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References lec 6
Chapter 3, Stewarts Clinical
Removable Partial Prosthodontics
(pages 51-83)
Chapter 6, Removable Partial
(pages 49-59)
We are going to talk about direct retainers they are thenext component of the metal frame work of the removable
partial denture design in which we are discussing
Retention in metal frame work of the removable partial
denture is different from other prosthesis for example for
the complete denture we get our retention from posterior
palatal seal how ever in metal frame work partial denture
we get our retention mechanically usually by engaging the
clasp under the maximum bulge of the tooth
There are two basic types of retention :
1-Intra coronal retentionWithin the contour of the tooth crown
Like the rests ,
precision attachments ( key and key lock
combination ), semi-precision
** we won't talk about these this semester they are
more advanced
2-Extra coronal retentionOutside the contour of the tooth crown
Cast*
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* wrought wires essentially prefabricated stainless
steal wires which we bend to form the tooth shape
clasps
Clasps above or below the bulge of the tooth aboveor below the survey line are
Supra bulge and Infra bulge
Infra bulge clasps give slight more retention than
supra bulge ones the reason is due what is called
push and pull theory
It's harder to push a table than to pull it so clasps
start from below the survey line when we try toremove it it will provide more resistance and retention
than the clasps which start from above the survey
line which is pulled so have less resistance
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The precision attachments provide support , retention
and reciprocation
Direct retainers usually form an assembly part of theassembly provide retention and the other parts of the
assembly give kind of a balance to the retention
which we need
so we have retention from this arm you
see this arm is on the survey line ( themaximum bulge ) on the facial surface
we have another arm on the lingual it's a mistake to
think that both of these arms provide retention
actually one of these goes to the undercut and the
other will not go it's function is different
the clasp assembly is composed of : the rest thatgives support , the retentive arm that goes above or
below the survey line to the undercut and that what
provide retention and the reciprocation arm which
balances what the retentive arm does or it's like an
abutment
( removable partial denture provides retention
because the clasps goes to the undercut )We have two types of clasps in dentistry the
prosthodontics and the orthodontics the ortho is
designed to move the teeth but in prost once it's fully
seated in the patient mouth it should not apply any
force to the patient teeth
However it's impossible to avoid some force on theteeth
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As the clasp is going down to be fully seated it's
going to go over the maximum convexity as the
clasps is going inside and outside the mouth it will hit
the teeth in very short seconds this short time as itgoes from above the survey line to below the survey
line can cause force on the tooth and this force can
cause movement even though it's only for short
period of time because it is repeated and that's why
we need something in the opposite side of the tooth
to hold it in place and that's what we call
reciprocation .
Here you can see the rest above and the
reciprocating arm is entirely above the
survey line where the retentive arm the tip is
below the survey line( the only part of the removable partial denture that
goes to the undercut is the tip of the retentive arm )
So Direct Retainers
Passive at rest
Activated only during dislodgement
basic requirements clasp assembly1- retention from the retentive arm2- support from the rest and keeps all the other
components in place when it's fully seated in the restplace
3- stability that resist horizontal movements from thebracing arm ,the rest and the minor connecter
4- reciprocation
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receprocating arm rest retentive arm
L B
If you take a look at this diagram if this is the tooth withthe lingual side ( L ) and the buccal side (b ) you will see
as the clasp is seated the reciprocating arm touches thetooth even before the rest and the clasp engage thatmeans as this retentive arm goes over the undercut startto press on the tooth to move it this reciprocating arm (balancing arm ) prevent movement of the tooth awayfrom the retentive arm it keeps the tooth in it's specificplace and here when it at rest it should be completelypassive no any force on the tooth
5- Encriclement which provide stabilization directlywhich prevent tooth migration same idea as thereciprocation the design )the rest ,the retentive arm, thereciprocating arm and sometimes the minor connector)should go around the tooth for at least 180 degreespreventing both the denture and the tooth from migration
6- Passivity once the partial denture is fully seated thereshould be no active force on the tooth
Passivity word is used in two concepts
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1- Preventing partial denture from moving the teeth2- Passivity of the alloyIn Co-Cr alloy the main component is cobaltCr is added to prevent corrosion we say that Cr providepassivity to the alloy
