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1. FORCES ACTING ON CAST RESTORATION
INTRODUCTION
The word biomechanics clearly denotes the application of mechanics to
biologic systems. It is often compared with engineering principles. Stinners
says that the engineers can definitely predict the amount of stresses to be
induced where as dentist cant predict it. Because we deal with human tissues
and do not always follow the rest of the dynamic nature of the actual biting
(forces) stresses during mastication, it is difficult to measure Skinner’s
mentions about the forces.
- Average of 77kg (170.16).
- Molars 41.91kg
- Premolar 23.46kg
- Cuspid 14.34 kg
- Incisors 9.25kg
Never the less the design of the tooth is somewhat of an engineering
marvel in that the tooth is generally able to absorb impact energies. The
modulus of resilience of dentin is greater than that of enamel and thus is better
able to absorb impact energy. Enamel is brittle with low proportional limit, low
modulus of resilience, but when it is supported by dentin, it does withstand the
forces, loss of support from and decreases its strength by 85%.
A healthy uncut tooth is the strongest stress bearing structure, when this
health is hampered by caries or trauma, the restorative dentistry comes in
picture. Here the interaction of orofacial complex with forces should be clear.

2. Let us consider about a cast restoration e.g., inlay, it exhibits mechanical
problems of resilience and retention.
When the forces are applied at right angles to the flat bur of a filling, the
filling by rolling it out of the cavity walls prevent the displacement. [The
degree of resistance to displacement offered by each type of preparation.
In case of a Class II elimination of one of the vertical 2nd
walls
considerably reduces the resistance to displacement. Even though the gi
extension is made the retentive qualities are not improved. Dropping the pulpal
floor at some also slightly increases the resist to displacement. Also a groove
placed in the gingival wall helps for same purpose Mcmath suggests that the
gingival groove should not be wider than half of the width of the gingival wall.
The depth of the groove end. The gingival groove prevents the lateral spreading
of the casting.
When considering the axioproximal walls. The compressive forces resulting
from vertical pressure have an important bearing on two retention of the inlay
the relationship of the vertical axial wall with buccal and lingual walls is
critical. The question arises whether they end flare axioproximally or parallel to
each other.
Let use see the degree of resistance to displacement offered by each type of
preparation.
In a preparation with 2nd
walls when force F is applied, there is a tendency of
point X to rise occlusally on arc XY but this is resisted by the dentin lying
within the area XYZ.
In case of slight divergence slight resistance is found for displacement. Little
available dentin in the area XYZ prevents rolling.

3. But in divergent walls, there is absolutely no dentin to prevent this
displacement hence the casting rolls out of the cavity.
Therefore, parallelism of the walls offers maxi rotational resistance. Taper of 2-
5° is also acceptable which is required for convinance.
Same principles can be applied to the buccal and lingual walls of the occlusal
dovedail.
Role of dentin
A well seated inlay contacts the walls and the base is in contact with the
dentinal floor when the vertical forces are exerted there is a tendency to force
through the custom because of the taper shape it gives a wedging effect. This
tends to distend the lateral walls outward if the lateral walls are also placed in
dentin like the floor, this elastic material exerts opposing stresses and grips the
casting making the dislodgement difficult.
Let us see the stresses and forces. The dentinal walls resist the tensile forces of
displacement. Contact is assumed to have been made between the inlay and the
walls of the cavity. If the inlay is further forced downward, it moves little
distance, if the tooth structure is prevented from deformal it in the metal which
deforms. But there are some opposing forces which are equal in magnitude i.e.
the force tending the stop the inlay being pushed further into the cavity will
exactly equal the force of friction tending to hold it there.
When the force is removed, the gold of the inlay expands maintaining the
contact with the tooth structure where as the tooth structure compresses,
inducing stresses within it; while the gold expands an equal amount receiving
stresses within itself it proceeds until a point has been reached where the
induced unit compressive stress in the tooth structure is exactly equal to the

4. residual compressive unit stress in the gold of the inaly. If these two stresses
are not equal movement of the differential area would continue.
Resistance to horizontal displacing forces
Application of a vertical force to the inclined plane will dislodge a filling
horizontal even in a cavity with flat pulpal and gingival walls, the filling is
rotated occlusoproximally with the rotation point being the gingival marginal
wall. These forces are always effective marginal wall. These forces are always
effective unless counteracted by an opposing movement. The counteracting
movements are in 4 ways:
1) Occlusal dovetail – properly prepared occlusal lock in between two
strong cusps is the strongest means of resisting the displacement of the
inlay.
2) Gingival wall gingival groove prevents the lateral displacement of the
inlay because of the inherent weakness of this groove, the chances of #
of gingival wall are more. Hence the gingival wall is prepared in two
planes, cavosurface and axial when the horizontal displacing force is
applied to the occlusal inclined plane the lateral displacement of the
filling will be resisted by the angular wedge portion of the inlay; which
extends into the acute angle formed by the axiogingival walls; until the
inlay is raised vertically to clear the ridge of the gingival wall. Simple
inward and downward slope of the gingival wall is not enough maxillary
angulation of 45° i.e. the ideal depth of the inner bevel resists the
displacement.
3) Pulpal wall offers no resistance to horizontal displacement other than
friction pulpal wall prepared in two planes i.e. with inclined planes helps

