Fortunately it
is possible to utilize certain preventive measures.
Prevention of impingements from lateral shifting:
1. Provide a slight space beneath the lingual bar by placing a thin block-
out material before duplicating the master cast.
2. Employ more rigid stabilizing units (reciprocal clasp arms, auxiliary
occlusal rests, indirect retaining units, etc.
3. Reduce the cuspal inclines of the opposing occlusal surfaces. When
unused teeth present cuspal inclines that are steeper because the teeth
have not been in function and have had no abrasive wear, such teeth
should be adjusted.
4. The height of their cusps and the steepness of their cuspal inclines
should be made to correspond to that of the remaining natural teeth.
5. Restore the best possible occlusal level of extruded teeth by
grinding,or by restoration when they must be shortened so much that
the dentin would be exposed. Occasionally, such teeth may have to be
extracted because their malposition is so extreme.
6. Relieve the major connector in an area of anticipated impingement
after the casting has been made (or when irritation has occurred);
reduction by grinding may make the major connector flexible, in
which case one trauma would be likely to replace another.
7. Since this type of lesion is associated with lateral appliance
movement, it is doubly urgent that the mandibular base be extended to
maximum flange length, especially on the lingual. If the ridge height
is subnormal and there is a sharp lingual edge, surgery should be
utilized to make possible a longer lingual flange by recontouring the
area.
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8. Splinting to provide multiple abutment support will effect more
adequate stabilization and reduce the possibility of appliance
movement.
It is interesting to note that of the above measures suggested for the
control of a very annoying and too frequent partial denture difficulty, all
but one may be said to be preventive. Six of the seven are planned and
executed before delivery of the prosthesis. Four of the seven are measures
to be completed at the mouth preparation stage of the proposed service.
Only one measure (relief of the casting) can be classed as remedial, and it
is suggested only in a limited way. With careful attention directed toward
the six preventive aids, the rehabilitation program will have improved
chances of success. If it fails, it will probably do so in spite of the lone
remedial measure.
c.Trauma from connector settlement
Removable partial denture without adequate occlusal rests is
seldom encountered in modern prosthodontics. A strong plea has been
made for utilizing tooth support (gained by the use of proper rest units) to
prevent gingival irritations. No less forceful is the claim that inadequacy
of occlusal rests can be cited as a cause of major connector impingement.
Particularly in the instance of a mandibular partial denture can it be
stated that even maximum extension of the bases will not alone be able to
gain sufficient support to avoid an occasional appliance settlement. Even
with the aid of tooth support, there still will be some situations where
such settlement will occur.
One such occasion is that in which one or both abutments have had
no recent occlusal work loads. An abutment of this category is certain to
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take a position (after assuming abutment service) which will alter its
relation to the alveolar walls that are to give it support. It may be
expected that this change will result in an increasing contact between the
lingual bar and the surface of structures subjacent to it.
When too little free-way space has been provided for a patient with
unusually heavy occlusal force loads, there may be intrusion of the
supporting teeth. Hypercontact of the connector is sure to follow.A
similar result is encountered when the bone of the alveolar process is
subnormal. If the patient is incapable of normally maintaining his bone
structure, it is certain that the bone tolerance limit will be more quickly
reached. Under this condition and with too heavy occlusal loading,
abutment intrusion is possible. In this connection it would be profitable to
review the section on determining the probable stability of the alveolar
bone.
The use of unsuitable materials to support an occlusal rest is about
the same as not using one. Obviously, a soft filling (such as silicate) in
the area of an occlusal rest site will reduce. As the support for an occlusal
rest is lowered, the appliance settles to closer contact with the mucosal
surface beneath it. The same effect can be the result of grinding the
supporting surface of an occlusal rest during the finishing and polishing
of a removable partial denture casting.
Avoiding major connector settlement:
1. The primary preventive measure to be taken is an attempt to adjust
for any metabolic imbalance, when there is evident failure to
maintain the alveolar bone.
2. If there has been previous loss of supportive bone, splinting of the
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adjacent teeth will be a major factor in avoiding overload after the
abutment service is added.
3. Adequate occlusal rest units should be provided. In polishing the
undersurface of these, no reduction is to be made burnish and
polish lightly.
4. Restorations made of easily abraded materials must be avoided in
locating primary occlusal rests.
5. When abutment teeth have not had recent occlusal function, digital
exercise will help to reduce the amount of positional adjustment
after prosthetic loading occurs. The patient should be instructed to
place his finger so that an occlusal force may be simulated as to the
amount and direction. Such exercises should precede the final
impression by a few days, during which time the exercise should
be frequently repeated.
6. In designing a removable partial denture, where the possibility of
overload is suspected, auxiliary occlusal rests can profitably be
included, in order to spread the work load more widely.
7. A lingual bar wax pattern should be thickened when there is a
chance that later reduction may be needed.
8. A supporting base should be extended as widely in a buccal
direction and over the retromolar area as possible, when an un-
stable condition is suspected. Include all of the basal bone surface
which can be used without encroaching on moving structures.
9. Finally, reduce the occlusal table (both in width and length) to
lessen the force loads which may be received in any single contact
with a food bolus. In establishing the occlusal pattern, care also
should be taken (especially in these situations) to avoid the
overreduction of free-way space. Continuous occlusal pressure
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from muscle tension must be avoided.
10.If, in spite of the above preventive measures, major-connector
settlement does occur, the impingement certainly will be of less
degree and perhaps can be entirely relieved by a reduction of the
under (or tissue) surface. Any such reduction, how ever, is quite
definitely limited—the bar must not be made flexible.
With the exception of the first of the above corrective measures,
which will often require specialist management, all are either to be done
in mouth preparation procedures by the prosthodontist or are to be under
his direction and executed by a technical assistant.
6.STRESSES WHICH TORQUE OR TWIST THE ABUTMENT
The stresses resulting in the various impingements of the major
connector, which have been discussed in the preceding paragraphs, may
be caused by a tooth-borne removable prosthesis as well as by the ex-
tension-base type. However, the stresses which cause torque or twisting
action will be found to operate to an exaggerated degree in the partial
denture having an extension base and practically not at all in the tooth-
borne appliance. This is because the prosthesis with the free end produces
twisting and tilting forces because of its lever action. Since this base is
supported by structures having some yield, both its lateral and vertical
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movement will need maximum control, even when the base is the best
possible.
The term "torque" is used in the following analysis to designate
that stress which tends to twist or turn an abutment in its alveolus, as
distinguished from a force which leads to the tilting of the abutment
laterally or proximally. Conditions which encourage the lateral movement
of the extension-base prosthesis make this stress a constant problem, and
its control one of the reasons that this prosthesis has been called the most
difficult of prosthodontic assignments.
Lateral movement of the extension base becomes aggravated when
the sub-basal ridge is low and flat in form. This movement results
principally from inadequate flange length. It also may be increased by the
presence of a flabby, movable pad of mucosal structures over the ridge.
Another critical factor in the development of torque stresses is the
presence of high cuspal inclines, especially if these are surfaces which are
not in occlusal balance. This lack of occlusal harmony occurs frequently
in the partial denture on which substitutes have been placed in relation to
teeth which had migrated from normal alignment, and which had been out
of occlusal function for a long period.
On these unused teeth, the cuspal height and inclination are both
excessive as compared to the existing condition of the remaining teeth
that have been subjected to abrasive wear. When such teeth govern the
excursive movements of the jaw, then the supplied teeth (and those with
which they occlude) cannot possibly be in harmonious balance until their
surfaces also have been made to conform.
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In addition to the above conditions, torque stresses will be most
destructive
When the occlusal loads are heavy; when the abutment has a
round, tapered root
When the abutment root is single (or fused).
When there has been previous alveolar bone loss about the
abutment teeth
When the occlusal table is long, and the number of remaining teeth
are few and
When the patient has a well established habit of bruxism.
Preventive measures in torque control:
Surgical recontouring of flabby and hypertrophic tissue on the
alveolar ridge.
Splinting the adjacent teeth, if the root is short or tapered,
which gives counter leverage advantage of multirooted
abutment.
Maximum extension of denture base within the physiologic
limit.
Use a rigid connector which extends to a remote anchorage in
order to effect adequate counter leverage.
Utilize a combination clasp to provide its stress breaking
action.