Magnitude of Retention
IMPORTANT: YOU MUST READ EXPLANATION
FOR
CLASP FLEXIBILITY/CLASP RETENTION FROM
THETEXTBOOK REFERENCE
The slide and the lecture are not enough
The equation is called law of beam and in our case it
is the law of the retentive clasp arm which can
handle an applied load and the clasp will be deflected
away from the tooth
There are dimensions of this beam it has a width , athickness and a length
It is important to understand these because the
design of the retainer will affect how many retention
and support
** If I want to make the clasp retainer longer it will
be more flexible and the shorter it is the more
rigid
** if I want to make the thickness thin or the width
narrow it will be more flexible
And this is what the equation stands for :
D = 4PL^3 / Ewt^3
D : deflection
P : applied force
L : length
E : elastic modulus w : beam width t : thickness
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by looking at the equation by increasing the
length you increase the resistance of the clasp
there are other factors other than the material
quality of the clasp which can affect the retention:
1- The shape of the tooth or the angle of convergent
If the angle of convergent is flat the surface is smooth that
means the amount of retention is less so the depth o f the
angle of convergence is important . the more convexity of
the tooth the more retention we have
Clasp flexibility1-the length, the longer it is the more flexible it will be
and it's increased by the curvature so molar teeth havemore retention2- diameter thickness and width when increased morerigid and when decreased more flexible .The actualmaterial of wrought wires is stainless steal is more
flexible than cast metal which is more rigid3-metal material, cobalt chromium is very rigid alloy whilenickel chromium is softer4-the shape the cross sectional geometry ,if its circular it
bends easily with flexibility in every dimension (wroughtwire) , if its square or semi circular( cast ) part of it will beflexible and the other part won't be it would be harderand the geometry will prevent the rotation you need to
read it from the text book .
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Direct Retainer Selection
Principal
Pick a retainer to suit the
existing teeth rather thanprepare the tooth to fit a
particular direct retainer
design
We talked about toothtissue and tooth tooth borne
partial denture. we said that one of the major concept
of a partial denture design is the fabrication of partial
denture rotation that is going to rotate around aspecific fulcrum and this fulcrum is a tooth this tooth
is going to take too much load and in our design
when we make the clasp we do not apply too much
force of the tooth which stand right at the end of the
fulcrum.
If we have a class one partial denture with a tooth in
one side and a tissue on the other side the stress willbe on the last tooth
Direct Retainers
Designed forTooth-Tissue
Borne.
Stress releasing.
Designed forTooth borne.
Non-stressreleasing.
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Types of clasps
We have something that reduces the stress of the
abutment and some which does not. some whichis very tight and one which release when there is
excessive force
non-stress releasing retainers1-cast circumferential clasp2- ring clasp which goes all the way around the tooth3-embrasure clasp but with two clasp next to each other
4-reverse action C clasp if It's adjacent to theedentulous place
1- cast circumferential clasp
Our first choice usually of a posterior tooth is actually
called-cast circumferential clasp which starts from
above the survey line which we called supra bulge
also known as Akers clasp ( name of the dentist who
designed it ) Simple to make, hygienic. And they
have excellent stabilization because we have good
encirclement around the facial and lingual with the
rest.And because they are supra bulge they are not
considered esthetic as they appear when the patient
smile they tend to be less hygienic from below and
it's more difficult to adjust the longer it is the easier toadjust and vise versa .
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The rest adjacent to the edentulous area and the
clasp is supra bulge we have something called the
shoulder which is above the survey line and the
terminus which engages the undercut and we have
the middle section. here in this design we start near
the edentulous area with the Reverse Action
Circumferential clasp we do exactly the same thingbut the rest is on the opposite side of the edentulous
area
2 Ring clasp we have another design called a Ring clasp
it engages the tooth not complete a 360 degree but maybe
300 degrees starting from one point and we remain above
the survey line we go around the tooth and we engage the
undercut in the lingual surface
Edentulous space
mesial to the molar.Typical Cast
circumferential retainer.
Undercut on the
distobuccal of the
molar (preferable
location).
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The thing that is significant about this kind of clasps is that
the bracing arm is both bracing and retentive. sometimesbecause it is too long ( flexible ) we can add an extension
(minor connector )to reduce this flexibility or usually
mandibular molars which are mesialy and lingually tilted
we add it to them as the pic on the left above.
Which tooth is one of the most extracted teeth ?
