6. forces of occlusion

Forces acting on Class II restorations

Classically, occlusal forces are evaluated based on local factors such as

periodontal health, surface area of periodontal support, clinical crown height, and the

contact angle to the opposing dentition. Also important is the number of posterior

tooth- to- tooth stops, which distribute the occlusal load. New insights have come

from computer modeling and orthodontic studies that indicate that other variables can

also be significant. Musculoskeletal factors can influence the forces placed on the

dentition. Proffitt has shown that an individual with a high Frankfurt mandibular

plane angle will generate half of the first molar occlusal forces as an individual with a

low angle. Additionally, Hannam and Wood have shown the first molar force to be

strongly influenced by the cross- sectional area of the masseter muscle. 21 Every tooth

has its own stress pattern, and every location on a tooth has special stress patterns.

These will be considered in detail.

STRESS BEARING AND STRESS CONCENTRATION AREAS IN ANTERIOR

TEETH26:

a) The junction between the clinical crown and clinical root bears shear components

of stress, together with tension on loading side and compression at the non-

loading side, during excursive mandibular movements.

b) The incisal angles, especially if they are square, are subject to tensile and shear

stresses in normal occlusion. Massive compressive stresses will be present in edge

–to-edge occlusion, and if the incisal angles are involved in a disclusive

mechanism, these stresses are substantially increased.

c) The axial angles and lingual marginal ridges will bear concentrated shear stresses.

d) The slopes of the cuspid will bear concentrated stresses, especially if the cuspid is

a protector for the occlusion or part of a group function during mandibular

excursions.

e) The distal surface of a cuspid displays a unique stress pattern as a result of the

anterior components of force concentrating compressive loading at the junction of

the anterior and posterior segments of the dental arch and micro lateral

displacement of the cuspid during excursive movements.

f) The lingual concavity in upper anterior teeth bears substantial compressive

stresses during centric occlusion, in addition to tensile and shear stresses during

protrusive mandibular movements.


g) The incisal edges of lower anterior teeth are subjected to compressive stresses.

Shear stresses are present during protrusive mandibular movements.

h) Resistance to stress fractures will be maximum at gingival end and decreases

incisally.

i) Cervical portions of anterior teeth with Class 5 lesions will have stress pattern

similar to posterior teeth and the intensity of these stresses increases with deep

bites.

j) Loss of an axial angle, incisal angle, or tooth structure at the neck of the tooth will

dramatically decrease the tooth’s ability to resist loading.

STRESS BEARING AND STRESS CONCENTRATION AREAS OF POSTERIOR

TEETH26:

a) Cusp tips, especially on the functional side, bear compressive stresses.

b) Marginal and crossing ridges bear tremendous tensile and compressive stresses.

c) Axial angle bear tensile and shear stresses on the non-functional side and

compressive and shear stresses on the functional side.

d) The junction between the clinical crown and root during function (especially

lateral excursion) bears tremendous shear stresses, in addition to compression on

the occluding contacting side and tension on the non-contacting side.

e) Any occlusal, facial, or lingual concavity will exhibit compressive stress

concentration, especially if it has an opposing cuspal element in static or

functional occlusal contact with it.

FORCES EXERTED DURING MASTICATION AND THEIR RESOLUTION

Various types of forces are exerted on the teeth during movement of the

mandible and during mastication. Since the tooth surfaces are curved or at an inclined

plane, forces are not only vertical but other types of forces may also be exerted on

these surfaces.

When a force acts perpendicular to a fixed horizontal surface, the resolving

force reacts perpendicular to the surface with an equal and opposite force. In curved

surfaces the resulting forces might not be exerted along the long axis of the tooth. The

reacting force of the incline is neither equal to the applied force in magnitude nor

opposite in direction. There is, therefore, a tendency of the cusp to slide down the


incline unless there is another force to prevent it. This is shown in the diagram where

AB is the contact on the inclined plane, M represents the force of mastication applied

on the incline, N is the reaction of the incline and H is the horizontal component of

the resolving force. The angle a is the angle made with the horizontal AC by the

tangent AB of the cuspal contact. It can be seen that as the inclination of the plane

increases, since M remains constant and N perpendicular, H increases rapidly and

may have unlimited magnitude.46

Frequently points of contact on two or more inclined surfaces with slopes

facing each other are involved, as in the case of a cusp of a tooth contacting the buccal

and lingual cusps of the opposing tooth or the buccal and lingual cusps and the

marginal ridges, the force of the bite is divided into two points and the opposing

horizontal forces are half as great as where there is only one point of contact.