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Since torque stresses originate with lateral appliance movement, it is
most important in the mandibular extension-base prosthesis to obtain a
stable base before the abutment has suffered damage from torque. Very
frequently the undersized lower ridge is also further handicapped by
having been out of function for many years. Until it has been
reconditioned, by having received work stimuli, it cannot assume the
support of functional loads at once without further resorptive loss.
There are two ways to handle this temporary instability. The
prosthesis may be completed and then "rebased",or a prosthesis without
teeth may be worn with only light digital exercise to stimulate the
alveolar process to become "re-organized". Further loss of basal
structures should be avoided. There is no more certain way to induce
torque stress loads, and there is no stress which is more destructive.
Since much of the control of torque stresses will be dependent upon
the amount of force received on the occlusal table, the matter of
achieving harmony in occlusion is a very vital factor in the control of this
type of stress. The need for adjusting the occlusal anatomy of opposing
unused teeth has been stressed.
Another matter, equally important, is to coordinate the occlusal
relations of the supplied teeth to those opposing so that there will be har-
mony throughout all ranges of excursive jaw movement. The method of
doing this by having the patient wear an occlusal wax record,during
which period jaw movements are exercised, is strongly recommended.
The measures for the reduction of stresses which place a twisting force
on the partial denture abutment diminish in effectiveness as the length of
the occlusal table increases. The most frequently occurring partial denture
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situation, the mandibular Class I, must make use of the bicuspid for
abutment support. This means that the appliance lever is long, while the
form of most bicuspid roots is least resistant to the turning action
stimulated by the torque stress.
At the same time it should be recognized that in this situation (the
most frequently occurring partial denture case), the measures for con-
trolling this induced stress are less than maximum. It would seem that this
unfortunate combination of circumstances attaches the greatest emphasis
to the need for reducing, at its origin, that force responsible for torque.
There is urgent need, then, for complete occlusal harmony, not only
during the voluntary effort of masticating but also throughout an
involuntary muscular contraction like bruxism.
7.STRESSES WHICH TILT AN ABUTMENT
It has been emphasized that stress loads can be transferred to the
supporting bone of the jaws most ideally, from a physiologic point of
view, through the periodontal membrane. But this is true only when the
force loads are received in a trajectory which is parallel to the long axis of
the abutment. When the tooth is tilted by forces that are not parallel to its
longitudinal axis, certain areas of the membrane fibers are compressed
instead of being tensed, and impingement trauma results.
Tooth-borne prosthodontic appliances often are made removable
and bilateral in design in order to avoid these lateral tilting stresses. In
bilateral design, the principle of cross-arch splinting can be applied to
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develop counter-leverage by which effective control of tilting stresses is
gained.
Control of forces which induce proximal (mesiodistal) tilting is not
so readily accomplished in the extension-base prosthesis, however. In
tooth-borne appliances there is little possibility of abutments being tilted
proximally; in the extension appliance this stress is a major problem. Any
slight yield of the mucosal pad structures, not to mention actual resorptive
change in the sub-basal supporting bone, tends to produce varying
degrees of vertical movement of the base, and proximal tilting follows.
The ultimate result of compressive trauma of the periodontium is
bone resorption in the area of the alveolar walls. As the tooth is tipped, it
assumes a position of increasing malocclusion, with the forces generated
by occlusal imbalance being added to the traumatic injury already
sustained.
As the abutment becomes mobile, lateral shifting of the prosthesis
may result to produce major connector impingement which further
accelerates the process of damage. Particularly in the maxillary extension
partial denture (because of gravity) there may also be a mesial proximal
tilting. Hence, in severely unfavorable situations, the supporting bone
may be overloaded from all directions because of lateral and proximal
tilting of an abutment.
It has been shown that extension partial denture rotates at its cross-
arch fulcrum line in two directions, toward and away from the sub-basal
structures.As a result, the periodontal pressure developed may be on
either the mesial or the distal surfaces of the abutment alveolus. The exact
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location of the pressure areas will be determined by the direction of the
tilting force.
a.Limiting Stress Caused by Base Movement Toward Ridge
Any movement toward the sub-basal structures would indicate lack of
sufficient support to sustain the occlusal load. If the partial denture is of
the type that is distally extended, the abutment will be tilted in that
direction. The first control is that of most direct approach improve the
support. There are two ways of doing this:
1. Improve the ability of the supportive structures to carry a greater
load. This can be done, in many instances, by surgical procedure at
the time of mouth preparation. If the mucosal pad shows excessive
mobility, it frequently may be improved by excision of some of the
hypertrophic mass to provide a more stable foundation.
2. If there has been prolonged lack of functional activity, a second
way of improving the support is the program of exercise therapy.
This has been found to so recondition the supporting bone that the
base may not require the usual rebasing procedure later.
Another effective aid in the problem of appliance instability is to
increase basal coverage. The beneficial result of increasing the
supportive area of the base is two fold. Not only can a greater occlusal
load be borne safely, but the wider distribution of the applied load will
lessen the possibility of resorptive change. Hence, proximal tilting of the
abutment may be avoided more frequently and for longer periods.
There is a very definite limit to extending the size of the base,
especially in the mandibular edentulous ridge area. Surrounded by
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functionally moving structures, the peripheral limits of the base are
reached much too soon before an adequate area of coverage has been
attained, in many instances.
One more important measure may be taken when this impasse is
reached. This is to relate the base to the supportive structures in such a
way that all units of the surface are giving support. Care should be taken
always to keep the applied load well within the limit of the physiologic
tolerance of sub-basal structures.
However, distributing the functional load as uniformly as the nature of
the various component structures will produces the least chance of
overloading the firmer areas so as to induce resorptive change in them.
As has been pointed out, these measures too frequently are not enough
to insure stability of the base. The available mandibular area is too
limited. There is, however, another approach which is as direct as the first
this is to reduce the load at its source. To accomplish this, it is better to
reduce the buccolingual width of the occlusal surfaces supplied on the
prosthesis rather than to shorten the mesiodistal length of the occlusal
table. If the most posterior of the opposing teeth is not given occlusal
contact, it would tend to extrude, a condition which must be avoided.
The reduction in occlusal width does not always solve the problem of
overload . While it greatly diminishes the force generated by any single
occlusal masticatory contact on a food bolus, it in no way eliminates the
overload which occurs during bruxism. In the latter stress, only one point
of contact on the occlusal table is needed to transmit the full load.
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Unfortunately, the bruximatic force is usually the more dangerous
because it may be continued for long periods without interruption. In
mastication, the occlusal forces are applied intermittently, usually with
less biting pressure and for shorter intervals. There is a very dependable
way to reduce the possibility of bruxism, however; this is to carefully
eliminate all occlusal prematurities. Occlusal imbalance is considered to
be a primary cause of the habit of bruxism.
When these methods have been utilized to the limit,some curtailing
induced stresses, others augmenting the quality and degree of the support
then the last defense is again called into play. It is best to assume that (at
least in periods of subnormal tolerance) the demand on the supportive
structures may approach or exceed their maximum capability.
Accordingly, every effort should be made to include some safety measure
such as stress breaking type of flexible clasp as a last resort.
b.Limiting Stress Caused by Base Movement Away From Ridge
This stress does not develop in a tooth-borne prosthesis when a
direct retainer is functioning at each terminal abutment. Any force
tending to cause the extension-base appliance to leave its contact with the
sub-basal structures does produce tilting stress on the abutment. It is
suggested that the section relating to the indirect retainer be reviewed at
this point, since it has a pertinent application with the problem relating to
the stress being considered at this time.
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The amount of the force tending to induce movement of an
appliance away from the supporting ridge will vary, but its magnification
always will be directly proportional to the extended length of the ap-
pliance. In general, the forces which tend to move the prosthesis away
from its supporting structures will be less than those of occlusal origin
which move it toward them.
These forces are: the pull of sticky substances (upon which one
may have bitten) when the jaws are again separated; the pressing of
circumjacent structures as they are in functional movement against the
border or side of the prosthesis; sudden expulsions of air from the the
lungs (such as coughing or sneezing); and the force of gravity in
maxillary extension base appliances. The effect of this leverage stress on
the abutment tooth is to cause it to be tilted proximally in its alveolus.