5. in preventing the lateral displacement lowering the grooves or pinhole
serve the same purpose.
4) Properly contoured and placed contact areas definitely prevents this
rotational movement of the proximal inlay.
During centric and excursive movements of the mandible, both the restoration
and tooth structure are periodically loaded both separately and jointly. This
brings about different stress patterns depending upon the actual morphology of
the occlusal area of tooth and the contacting tooth elements.
Let us classify the loading situation and their induced stress patterns in the
following way:
A cusp contacting the fossa away from the restored proximal surface in
proximoocclusal reaction at a centric closure.
Because of elasticity of dentin the restoration will bend at the axiopulpal line
angle which creates tensile stresses at the isthmus portion and compressive
stresses in the underlying dentin.
A large cusp contacting the fossa adjacent to the restored proximal surface in a
centric closure. This large cusp will tend to separate the proximal part of the
restoration from the occlusal part this creates tensile stresses at the isthmus
portion and compressive forces in the tooth structure apical to the restoration.
Occluding cuspal elements contact facial and lingual tooth structure
surrounding a proximoocclusal restoration during all movements, there will be
a conclusion of the occluding cuspal elements contact facial or lingual parts of
the restoration completely replacing the facial or lingual tooth structures during
centric and excursive movements.

6. Occluding cusp contacting a restoration MRs, there will be concentrated tensile
stresses at the junction of MR and the restoration of the rest.
Similarly there will be conclusion of tensile stresses underneath the
corresponding areas if the cusp occludes the grooves or crossing ridges; axial
protions.
Plus the remaining tooth structure does show some stress pattern.
It is necessary to know the possible displacements that can happen to the
restoration the forces that can cause them and the fulcrum of these movements.
There are four such displacements for a Class II restoration.
1) Proximal displacement of the entire restoration. An obliquely applied
force ‘A’ develops a compact ‘V’ in a vertical direction and a
component ‘H’ in horizontal direction ‘V’ will try to seal the restoration
where as ‘H’ will try to rotate it proximal around axis ‘X’ at the gingival
walls.
2) Proximal displacement of proximal portion. If a restoration is considered
to be L shaped with a long arm of L occlusally and short arm proximally
when the long arm is loaded by vertical force ‘V’ it will seat the more
into the tooth. Because of the elasticity of d, it changes its position from
1 and 2. But as the metallic restorations are more rigid than dentin, the
short arm of L moves proximally and the fulcrum of the rotation is the
axiopulpal line angle to prevent these displacements facial gingival
grooves are given. But again there might be connection of stresses along
with these grooves.

7. 3) Lateral rotation of the restoration around hemispherical floors (pulpal
and gingiva). These floors instead of hemispherical they should be made
inverted tuncated cone shaped.
4) Occlusal displacement An inverted tuncated cone shape helps to seat the
restoration. Although magnitude of these four displacements is minute,
it has been repeated thousands of times per day. This can increased
microleakage .
Also the axiopulpal line angle is rounded (avoids stress) connection improves
visibility for the facial and lingual gingiva axial corners and increase mate
outk).
Flat pulpal and gingival floors at the isthmus should be perfectly flat resist the
forces at the most advantageous angulation.
All the parts of restoration should have individual retentive modes. E.g.,
dovetail fox O, grooves for proximal.
There should not be any occlusal discontinuity. Material strength with low
modulus of elasticity deforms if excess stress restoration where cast restoration
behaves exactly transmitting forces to the both Ans might lead to tooth.
Margins – No unsupported enamel (enamel if not supported by denticles its
strength by 85% flashes are advocated only in mate with high edge strength.
So one has to look out for the design features for the protection of the
mechanical integrity of the restoration and also of the remaining tooth
structure, in short resistance and retention form.
Design features for the protection of the mechanical integrity of the restoration.

8. 1. Isthmus in between O part and proximal or facial or lingual parts.
Mathematical, mechanical and photoelastic analysis of stresses at this
particular part reveal four things: a) the fulcrum of bending occurs at the
axiopulpal line angles, b) stresses are more to the surface of the
restoration i.e. away from that fulcrum, c) tensile stresses predominate at
the marginal ridge areas of a Class II restoration.
Materials tend to fail, starting from surface near MR and proceeding internally,
towards axiopulpal line angle.
To solve this problems 2 solutions tried i.e. i) by increasing mate bulk at the
axiopulpal line angle and placing the surface stresses away from the fulcrum
but this needs undue deepen of the cavity and also increased stresses in the
material.
ii) by origing axiopulpal line angle closer to the surface to decreases tensile
stresses near MR, but tooth in of a material lacks resistant to O. forces.
Ultimately combining these solutions i.e. the mate bulk can be increased and
bringing the axiopulpal line angle closer to the surface by just slanting the axial
wall towards pulpal floor.
Design features for the protection of physiomechanical integrity of the tooth
structure.
1. Isthmus should be as narrow as possible to minimize the tensile stresses
in the tooth structure.
2. Occlusal surface preserving sound tooth structure O dovetail. Box or
deepening of the O surface away from the lesion.

9. 3. Proximal surface prepared in a stepped manner. Two planed gingival
floor. The inner angulated dentinal plane (45° ideally).

10. 3. Proximal surface prepared in a stepped manner. Two planed gingival
floor. The inner angulated dentinal plane (45° ideally).