It is this wedging force which is of concern in the retention of the proximo –

occlusal restorations since if one point of contact is on the tooth and one on the

restoration, there is a tendency to wedge them apart. 46

FORCES ACTING ON THE TOOTH

A. IN CENTRIC OCCLUSION

In this case the teeth are under a pure closing effect and only axial forces are

applied to the tooth.

R ab is the resultant of forces a and b. R ab and c are the two adjacent sides of

the parallelogram passing through a given point as shown in the figure. The resultant

force is represented by the diagonal line passing through the same point i.e. V abc.

Hc is the horizontal component of force c. H ab is the horizontal component of the

force a and b. H c should be equal to H ab to meet the condition of equilibrium. V abc

is then the only force acting on the tooth as a whole and is equal to the sum of the

vertical component of all the applied forces.

B. DURING CHEWING

When the mandible moves from a lateral to centric occlusion, the resultant of

forces acting is not vertical but inclined vertically.


When tough food is compressed or all cusps are in intimate contact at the

three points, the forces a and b are decreased and c is increased with the resultant

changes in horizontal and vertical components. Since during chewing, H is greater

than H ab , the net resultant force is H abc. .so the net horizontal component is along the

direction of c. By using triangle of vector addition, the resultant is represented by R

abc. The resultant R abc. is a thrust inclined palatally on the maxillary teeth and buccally

on the mandibular teeth, whose horizontal component is H abc..

MECHANICAL FUNCTIONS OF THE MARGINAL RIDGES

The most severe loss of strength of a tooth is caused by loss of marginal

ridges. 38

1. NORMAL MARGINAL RIDGE

Forces 1 and 2 act on marginal ridges of teeth A and B respectively the

horizontal component of 1, H 1 and that of 2, H 2 , counteract each other. The vertical

component V 1 and V 2 are resolved normally by the underlying tissues.46

2. NO MARGINAL RIDGE

In this case H2 is missing in tooth B because force 2 is mainly directed toward

tooth A. The horizontal component H2 will drift the tooth A apart and the vertical

components V1 and V2 of both forces 1 and 2 will help the tooth impact vertically.

The vertical force V2 will be more than required and there may be slight tilting of the

tooth B. This will lead to further deteriorate the resolution of forces and lead to further

food impaction.


3. MARGINAL RIDGE WITH WIDER OCCLUSAL EMBRASURE

In spite of putting optimal pressure on the marginal ridges of tooth A and B,

the forces 1 and 2 act on adjacent teeth causing drifting of both the teeth. The vertical

component of forces will wedge the food in-between the two teeth. Similar effect is

seen when one marginal ridge is higher than the other.

4. NO OCCLUSAL EMBRASURE

In totality the vertical component of forces 1 and 2 will be more concentrated

than horizontal components. Though there may not be any vertical compaction of

food, the continuous impact of higher concentration of vertical component of forces

may lead to changes in alveolar bone after sometime.


VERTICAL LOADS AND DISTRIBUTION OF STRESSES

As the load is applied over the teeth, stresses are distributed

Parallel to the long axis

Perpendicular to the long axis

The force or load is applied at different areas at a time and stress distribution

depends upon various factors:

If the cross- section of that area is constant, stress distribution is practically

uniform.

If there is variation in cross-section (such areas are termed as prisms), stress

varies from point to point, being inversely proportional to the area.

If change of cross-section area is abrupt, greater concentration of stress occurs

at that point.

In vertical loading, there will be shearing stresses in prism in any plane. This

shearing stress increases to a maximum at 45 and then decreases to zero at 90.