The direction of the stress application will, be such as to tip the
abutment away from the edentulous area. Again, this stress will cause
zones of compression in the periodontal membrane, as did the stress
developed by movement of the base toward the ridge surface. While this
impingement may be less in magnitude, it may be more prolonged. The
effect of gravity (in displacement of the maxillary appliance, for instance)
is continuous for most of the time whenever the teeth are not in occlusal
contact.
Certain factors in the control of this stress are favorable, however.
The extension-base partial denture is predominantly of the Class I or II
variety. This means that a stress resulting from movement of the base
away from ridge contact would tend to tilt the abutment in a
mesioproximal direction. Usually, the abutment will make contact
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mesially with an adjacent tooth. The tilting force will, therefore, be
partially shared by this contacting neighbor.
There may be two or three such contacting and supporting teeth in
the arch. Also, when the abutment is multiple that is, adjacent teeth are
rigidly splinted not only is there wider distribution of this stress, but also
a more favorable leverage advantage is developed. Both of these
influences tend to reduce this type of stress now being considered.
This statement is not presented to minimize the importance of
seeking to reduce this type of stress, however.The end result of its
continuation can be very destructive, culminating in permanent injury to
the periodontium.
In order to limit the stress caused by the free end of an extension base
appliance tending to loose ridge contact:
1. Reduce the weight of the maxillary partial denture of the extension
type to lessen the effect of gravity.
2. Avoid peripheral encroachment on moving circumjacent structures
in the attempt to enlarge the area of the base.
3. Reduce the base peripheries, if there has been overextension.
4. Contour and finish the appliance so that there is less chance of a
sticky bolus adhering to its surface. Position the supplied teeth so
that the contact of tongue and cheeks will tend to displace the
appliance least.(Reduce the lower teeth, if necessary, on their
lingual surfaces.)
5. Employ complete palatal coverage to obtain surface tension
support (as for a complete denture) as an aid to the less effective
indirect retainer in extensive Class I cases
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6. Utilize the most efficient indirect retention which can be obtained
under the existing conditions.
7. Dissipate the remaining leverage stresses by the use of flexible
retentive clasp arms as stress-breaking units.
8. If a problem is anticipated in the adequacy of the control measures,
or if a weakened abutment tooth must be used, it is well to utilize a
multiple abutment if splinting is possible.
It would seem, then, that the most destructive stresses induced by
the partial denture are those which twist or tilt an abutment tooth. This
is because the functional forces produced on the occlusal table are
magnified by the appliance, acting as lever, and are then passed on to
the abutment.
Certain measures can be taken to prevent this and, in fact, to
accomplish a reduction of the stress load in many cases. If the various
measures for controlling these stresses, as outlined in the foregoing
pages, are applied with meticulous insistence at the time of mouth
preparation, during design and construction of the prosthesis, and at
each appointment for maintenance service, then the removable partial
denture so produced will be most likely to give a long period of
satisfactory service.
SPLINTING TO IMPROVE STRESS CONTROL
A very common predisposing cause of alveolar breakdown is a
previous loss of bone support. As the bone level at the alveolar crest is
reduced, so also is the surface of the alveolar wall remaining for root
support. The stress assumed by each square unit of bone surface becomes
greater as the depth of the alveolus decreases. If the abutment happens to
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be one which has a tapered root form, this decrease in percentage of area
of remaining alveolar support is quite rapid. Add to this situation an
unfavorable root form and it need not surprise one to find the surrounding
tissue overloaded.
An abutment with a single root is always more vulnerable because
of its reduced area of surface support, but when this one root is round it
also becomes very susceptible to torque. Often a single root is round and
tapered and this form is accompanied by a previous loss of bone around
it, indicating a susceptibility to alveolar atrophy. To use such teeth for
abutment support is a matter of questionable wisdom to say the least.
Stress control actually starts,at the diagnosis stage of partial
denture service.The wise prosthodontist will prescribe complete denture
service for a case in which he knows that it will be impossible to control
the stress load which the contemplated partial denture is likely to induce.
Where there are especially urgent reasons for retaining the
remaining teeth under some or all of the above conditions, there is one
possible way of controlling the stress load with reasonable success. This
is by the use of multiple abutments. The most effective way of
accomplishing a division of abutment work is by the actual union of two
or more teeth. Restorations in the adjacent teeth may be soldered at their
contact points to make such union.An other application of this idea is to
use the fixed bridge, uniting a tooth which is standing alone to one which
is separated by only a one or two tooth space.
This splinting of weak teeth produces an abutment support which is
comparable to that of a multirooted tooth. A molar with two or more
widely separated roots is accepted as an ideal bridge abutment. By such
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union a great advantage is gained against torque and a favorable leverage
is developed to combat proximal tilting.
Damage which formerly was attributed to the rigid fixation of
teeth, in the light of present-day knowledge seems to have been caused by
lack of attention to some other phase of stress control. Until alveolar
atrophy can be controlled through other remedial measures, the splinting
of these weakened teeth offers hope for saving at least the majority of
them.
THE COMBINATION CLASP IN STRESS REDUCTION
A combination clasp is one in which the retentive arm is made of a
round, flexible, wrought structure. In spite of all efforts at stabilization,
the base of an appliance that depends upon the subjacent structures for its
major support will have a variable amount of lateral and vertical motion.
Some device, therefore, is necessary to eliminate, or at least reduce, the
resulting stress before it is transmitted to an abutment tooth and the
surrounding area of supporting tissue.(Fig-41)
Stress-breakers of varied types have be entried from time to time to
reduce the work of a partial denture abutment. Most of these have
incorporated an idea of a broken joint between the clasp and the
appliance. This device allows some movement laterally or toward the
ridge but does not let a prosthesis move away from the tissue.
There is, usually, too much movement allowed, stresses are not
uniformly distributed and the very valuable stabilizing leverage of the
bilateral design is then lost. This type of moveable attachment is
complicated to make and adds materially to the cost.
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A simple, inexpensive but very effective approach to the stress-
breaker control of forces which escape all other means of elimination is
the combination clasp.Because this clasp uses a retentive arm made of
wrought alloy, it is flexible and, being fibrous, has a toughness that
permits its use in very small gauges.
Also, a round form of retentive arm is given to this clasp to make it
equally flexible in any direction. For this reason it is as effective against a
twisting stress as one which tends to tilt out of vertical. Hence, no stress
which would shift the abutment can pass through this flexible arm. Any
stress which can reach this point is simply dissipated, because the
wrought arm will yield (flex) before the pressure generated against the
tooth is enough to cause periodontal injury.
The wrought arm of a combination clasp has the additional and
very practical advantage of being adjustable. An increase or decrease in
the amount of retention requires that the clasp arm be moved cervically
(into the undercut) or occlusally to a level nearer the height of contour. If
the arm is half-round, as the cast clasp is, the above adjustment would
require an edgewise bend. This is a most difficult change to make in a
cast structure without permanently injuring it.
METHODS OF STRESS ANALYSIS
BRITTLE LACQUER COATING TECHNIQUE
This technique was developed by DeForest et al in the 1940s.
It gives a qualitative and roughly quantitative analysis of the strain
patterns in a previously deformed body. The technique is particularly
useful in detecting and measuring strains at the surface of a
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structure as well as indicating the direction and sequence of the
tensile strains. The qualitative analysis involves spraying a lacquer
onto the surface of the body to be tested. This is allowed to dry
and loads are applied in the desired way.(Fig-31)
Cracks appear in the lacquer in areas of maximum tensile
stress. Increasing the load causes cracks to form at other points
where the tensile stress has exceeded that required to fracture the
lacquer. The lacquer selection is critical and depends on the
humidity,temperature and sensitivity required. The prime constituent
of the lacquer used is colophony resin and spraying is carried out
using the equipment specifically designed for this purpose. The
major drawback with this technique is that the cracks which are
relatively easily seen on load application disappears once the load is
removed.
Additionally as the cracks do not run out onto the surface,
the top surface has to be etched away before developing with a
dye. The technique is also sensitive to fluctuations in temperature
and humidity and only gives a qualitative assessment of the stresses.
Due to its simplicity this method has been widely used in dentistry
and was first applied to examining stresses in dentures by Matthews
and Wain.
Qualitatively. the technique gives a quick and easy test as a
guide to the need for primary modifications. It has been suggested
that for quantitative measurements in dentures the brittle lacquer
technique is used in conjunction with electrical strain gauges.