A : 6 molars because of bad oral hygiene with the
lack of knowledge that it is not a deciduous tooth so
extracted early in life because of caries then when the 7
erupts there is nothing in the place of 6 so the 7 tilts
mesially and lingually with such a tooth it is very difficult
to put a clasp in the right position so we use the ringclasp
Disadvantages of a ring clasp: they are very difficult to
adjust; they have poor hygiene (especially with the
presence of the strut).
They are contraindicated when there's excessive tissue
undercut: sometimes the lower piece of metal has to gonear the bone, so if there is excessive undercut they are
difficult to extend.
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3- Double Embrasure Clasp
the rests are back to back . some people like it a lot
some hates it a lot because they require a preparation ofthe tooth so the clasp will go to the embrasure.
however if I don't have an edentulous space how can I
put a rest for the minor connector ?? as u can see that
the embrasure clasp is
going to the facial
surfaces of both of the
teeth
4- C clasp ( Hair pin clasp )
It is the least desirable design
When we survey a tooth we mark the maximum convexity
of the tooth . some teeth are a symmetrical when the under
cut of the same tooth is law in one side and high on the
other . so sometimes we are forced to start the clasp from a
point and goes to the undercut to the rest distal corner . the
question is how to start from above from the distal and
finish here ?? ( look at the pic )
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So we have to go and then come back and it is very difficult
to adjust and not hygienic and technically difficult to make
But there are other solutions is putting the rest on the
mesial and bring the clasp down, there is different choices.
There is an unusual type of clasps where there is a
concavity in the occlusal plane we can solve it by adjusting
the occlusal plane using metal on top of the tooth ( not
important )
Now we finished the non stress bearing design we will
start talking about three Stress bearing design
when the denture goes away from the tissue or goes down
to the tissue we will end up with excess amount of force
on this tooth . the clasp is designed not to over load this
tooth
There is something called mesial rest concept when I
have an edentulous area the best place to put the rest
mechanically to put it in the point as close to the
edentulous area except in tooth tissue borne prostheses
we usually put the rest away from the edentulous area. if u
place it on the distal you will over load the tooth as the
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force vector is almost horizontal and the posterior teeth
and the tissues are not designed to get lateral forces. But
on mesial rest concept because the arm of rotation is
larger we have more vertical forces and this what happenswhen we change the position of the rest .
What happens with the clasps when we put the rest on the
distal side when the patient bite on the partial denture?
the clasp will extract the tooth .
the retention is needed when the patient opens his mouth
and when there is a sticky material in between his teeth soI don't need retention now ( we get retention from the
clasp ) .you can see that the clasp is working while the
patient is biting and we don't won't this to happen so as he
bite he extract his own tooth . but if we put the rest
mesially and when the patient bite the clasp will go away
from the tooth ( stress relief ) it works when it must and it
disengage when we don't won't it to work .
1- and this is what is called RPI system
"R" Rest (always mesial)
"P" Proximal Plate (distal)
"I" I - Bar (buccal)
We essentially have a bar or an infra bulge which comes
from below it provides aesthetics
bar
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2- RPA Clasps
in RPA we have the R ( mesial ) the P but the A here is
from wrought wires where in PRI was cast metal from I bar
now the A here is coming from above so it is supra bulge
and what they did actually they brought wrought wires and
bend it instead of metal clasps
Used where infrabulge approach not possible
"R" Rest (always mesial)
"P" Proximal Plate (distal)
"A" Aker's retentive arm (always
wrought wire)
3- Combination Clasp
Circumferential clasp with wrought wire clasp
Bracing and retentive arms originate from distal rest
Guide plane must not run entire occluso-gingival
height
Used when : 1- Tooth-borne cases with poor
prognosis for posterior abutments
2- Mesial rest and infrabulge approach not possible
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Notes about the lecture :
1- The dr emphasized to read the lecture from the
book as he was talking briefly
2- The dr changed the syllabus so this lecture is not
as 2009 one he said that everything in this lec will
be easier when we take the biomechanics lecture
which is on this Sunday
3- He was talking in Arabic all the time so I tried my
best to make everything clear4- Forgive me as there are not enough pictures in the
script but I don't have the slides
Finally I would like to say hi to my lovely adorable
friends Ala2 , hashim , sawsan and Areej love u