Therefore, materials that are weaker in shear than in compression or tension rupture in

planes at 45 to the axis.

The modulus of elasticity of the material is thus an important property that

should be taken care of. If the modulus of elasticity varies between the tooth and the

restorative material, with the vertical force exerting on both, the stiffer material will

be highly stressed.46

When force is applied perpendicular to the prism axis the resultant resolution

is known as a beam. Beams can be supported from both ends (simple beam) e.g. MOD

preparations, or from one end (cantilever beam) e.g. MO/DO preparations. The


retention of the restoration depends upon these beams, although the strength and the

deflection of the material also play a part.

Moment of force = force perpendicular distance

The bending moment is at the axiopulpal line angle, which tends to rotate the

restoration out of the cavity. Gingival retention with a moment equal to F L is

required to counteract this moment. The total retentive force (R) is equal to F L / l

where l is the depth of the axial wall. If we take the depth of the gingival wall (d) into

account, then R and d will be in the same direction, so their moment of force is zero.

Therefore, the depth of the gingival wall does not take part in retention.

In MOD preparation, the force (F) is divided equally on both the sides. The

mesio- distal distance (L) is also divided into two. The moment of force at the mid

point is

F/2 F/2 = FL/4

If this moment of force is divided into two (because it is usually acting on both

the ends) then the moment of force is FL/ 8.

Since the beam forms a concave downward curvature between the load and the

fixed end, therefore, by sign convention, this end moment is taken as negative.

By equation R l = FL /8

So R = FL / 8l

The negative sign is used only in vector form and in magnitude only positive

sign is used.


Similarly, as in MO/ DO preparation, if we take depth of gingival wall (d) into

account, then R and d will be in the same direction, so their moment of force remains

zero.

It is presumed in MOD preparations that the length of the axial wall (l) is kept

equal on both the ends. If there is marked discrepancy between the two ends, the end

result may not be the same as is described earlier.46

FORCES ACTING ON RESTORATIONS

ANTERIOR TEETH:

For any proximal restorations in anterior teeth (class 3, and class 4 restorations),

there are two possible displacing forces:

a) Horizontal forces - displace/rotate restoration in a labio-proximo-lingual or

linguo-proximo-labial direction. The fulcrum is almost parallel to the long axis of

the tooth being loaded.

b) Vertical forces - displace/ rotate the restoration proximally (sometimes facially

and lingually).The fulcrum is at the gingival margin of the preparation. The loading

arrangement is similar to occluso-proximal restorations in posterior restorations.


The mechanical picture can be summarized as follows:

In anterior teeth with normal overbite and overjet during centric closure of the

mandibular (from CR to CO), mainly horizontal forces will be in action. If loading

the proximal restoration directly, these would try to move it linguo-proximo-

labially (for the upper restoration) and labio-proximo-lingually (for the lower

one).The magnitude of horizontal forces at this stage of mandibular movement is

not very substantial, and the vertical one is almost nil.

In protrusive and lateral protrusive movements of the mandible, directly loaded

proximal restorations in anterior teeth will be subjected to substantial horizontal as

well as vertical displacing forces, especially in restorations replacing the incisal

angle. Restorations in upper teeth will be rotated labially and proximally and those

in lower teeth will be rotated lingually and proximally.

If anterior teeth meet in an edge-to-edge fashion at CO, loading of the proximal

restoration, involving incisal angles(class4)will be similar to any class 2

restoration, i.e. vertical displacing forces with very limited horizontal

components. This loading will continue during all centric closures and excursive

movements of the mandible. However, if the incisal angle is intact (class 3), these

displacing forces will be minimal.

In Angles’ class 3, the loading conditions will be similar to cases with normal

overjet and overbite except that horizontal loading will tend to displace/rotate

restorations labio-proximo- lingually(for upper teeth) and linguo-proximo-labially

(for lowers).

In occlusions with anterior overbite or severe overjet, or any other condition that

created a no-contact situation between upper and lower anterior teeth during CO

and excursive movements of the mandible, the proximal restorations will not be

loaded directly either vertically or horizontally. However there may be momentary

contact during excursions from one location to the other in some cases and this

should be noted.