ELECTRICAL STRAIN GAUGES
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Electric strain gauges have been used to give a quantitative
analysis of the stresses encountered. For any material a well-defined
relationship between stress and strain exists. If the strain in a certain
area of the denture is measured,the value of the stress can be
calculated provided the elastic modulus of the material is known. It
is this principle that is utilized in employing the electrical strain
gauges for the measurement of stress.(Fig-32)
An instrument measures the strain and using the relationship
described above,the stress is calculated. This instrument belongs to a
class of strain gauges which depend on the alteration of some
parameter associated with the flow of an electric current for the
measurement of strain.
When the load is applied strains in the surface of the
specimen under examination are transmitted to the wire filament via
a paper backing cemented onto the surface. This results in a change
of resistance of the wire filament which is then measured by some
associated electrical current.
Three factors have to be considered before the gauges are
fixed to the surface of the specimen:
(1) location of the gauge.
(2) size of the gauge to be used
(3) orientation of the gauge with regard to the specimen.
Strain gauges have been widely used in clinical research on
dentures. Studies have been carried out which examine both
mandibular and maxillary denture rigidity and denture deformation.
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Kegli and Kyddx were the first to apply the strain gauges to study
mandibular base deformation.
Regli and Gaskill” studied the deformation of plastic denture
bases using strain gauges and concluded that dentures with high
ridges exhibited torsion deformation during mastication and those
with flat ridges exhibited compression. They also found that the
ability of the denture base to resist deformation was an important
factor in adequate stress distribution to the supporting structures.
ADVANTAGES
The main advantage of using this technique is the relatively
small size of the gauges which causes minimal interference during
use. However the disadvantages encountered are far greater than the
advantages.
DISADVANTAGES
The gauges have to be sealed effectively from the oral
tissues. if used intraorally, to prevent short circuits and they must
be adhered firmly to the surface of the appliance.They must also be
placed in relevant parts of the denture as well as aligned in the
correct direction. Additionally with these gauges only the surface
strain at selected points is measured and although stresses may be
calculated from the strain measurements,this requires time.
Strain gauges have been used in conjunction with the brittle
lacquer coating to overcome some of the problems discussed above.
The initial analysis with the lacquer indicates the areas of high
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stress concentration thereby enabling location of the best areas to
cement the gauges.
The coating also gives the direction of the tensile stresses
and hence aids in the alignment of the gauges. Wain”’ employed a
combination of the two methods. The lacquer was coated onto the
denture and loads were applied. The gauges were then applied
where the cracks were seen and strains measured.
PHOTOELASTIC ANALYSIS
The photoelastic method is a well-recognized engineering
method of stress analysis and was first applied to dentistry in 1949
by Noonan. His study employed this method to evaluate amalgam
restorations and cavity design. Since its initial application. the
method has been used widely in the field of dentistry. The technique
involves construction of a model of the structure to be investigated
from a photoelastic material.(Fig-33)
The direction and magnitude of the applied forces and the
way it is supported and its shape must simulate the conditions of
the actual structure to obtain a true analysis of the stresses. The
temporary double refraction under stress of photoelastic materials is
utilized for photoelastic analysis. The incident ray of light is
resolved into two rays which travel at different velocities along the
principal plane of the material and emerge retarded with respect to
each other.
The amount of retardation is directly proportional to the
difference between the principal stresses and is measured using a
polariscope. The coloured fringes obtained are used for the stress
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determinations. The application of the method to dentistry was
reviewed by Mahler and Peyton. They concluded that the technique
was particularly applicable to dental problems because of the
irregular shapes encountered.
Initial studies utilized two dimensional models but with
improvements in technology the three-dimensional model is being
used. Despite this three dimensional photoelastic studies have been
limited due to the complexity involved in the determination of the
complete state of stress in three dimensional irregularly shaped
structures.
Although the photoelastic method is widely used in dentistry
there is little documentation of its use in dentures. The majority of
the studies relate to partial dentures and few have been reported in
complete dentures.
ADVANTAGES
The advantage of the photoelastic method over the earlier
methods discussed is that it provides a visual display for the
observation and measurement of stress distribution throughout the
model under investigation. However the method requires special
equipment and expertise to perform adequately. Since the
introduction of the computation era,stress analysis of dental
structures has been made easier with the use of the finite element
method.
DRAWBACKS:
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The specimen preparation for this method is arduous since it
is critical that the model is of uniform thickness. Tanner” has
examined the factors that affect the design of photoelastic models
for two-dimensional analysis. He concluded that the mode of support
was the most important factor affecting the relationship between the
experimental model and the original structure.
The elastic modulus of the material used for specimen
preparation may not conform to the actual material used for the
prosthesis. Additionally no absolute value for the magnitude of the
stress is obtained and only the maximum shear stresses can be
analysed. Separation methods have to be used to obtain other
components of the stress tensor which can be lengthy and
demanding.
REFLECTION PHOTOELASTICITY
This is a new method of detecting stresses in prosthetic appliances.
Through the observations of fringe patterns created upon loading,
reflection photo elasticity gives immediate identification of stress fields
in parts of the studied object accessible to normally incident light. This
method has been widely used in testing industrial prototypes but
experiments in dentistry are limited.
FINITE ELEMENT ANALYSIS
The finite element analysis is a computerized numerical
method used to determine the distribution of stresses and
displacements in a structure subjected to mechanical load. Initially
developed for use in the aircraft industry.The method has seen
widespread use not only in aerospace engineering but also in civil
engineering. Prior to the advent of the computer the technique
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required considerable mathematical ability. However. with the
availability of a number of software programs the method has
become more versatile.(Fig-34)
The basic concept of the method is the idealization of the actual
continuum as an assemblage of a finite number of discrete structural
elements,interconnected at a finite number of points called the nodal
points. The finite elements are formed by figuratively cutting the
original continuum into a number of appropriately shaped sections
and retaining in the elements the properties of the original material
(such as the elastic modulus and poisson’ s ratio). In structures
having a regular simple geometry relatively small numbers of
elements will be adequate, however in more complex shapes a
higher number of elements would be required to improve the
accuracy of the analysis.
The analysis process consists of satisfying compatibility within
each element and equilibrium conditions at the nodal points. By
concentrating the equivalent forces at the nodes, equilibrium
conditions are satisfied in an overall sense.
The information required to calculate the stresses is:
The total number of nodal points.
The total number of elements.
The type of boundary conditions.
Evaluation of the forces at the external nodes.
Coordinates of each nodal point.
The elastic modulus and poisson’ s ratio.
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Once these are specified, the displacements. as well as the stresses.
can be immediately calculated with the help of the program. The
validity of the finite element results depends on the precision by
which the geometry, material properties and interface conditions,
support and loading are in accordance with the physical reality.
The finite element method, due to its simplicity and relative
ease of use is becoming more popular for the stress analysis of
dental structures. Additionally its other advantages are that the oral
conditions can be simulated reasonably easily and different
parameters can be altered relatively simply. Although initially used
in two dimensions the popularity and improved accuracy of the
three dimensional model is becoming more apparent. The three-
dimensional model has been used in the stress analysis of the
mandible and other structures.
This method has proved to be valuable in stress
analysis.However the limitations of the method lie in the validity
and accuracy of the model. The latter problem can be overcome by
the use of convergency tests where subsequent mesh refinements of
the model make the results concerge. The validity of the analysis
should be established by either comparing results with clinical
observations or laboratory tests.
The limitations of a two- dimensional design must be
appreciated where the analysis involves these models. Additionally in
the finite element analysis it is assumed that the interfaces between
different materials are in perfect adhesion, with the elements
comprising different materials being joined at common nodes.
Another drawback is that the computer package for the analysis can
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be quite costly. Despite these limitations the method seems to be
promising and appears to play a valuable role in the stress analysis
of all dental structures.
In the finite element method all the stress components can be
calculated and these can be calculated at each point in the model.
Changes of relevant parameters and loads can be easily incorporated
into the calculation and hence conducting the analysis and
assimilating the results is quicker than the photoelastic method.In as
much the accuracy of the calculation results can be easily increased
by increasing the number of elements and the three-dimensional
analysis is likewise easily within its range of possibilities.
HOLOGRAPHIC INTERFEROMETRY
While holography is often used to obtain recreations of 3-
dimensional objects, many industrial applications of holography make
use of its ability to record two slightly different scenes and display the
minute differences between them. This powerful technique, called
interferometry,is an invaluble aid in design, testing, quality control, and
stress analysis.(Fig-35)
Holographic techniques are non destructive, realtime,and
definitive in allowing the identification of vibrational modes,
displacements, and motion geometries.If the object under study is
changed or disturbed in some way during the hologram exposure or from
one exposure to the next,then a pattern of “fringes” will appear on the
image itself, making the object look striped.
These fringes really represent maps of the surface displacement
caused by the force or stress that disturbed the object.Such a displacement
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map represents an extremely sensitive picture of the actual motion the
object has experienced, with a single fringe contour representing lines of
equal displacement.
Holograms can record motions and displacements, deformations
and bends, and expansions and contractions on virtually any object. The
typical optical laser used in holographic interferometry gives an accuracy
better than a half wavelength (about 10 millionths of an inch), and both
qualitative and quantitative information can be derived from the fringe
patterns.
This allows us to look at the effects of vibration, temperature,
stress and strain,and other physical forces in an entirely nondestructive
way. A powerful feature of holographic interferometry is that information
is obtained over the entire illuminated surface of the object being
studiedas a full and continuous field, which is important in understanding
what is happening to the object as a whole.
Holographic interferometry is used in vibration and modal analysis,
structural analysis, composite-materials and adhesive testing, stress and
strain evaluation, and flow, volume/shape, and thermal analysis.All these
applications derive from one or more of the three basic methods of
applied Holographic interferometry
Real-time,
Multiexposure, and
Time average holography.
Interferometric nondestructive testing can be accomplished with either
continuous or pulsed lasers of almost all wavelengths.Continuous lasers
are ideal for real-time studies of displacement and motion. Pulsed lasers
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can be synchronized with motion and also can record holograms of
extremely fast transient phenomena.
REAL-TIME HOLOGRAPHY
Real-time holography allows one to observe instantaneously the
effects of minute changes in displacement on, or in,an object as some
stress affects it. This is done by superimposing a hologram of an object
over the object itself while it is being subjected to some small force or
stress.
MULTIEXPOSURE HOLOGRAPHY
Multiexposure holography creates a hologram by using two or
sometimes more exposures. The first exposure shows an object in an
undisturbed state. Subsequent exposures, recorded on the same image, are
made while the object is subjected to some stress. The resulting image
depicts the difference between the two states.
TIME AVERAGE INTERFEROMETRY
The third technique,time average holography, involves creating a
hologram while the object is subjected to some periodic forcing function.
This yields a dramatic visual image of the vibration pattern.All these
techniques reveal the shape,direction, and magnitude of the stress induced
displacements in the structure under study. An important key to
holographic interferometry’s success is that it allows the use of very low
level, non destructive stress to gather data that once required destruction
of the material.
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STRESS CONTROL BY DESIGN CONSIDERATIONS
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It is often argued that the theoretical aspects of partial denture
design are of primary importance. In reality,clinical observation and
experience must be used to balance what should happen with what will
happen. It has been stated that "No removable partial denture can be
designed or constructed that will not be destructive in the mouth." This
statement can be fully justified if all forces and movements are
considered.
There is no mechanism to counter all forces that may be applied to
a removable partial denture. Nevertheless, a design philosophy that
strives to control these forces within the physiologic tolerances of the
teeth and supporting structures can be successful. Therefore,the design
philosophy of this book is a combination of theoretical and clinical
knowledge that a practitioner can learn and then use to achieve
predictable results.
Past arguments about partial denture design philosophies have
resulted in noticeable confusion. As a result, many practitioners have
abandoned their design responsibilities. The stresses induced by a
removable cast partial denture can be managed by keeping design
considerations for the various components of the partial denture in
mind. They are as follows
I. Direct Retention
The retentive clasp arm is the element of a removable partial
denture that is responsible for transmitting most of the destructive forces
to the abutments. Consequently, a removable partial denture should be
designed to keep clasp retention at a minimum, and yet provide adequate
retention to prevent dislodgment of the denture by unseating forces.
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Other components of a removable partial denture may contribute to
the retention of the prosthesis,thereby allowing a reduction in the amount
of retention provided by clasps. Exploiting this retentive potential in
widely separated areas of the mouth can re-sult in reduced loads on the
abutment teeth. As a result, the support and stability of the prosthesis also
may be improved.
POTENTIAL SOURCES OF ADDITIONAL RETENTION
a) Forces of adhesion and cohesion
For prosthetic purposes, adhesion may be defined as the
attraction of saliva to the denture base and soft tissues, and
cohesion may be defined as the attraction of saliva molecules for
one another. Although it is impossible to develop a peripheral seal
around the borders of a removable partial denture, adhesion and
cohesion can still contribute to retention. To maximize this effect,
each denture base must cover the maxi-mum area of available
support, and it must be accurately adapted to the underlying
mucosa.
b) Frictional control
The partial denture should be designed so that “Guide planes”
are created on as many teeth as possible. Guide planes are areas can
the teeth that are created so that they are parallel to each other to
the path of insertion and withdrawal from the mouth. These planes
may be created on the enamel surfaces of the teeth or in restorations
placed on the teeth. The frictional contract of the prosthesis against
these parallel surfaces can contribute significantly to the retention
of the denture.
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d) Neuromuscular control
The innate ability of the patient to control the actions of the
lips, cheeks, and tongue can be a major factor in the retention of
a removable prosthesis. A patient who lacks the ability or
coordination to control the movement of these structures may
not be able to retain a prosthesis. The design and contour of the
denture base can greatly affect the patient's ability to retain a
removable partial denture.
Any overextension of the denture base can contribute to
displacement of the prosthesis. As a result, clasping mechanisms
will no longer be passive and will apply undesirable forces to the
abutments. These forces may produce noticeable tooth
movement and /ordiscomfort. Properly contoured denture bases
prevent such difficulties and can enhance retention and stability
of a removable partial denture.
d) Clasp position
The position or the relation of the retentive clasp to the height
of contour is more important in retention and in controlling
stresses.
The number of clasps used in the design will determine the
type of stress developed within a denture. Removable partial
dentures with four clasps are described to have a “Quadrilateral
configuration”. Similarly RPD with three and two clasps are
described to have “tripod” and “bilateral” configuration
respectively.
Quadrilateral configuration
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The quadrilateral configuration is indicated for Class-III arches,
particularly when there is a modification space on the opposite side of the
arch. A retentive clasp assembly should be positioned anterior and
posterior to each edentulous space. This creates a stable mechanical
situation in which leverage is effectively neutralized.(Fig-38)
For a Class III arch in which no modification space exists, the goal
should be to place two clasp assemblies adjacent to the edentulous space,
and two clasp assemblies on the opposite side of the arch. The clasp
assemblies on the intact side of the arch should be separated for
additional mechanical stability. Consequently, one clasp assembly should
be placed as far posteriorly as possible, and the other should be
positioned as far anteriorly as space and esthetics will permit. This
maintains the quadrilateral concept and represents an effective method of
controlling loading.
Tripod configuration
This design is used primarily for class II edentulous arches. If there
is a modification space on the dentulous side, the teeth anterior and
posterior to the space are clasped to bring about the Tripod configuration.
If the modification space is not present, one clasp on the dentulous side of
the arch should be positioned as far posterior as possible, and other as far
as anterior as factors. Such as interocclusal space, retentive undercut and
esthetic considerations will permit.(Fig-37)
The design is not effective as quadrilateral configuration but is
most effective in neutralizing leverage in class II situation.
Bilateral configuration
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In case of bilateral distal extension group, or class I ideally
the single retentive clasp on each side of the arch should be located
near the center of the dental arch or denture bearing area. In
bilateral configuration the clasp exerts little neutralizing effect on
the leverage induced stresses generated by the denture base.(Fig-
36)
e) Clasp design
1) Circumferential cast clasp
The conventional circumferential cast clasp originating from a
distal occlusal rest on the terminal abutment tooth and engaging a
mesiobuccal retentive undercut should not be used on distal extension
RPD. The terminal of this clasp reacts to movement of the denture base
towards the tissue by distal tipping, or torquing forces on the abutment
tooth.(Fig-39)
A cast circumferential clasp that approaches a distobuccal
undercut from the mesial surface of the terminal abutment tooth is
acceptable. As an occlusal load is applied to the denture base, the
retentive terminal is moves further gingivally into undercut area
and looses contact with the abutment tooth. In this manner the
torque is not transmitted to the abutment tooth.