In cases when the proximal restoration of anterior tooth is a part of a mutually

protective occlusion, i.e. an incisor and the adjacent cuspid are involved in an

anterior lateral disclusion mechanism, the teeth and the restoration will be part

of that disclusion mechanism with excessive horizontal and vertical loading

forces. These forces work together and simultaneously. However, they may


differ in magnitude at different stages of mandibular movements. Loss of

incisal angle of tooth will lead to definite direct loading of the restorations.26

As incisal retention cannot be made in may cases due to thin labio- lingual size of

the tooth, a lingual lock may be placed to prevent rotation. Such a lock should be

as close to the incisal edge as possible and still be in dentin to reduce stress.

POSTERIOR TEETH

Class 1 restorations

Among the forces “shearing forces” tend to separate the buccal and cuspal

elements i.e. try to split the tooth. In order to overcome this, it is advantageous to

have a mortise- shaped preparation in an inverted cone shape.

When caries cone penetrates deeply into dentin, removing undermined and

decayed tooth structures can lead to a conical cavity preparation. If the occlusal

loading is applied centrically, the restoration may act as a wedge, concentrating

forces at the pulpal floor and leading to dentin bridge cracking, and an increased

tendency for tooth splitting. The other thing that could happen is that the restoration

will have the tendency to rotate laterally, for there would be no lateral locking walls.

This could lead to microleakage or fracture of marginal tooth structure. 26 The

problems in cast restorations are also related to movement and rotation of the

casting if the floor is not flat. Additionally, since the depth of the cavity is less and

walls are diverging occlusally, the chances of rotation are higher than for direct

restorations. Such rotational forces are counteracted, to some extent, by adhesive

materials such as composites and glass ionomers.

Class 2 restorations

There are four displacements for a class 2 proximo-occlusal restoration:

a) Proximal displacement of the entire restoration

In analyzing the obliquely applied force ”A” in to a vertical component “V” and

a horizontal component ”H”, it can be seen that “V” will try to seat the restoration

further in to the tooth, but “H” will tend to rotate the restoration proximally around

axis ‘X” at the gingival cavosurface margin. To prevent such displacements, self-

retaining facial and lingual grooves are necessary, in addition to an occlusal

dovetail.


b) Proximal displacement of the proximal portion

If one were to consider the restoration as being L-shaped, with the long arm of

the L occlusally and the short arm proximally, when the long arm is loaded by

vertical force “V”, it will seat the restoration more into the tooth. However, the short

arm of the L will move proximally. The fulcrum of this rotation is the axio-pulpal

line angle. In order to prevent such a displacement, proximal self-retention is the

form of facial, lingual and/or gingival grooves are required provided there is

sufficient dentin bulk to accommodate them.

Forces acting on Class 2 Restorations


c) Lateral rotation of the restoration around hemispherical floors (pulpal and

gingival)

As in class1 cavity this can be prevented by definite point and line angles.

d) Occlusal displacement

This can be prevented by directing occlusal loading to seat the restoration and

by inverted truncated cone shaping of key parts of the preparation.

Although the magnitude of these four displacements is minute, they are

repeated thousands of times per day leading to microleakage and mechanical and

biological failure. To prevent this every part of the cavity should be self-retaining, if

possible.26

to the tangent of the planes. The horizontal component (H) acts approximately at the

centre of the tooth. The vertical component (V) is constant. The deflection is mainly

by the horizontal component which depends upon the height of the axial wall (L) and

the depth of the occlusal (d1) and cervical walls (d2).

Since bending moments depend upon modulus of elasticity, a young tooth may

deflect more because of its less modulus of elasticity.

Threaded posts can produce high stress concentrations during insertion and

loading, but they have been shown to distribute stress evenly if the posts are backed

off a half- turn and when the head contact area is of sufficient size.

a. The cement layer results in a more even stress distribution to the root with less

concentrations. 41

6. forces of occlusion

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