2) Vertical projection clasp or bar clasp
This clasp is used on the terminal abutment tooth on a distal
extension partial denture when the retentive undercut is located on
the distobuccal surface. It is never indicated when the tooth has a
mesiobuccal undercut. The bar clasp functions in similar manner to
reverse circumferential clasp. As the denture base is located
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towards the tissue, the retentive tip of the bar clasp rotates
gingivally to release the stress being transmitted to the abutment
tooth.(Fig-40)
3)Combination clasp
When a mesiobuccal undercut exists on a abutment tooth
adjacent to a distal extension edentulous ridge, the combination
clasp can be used to reduce the stress transmitted to the abutment
tooth. Wrought wire clasp by virtue of its internal structure is more
flexible than a cast clasp. It can flex in any spatial plane, where as a
cast clasp flexor in horizontal plane only. The wrought wire
retentive arm has a stress breaking action that can absorb torsional
stress in both the vertical and horizontal planes. A cast
circumferential clasp under some situation would transmit most of
the leverage induced stress to the abutment teeth.(Fig-41)
f) Splinting of the abutment teeth
Weak abutment teeth should be splinted with the adjacent
teeth for strength and stability. Splinting helps to share the stresses
produced in a weak abutment tooth. It will stabilize the weak teeth
in mesiodistal direction. Usually splinting is done by fabricating
full veneer crowns over the teeth to be splinted or by clasping more
than one tooth on each side of arch with numerous rests for
additional support and stabilization.(Fig-42)
Guide planes helps to increase the horizontal stability of the
denture. Hence, additional clasps can be used to increase the guide
planes and also increase the cross arch stabilization.
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Indications
Abutments with tapered or short roots.
Terminal abutments located on the edentulous side of a distal
extension denture base.
Fixed splinting is given if there is some loss of periodontal
attachment, after a periodontal disease or therapy.
II. Indirect Retention
An indirect retainer is a part of removable partial denture that
helps direct retainer to prevent displacement of the distal extension
denture by resisting the rotational movement of the denture around
the fulcrum line established by the occlusal rests. The indirect
retainer is located on the opposite side of the fulcrum line from the
denture base.(Fig-43)
Indirect retention is based on lever principle. It is produced by
moving the axis of rotation of the denture away from the point of
application of force.
In class I situation, indirect retainers are necessary and they
should be positioned as far anteriorly to the fulcrum line as
possible.
In class II situation, the fulcrum line runs through the most
posterior abutment on the dentulous side and the terminal
abutment on the distal extension side. Adding another rest
perpendicular to this fulcrum line provides indirect retention.
In class III situation, indirect retention is usually not
required. In some case there is a buccolingual placing of the
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
denture, which is prevented by placing rest on the dentulous
side, perpendicular to longitudinal axis of rotation of denture.
III. Denture Base
- The denture base should be designed to cover the maximum
amount of soft tissue available.
- The denture base should have long flanges, within the
physiological limits of the soft tissues in order to stabilize the
denture against horizontal movement.
- Distal extension denture base must always extend onto the
retromolar pad area in mandibular denture and cover the entire
tuberosity in the maxilla.
- The denture base will displace the soft tissues on the ridge
during functional occlusal load. A functional impression is
recorded to fabricate the denture in order to improve its
adaptation and avoid excessive tissue displacement.
IV. Major Connector
In the mandibular arch the lingual plate major connector that
is properly supported by rests can aid in the distribution of stresses
to the remaining teeth. It is particularly effective in supporting
periodontally weakened anterior teeth. It also contributes to the
effectiveness of cross arch stabilization.
In the maxillary arch the use of a broad palatal major
connector that contacts several of the remaining natural teeth
through lingual plating can distribute stresses over a large area.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The major functions o the major connector includes rigidity,
retention and stability.
V. Minor Connector
The major connector joins the major connector to the clasp
assembly and the guiding planes located on the abutment tooth
surface. The minor connectors used for auxillary rests aid in
indirect retention.
- These provide horizontal stability to the partial
denture against lateral forces on the prosthesis.
- The abutment tooth receives stabilization
against lateral forces by the contact of the minor connector.
VI. Rests
Properly designed rests help in control the stresses, by
directing the forces, acting on the denture to the long axis of the
abutment tooth. The floor of the rest seat should be less than 90° to
a tangent line drawn parallel to the long axis of the tooth(Fig-46).
Adding rests on the additional teeth decreases the amount of
occlusal load on each tooth and helps to distribute the occlusal load
equally to all the abutment teeth.
STRESS BREAKERS
A stress breaker is defined as "A device which relieves the
abutment teeth of all or part of occlusal forces" GPT-6.
All vertical and horizontal forces, applied to the artificial
tooth are distributed throughout the supporting portions of the
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
dental arch. Broad distribution of force is accomplished through the
rigidity of the major and minor connectors.
In distal extensions situations, the use of rigid connection
between the denture base and supporting teeth must account for the
base movement without, stress on the abutment teeth and residual
ridge is minimized through the use of functional basing, broad
coverage, harmonious occlusion and correct choice of direct
retainers.
The concept of stress-breaking exists that insists on
seperating the action of the retaining elements from the movement
of the denture base by allowing independent movement of the
denture base for its supporting framework and direct retainers
Aims:
1) To direct occlusal forces in the long axis, of the abutment
teeth.
2) To prevent harmful forces being applied to the remaining
natural teeth.
3) To share the forces as evenly as possible between the natural
teeth and distal extension area according to the ability of these
different tissue to accept the forces.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
4) To ensure that the part of the load applied to the distal
extension area is distributed as evenly as possible over the
whole mucosal surface.
Dentures with a stress breaker are also called as a "broken
stress partial dentures".
In a tooth tissue supported partial denture, when an occlusal
load is applied, the denture tends to rock due to the difference in
the compressibility of the abutment and soft tissues. As the tissues
are more compressible, the amount of stress acting on the
abutments in increased, which can produce harmful effects on the
abutment teeth.
To protect the abutment from such conditions, stress breakers are
incorporated into the dentures. A stress breaker is a hinge like joint
placed with in the denture framework, which allows the two parts of the
framework on either side of the joint to move freely.
I. Movable joint between the direct retainer and denture
base. This group includes hinges, sleeves, and cylinders and
ball and socket devices. Being placed between the direct
retainer and denture base, they may permit both vertical
movement and hinge action of the distal extension base. This
prevents direct transmission of tipping forces to the abutment
teeth as the base moves tissue-wards under function. E.g.
Dalbo attachment, Crismani attachment, ASC 52 attachment.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
II. Flexible connection between the direct retainer and
denture base includes the use of wrought wire connector and
split major connector(Fig-47).
Advantages:
- Vertical forces acting on the abutment teeth are
minimized and alveolar support of abutment teeth is preserved.
- Intermittent pressure of denture bases massage the
mucosa thus providing physiologic stimulation, which prevents
the bone resorption and eliminates need for relining.
- Minimal requirement of direct retention.
- Weak abutment is well splinted even during the
movement of the denture base.
Disadvantages:
- Design is complicated and expensive.
- The assembly is very weak and tends to fracture very
easily.
- Difficult to repair.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
STRESS BREAKERS OR STRESS EQUALISERS
Stress Breaker
A stress breaker is a device which relieves the abutment teeth of all
part of the occlusal forces(GPT-2005).
Stress Director
A stress director is a device that allows movement between the
direct retainer which may be intracorornal or extra coronal.(GPT-2005)
Introduction:
The resiliency of the tooth secured by the periodontal ligament in
an apical direction is not comparable to the greater resiliency and
displaceability of the mucosa covering the edentulous ridge.Due to this
forces are transmitted to the abutment teeth as the denture bases are
displaced in function.
It is agreed that a rigid connection between the denture and the
direct retainer on the abutment tooth is damaging and that some types of
stress director or stress equalizer(a flexible or movable joint between
teeth and metal frame work so that the clasp) is essential to protect the
vulnerable abutment teeth.It allows independent movement of the denture
base and the direct retainers separates the action of the retaining elements
from the movement of the denture.
The need for stress breakers on free end RPDs has been recognized
on the basis that the resiliency or displaceability of the mucosal tissue
ranges between 0.4 mm to 2mm, while the vertical resiliency of a normal
healthy tooth in its socket is approx. 0.1mm. This tissue resiliency
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
differential of 20 to 40 times the axial displaceability of a normal tooth in
its socket dictates the necessity for some form of stress direction in the
partial denture design.
TYPES OF DESIGNS:
RESILIENT JOINTS AND HINGE JOINTS
Joint attachments are used as retainers for unilateral and bilateral
distal extension partial dentures.They allow various degrees of movement
between the body of the prosthesis and the abutment teeth. The
movement
may be:
Rotation around a transverse axis
Vertical bodily movement
Based on the type of movement, joints are classified either as:
Resilient hinge joints that allow both vertical bodily movement and
rotation around a transverse axis
Pure hinge joints that only allow rotation around a transverse axis
RESILIENT HINGE JOINTS
Joints can be connected directly to an abutment crown through
either the female or the male part. In these cases the two parts are
separated in the mouth as the denture is removed.
There are, however, joints in which the entire joint construction
can be separated from the abutment. The male and female elements,
which comprise the resilient part,are connected to the abutment tooth by
means of a sliding attachment (sliding attachment resilient joint). Joints in
which one part can be removed from the other at the abutment crown are
called separable joints. Those that cannot be separated in the mouth are
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
designated as linked joints and are connected to the abutment teeth by
means of additional retainers, such as cast clasps or double crowns.
Each individual form of joint is offered by the manufacturers in
many variations, allowing a wide range of applications. Thus, there are
special types for extracoronal installation, and for both unilateral and
bilateral use.
When using a jointed connection between the body of the
prosthesis and the abutment teeth, the compressibility and the resilience
of the mucosa must be taken into account The stress-breaker effect
prevents the transmission of excessive forces to the abutment teeth during
chewing. The springs built into the joints cushion the loading forces and
return the denture bases to their rest positions.
Dalbo attachment
This attachment is one of the oldest and most successful
extracoronal attachments and is classified as an adjustable, directed-hinge
distal extension attachment.This system features lateral stability, vertical
resiliency, and hinge movement.The advantages of the Dalbo system are
the intrinsic direct retainer and excellent stability owing to the vertical
beam. The attachment may be used in unilateral or bilateral applications.
The unilateral configuration provides a larger vertical bar for enhanced
lateral stability. The attachment is offered in two sizes, although the mini
version lacks vertical resiliency.(Fig-48)
The resilience hinge joint by Dalla Bona is available in separable
(Dalbo extracoronal attachment) and linked(Dalbo-Fix) forms.With the
separable variety,the ball-shaped male section is attached to the abutment
crown,either by soldering or by being luted to the wax pattern
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
beforecasting. The removable female housing with its enclosed coilspring
is embedded in the denture base.
The vertical resiliency is rendered through the presence of a spring
and found only in the standard unilateral and bilateral designs.The
difference between the standard and the mini is approximately 2 mm in
clinical crown height requirement, 1.7 to 2.0 mm in preparation depth,
and 1 mm in faciolingual width requirement. As in all extracoronal
attachments, the amount of space required in the denture base is
approximately 5.5 to 6.0 mm.
This often creates difficulty with tooth placement and inadequate
strength for the resin. The minimum amount of resin recommended
should be strictly adhered to so as not to compromise the strength of the
denture base in the region of the attachment. This extracoronal retainer
offers a mechanism to "lock" the attachment for reline procedures.
ASC-52 ATTACHMENTS
The functional properties of the ASC 52 resilient joint attachment
stressbreaker is based upon the original adaptation of the CARDAN
JOINT principle. The ball screw spring joint ASC-52 from Degussa is a
separable attachment, that is,it can be disconnected in the mouth. The
female part is attached to the abutment crown, and the male unit, to the
removableprosthesis.The male unit is made up of a ball tipped sliding
bolt, enclosure, spring,and screw(Fig-49).
It is most useful due to the following reasons:
The removable part of the prosthesis can accomplish a wide variety
of movements according to the specific case
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
This removable part is well anchored to the abutment teeth, but it
does not overload them.
In this way a perfect retention of the partial prosthesis in the mouth
is assured,the prosthesis can be easily inserted and removed
without any risk for the abutment teeth.
The action of the inner part can be regulated according to the
specific needs.
An increase or decrease in prosthesis moveability is achieved by
adjusting the spring tension(screw or unscrew the small nut),
It is possible to replace any detail of the inner part at any time
(wear, damages, accident),dental technicians will find ther joint
attachment easy to handle.
DSE HINGE
The DSE Hinge is intended for use on bilateral clasp retained free
end removable partial dentures to reduce loading or torquing of
abutments. The small size is easy to work with and eliminates multiple
inventory requirements.The unique design provides for easy freeing after
casting and provides total lateral stability.For patients, it allows patient
comfort and abutment protection by allowing independent unilateral
function eliminating torquing leverage on the abutments on the
nonfunctioning side. The miniaturized size allows utilization in short
vertical spaces and provides for good esthetics(Fig-50).
The FR system
The FR system for removable partial dentures is a technological
revolution for what many call semi-precision or "mill-ins." A
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
tapered,friction retained intracoronal precision attachment for removable
partial dentures with a lingual arm and for segmented bridgework or non-
parallel abutments in fixed bridgework.The female is actually cast against
the prefabricated male for total accuracy, improved proximal wall
contacts, and tremendous time and labor savings(Fig-51).
This simple and inexpensive attachment uses a single investment
technique, eliminates porcelain in the female due to the silicone male,
allows for easy and accurate duplication, no soldering is needed, easy
separation of male and female, and the miniature size allows for use in
‘close bite’situations. Easy insertion because of the tapered male,
improved esthetics (no metal on the occlusal),excellent retention and
reduced wear account for this being one of the most popular attachments
in dentistry.
The UNOR
The UNOR is a screw adjustable retention precision attachment for
intracoronal use. The beveled male is adjustable so retention may be
eitherincreased or decreased, allowingfor easy patient insertion and
removal, thus less wear. Vertical height may also be altered for short or
‘close bite’ situations. The female may be directly cast with precious or
semi-precious alloys for easy fabrication.(Fig-52)
A female ceramic former is available for creating a female in non-
precious castings. The excellent external wall contact allows for guide
plane stability. The male may be connected to the cast frame by acrylic
resin, composite resin, or solder.The system also allows for conversion of
a fixed bridge to a removable partial denture if distal abutments are lost.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
CEKA ATTACHMENT
Definition:
An attachment which has a patrix conical portion with a split head
for activation and a matrix cap portion.
Indications
The Ceka attachment was developed as an extracoronal attachment.
However, it can also be used for both root face abutments and bars. In the
latter case it allows increased retention of the superstructure where a clip
may not be provided. If the bar is short the placement of a clip may not be
possible and therefore the use of such an adjustable attachment can
provide the solution.
Advantages
The attachment can be used for many different clinical situations . The
matrix ring retainer can be placed in a variety of locations and the patrix
component comes in different forms allowing it to be cast, soldered or
bonded into place. The patrix has a cross split allowing for activation of
this attachment with wear.
The Traditional Ceka and Ceka Revax systems provide hinge,
vertical, and rotational movements to provide maximum abutment
protection. Each attachment consists of three angulations of plastic
female profiles with precision metal insert,male spring pin, and retention
component. The three angulations allow the user to design the case for
the patient’s needs(Fig-53)
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Disadvantages
The attachment requires adequate space and the correct angulation
relative to the path of insertion of the denture.
VERTIX
The Preci Vertix and Vertix "P" provides for hinge and vertical
movements. It should besupported by a broad based ridge. TheVertix is a
very inexpensive and popular system that provides patient comfort and
abutment protection for both mandibular and maxillary bilateral
removable partial dentures. Unilateral free end removable partial dentures
should be cross arch stabilized(Fig-54).
The Vertix features time-saving and simple routine techniques,
requires no additional tools, may be cast in any alloy to eliminate
soldering and dissimilar alloys, and provides outstanding space-saving
aesthetics. The plastic female absorbs negative movements to protect
abutments and provide patient comfort. It requires only a routine full
coverage abutment preparation and provides easy patient insertion and
removal. The only servicing requirement is the occasional, fast, easy
female replacement. Three different female retention clips are available to
accommodate all your retention needs.
O-SO Distal extension
The OSO is a popular extracoronal attachment that provides free
movement in all planes for maximum protection. A proven retentive
system utilizing an easily replaceable rubber O ring. This is a resilient
attachment with vertical and hinge stress-breaking action for free-end and
bounded partials. Maybe used in conjunction with other attachment(Fig-
55).
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Universal Ic attachment
The IC attachment is a popular spring loaded retaining attachment
that provides free movement for abutment protection without requiring an
abutment crown. The IC attachment requires a 180 degree reciprocal
lingual arm. The attachment consists of a male anchor and female inlay.It
is made of a stainless, chrome-alloy like those used for casting partials. It
will not tarnish or corrode, and when properly installed,will not
malfunction even after years of wear.Other benefits include no pulpal
involvement,no gingival retraction before impressions, easy to adjust at
the chair ,and this is a reversible procedure.
Mays unilateral attachment
Designed specifically for the unilateral distal extension, the Mays
is the first attachment with a lingual locking arm. It can not be
dislodged,but yet is easily removed for patient hygiene. Does not require
a lingual or palatal arm. No cast chrome framework required or soldering;
the male portion casts with the crowns. No parallelism is necessary ,even
on bilateral cases.(Fig-56)
HINGE JOINTS
Hinge joints exhibit only one type of freedom of movement,
namely, rotation around a transverse axis. Since there is no vertical bodily
movement, they have a somewhat more favorable topographic and
dynamic relationship to the distal abutment teeth than do joints with both
rotation and bodily movement. There is no direct mechanical irritation of
the gingival margin.Hinges that cannot be separated in the mouth (linked
hinges) can only be used on removable prostheses (partial dentures and
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
telescopic bridges). A bilateral application is also conceivable provided
the two denture bases are not rigidly connected to each other.
Hinges that cannot be separated in the mouth (linked hinges) can
only be used on removable prostheses (partial dentures and telescopic
bridges). A bilateral application is also conceivable provided the two
denture bases are not rigidly connected to each other.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
SPECIAL CLASP DESIGN TO CONTROL STRESS
RPI system
In 1963, Kratochvil introduced the “I-bar design philosophy”.This
philosophy was based upon use of an I-bar retentive element, a mesial
rest, and a distal proximal plate. Proponents of the I-bar philosophy
claimed that the resultant clasp design minimized torquing forces and
directed occlusal loads parallel to the long axes of abutments.The I-bar
rationale, especially the use of a mesial rest, emerged as a popular by-
product of Kratochvil's design principles(Fig-57).
The RPI clasp is a current concept for bar clasp design, as the full
“T” bar should not be used since it covers an unnecessary amount of
tooth structures compared with the RPI clasp.
Components of the I-bar System
Kratochvil's I-bar system includes a mesial rest, I-bar retainer, and
a long distal guiding plane that extends to the tooth-tissue junction. Each
component must function properly to ensure success of the I-bar system.
The RPI clasp fulfils the requirements of proper clasp design The
practitioner must understand that the I-bar retentive clasp is only one
element in the design equation.For this clasping system to function
effectively, all components must be properly designed, constructed, and
fitted.
The rest, located on the mesial occlusal surface of the abutment
tooth, acts as the point of rotation and exerts a mesial force on the tooth
rather than a distal displacing force. Pressure exerted on the extension
base moves the proximal plate tissueward without torquing the tooth. The
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
I bar also moves mesiogingivally away from the tooth under masticatory
load.
Rests
For distal extension base partial dentures where a bicuspid serves
as the abutment tooth, a mesial rest preparation is made.
For posterior teeth, where restorations are not placed,the rest seal
can be prepared in the appropriate triangular fossa.
Sufficient bulk of metal must be provided to permit the rest to
function without fracturing or bending.
Gold requires larger and deeper preparations than the non-precious
metals (chrome cobalt, nickel cobalt, etc.).
This preparation should be rounded and fully polished to permit
some rotation when depression of the extension base occurs.
If a cuspid is to serve as the abutment, a mesio lingual rest
preparation is made.
The rest seat must be deep enough to prevent the mesial rest from
slipping gingivally.
As a general rule, mandibular cuspids have a thin enamel covering
and when preparing an adequate rest seat, penetration into the
dentin is often inevitable.
If dentin is exposed, the preparation should be deepened and
modified to accept a gold foil, amalgam, or other restoration which
can be properly contoured.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Guide Planes
A guide plane is prepared on the distal surface of the abutment
tooth at the occlusal one third as proposed by Potter.
It should extend lingually just far enough so that the proximal
plate together with the mesial minor connector will prevent lingual
migration of the tooth.
The guide planes should be approximately 2 to 3 mm. in height
occlusogingivally.
This guide plane will often permit the proximal plate and the
mesial minor connector to contact the tooth simultaneously and
provide proper reciprocation against the force exerted by the
retentive buccal clasp arm during the seating and removal of the
denture.
If the mesial minor connector and proximal plate cannot contact
simultaneously, as may occur with cuspid abutments, then the
retentive I bar should engage the mesiobuccal undercut and receive
its reciprocation from the proximal plate alone.
Proximal Plate
It is placed on a distal guiding plane, extending from the marginal
ridge to the junction of the middle and gingival third of the
abutment tooth.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The proximal plate minor connector should contact approximately
1 mm of the gingival portion of the guiding plane in distal
extension cases.
The bucco-lingual width of the proximal plate is determined by the
proximal contour of the tooth.
The proximal plate extends lingually just far enough so that the
distance between the minor connector and proximal plate is less
than the mesiodistal width of the tooth.
It should be 1mm thick and join the framework at right angle.
The proximal plate together with the mesio-lingually placed minor
connector provides stabilization and reciprocation of the assembly.
I Bar
The approach arm of the I bar extends from the framework so as to
remain at least 3 mm from the gingival margin and then crosses
the gingival margin at right angles.
Approximately 2 mm of the I bar contacts the tooth surface,
usually at the gingival one third of the tooth.
The bottom ponion of the I bar contacting the tooth surface should
engage 0.01 inch undercut.
The I bar should taper slightly from the base to the tip. It is usually
placed at the greatest mesiodistal prominence on the buccal surface
or towards the mesial, but not toward the distal.
Slight relief is necessary when the arms crosses the gingival
margin.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
Advantages
Vertical masticatory force on the distal extension base causes the I-
bar to move mesiogingivally away from the tooth and the proximal
plate to move further into the undercut of the tooth.
Thus, both the I bar and the proximal plate disengage the abutment
and thereby reduce torquing of the tooth.
The mesial minor connector together with the proximal plate
provide the necessary reciprocation and eliminate the need for a
lingual arm.
The mesial rest eliminates the potential "pump handle" effect
that a force. on the base often induces with a distal rest.
The RPI clasp contacts the tooth minimally and is advantageously
used on caries prone patients.
The I bar itself makes very little contact with the tooth, it is usually
more esthetic than most other clasp arms.
Indications
The RPI clasp is indicated
In distal extension cases, as it provides a stress releasing action.
When tissue undercuts are not severe.
Contraindications
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
The RPI clasp is contraindicated with
Shallow vestibule (the base of the I-bar should be at least 3mm from the gingival margin).
High floor of the mouth which necessitates the use of lingual plate.
When buccal undercut is absent or only distobuccal undercut exists.
In cases with severe tissue undercut to avoid food or tissue trap.
If the facial surfaces of teeth are facial to the tissue surface, the RPA Clasp may be used.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
CONCLUSION
Many individuals have contributed to the progressive
advancement of partial denture service and have written extensively
to document their experiences and philosophies in this field. Their
objective are universal, to provide means of restoring function,
esthetics and comfort, which promotes the oral health.Partially
edentulous arches exist in a great variety of forms.
A thorough knowledge of the mechanical principles involved is
very important and it should be understood properly because it is an
integral factor in design of removable partial denture.Designing of the
appliance play an important role because it is through the structure
of the denture that the forces of mastication are transmitted from
the occlusal surfaces of artificial teeth, natural teeth and underlying
tissues.
Generally it is very important that, the design which provide
broad bases, rigid connectors, multiple rests and properly selected
retainers are most likely to effect favourable distribution of force
and maintain the integrity of remaining tissues. Hence while
designing removable partial denture biological as well as physical factors
should be considered.The biological factors includes the denture support,
avoiding the deleterious effect on the abutment teeth, proper stress
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
distribution and the physical factors includes the strength of the denture
base used, whether it accommodates the future relining etc.
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE
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