word on forces acting restorations

1. Introduction
In order to ensure the success of the restoration placed
in the oral cavity. The physico-mechanics of the forces
acting on it has to be understood, by restoring the tooth
form, we aim at maintaining the integrity and continuity of
dental arch which is very important as far as mastication is
concerned.
Therefore, the basic aim of cavity preparation design
should be to establish the best possible shape that can cope
with the distribution of stresses in tooth structure and
restoration without failure. for this one should understand
the nature of forces acting on it and resistance to such
forces.
Both resistance and retention form is very important
as far as success of restoration is concerned.
Resistance form is defined as the architectural form
given to a tooth preparation which enables both the
restoration and the remaining tooth to resist structural failure
from occlusal loading stresses.
1

2. Building a restoration is similar to building any
mechanical structure, in that the stress patterns of the
available foundation and the contemplated structure must be
predetermined.
Accordingly, the following items should be considered.
A STRESS PATTERNS OF TEETH
Every tooth has its own stress pattern, and every
location on a tooth has special stress patterns. Recognizing
them is vital prior to designing a restoration without failure
potential.
I. STRESS BEARING & STRESS CONCENTRATION
AREAS IN ANTERIOR TEETH
a. The function between the clinical crown and the
clinical root bears shear components of stress
together with tension on the 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
2

3. occlusion massive compressive stresses will be
present in edge-to-edge occlusion, and if the
incisal angles are involved in a disclusive
mechanisms, these stresses are substantially
increased.
c. The axial angles and lingual marginal ridges will
bear concentrated shear stresses. In addition on
the loading side tensile stresses are present, and
on the non-loading side compressive stresses are
found.
d. The slopes of the cuspid will bear concentrated
stresses, especially if the cuspid is a protector for
the occlusion or part of a function during
mandibular excursions.
e. The distal surface of a cuspid exhibits a unique
stress pattern as a result of the anterior
components of force concentrating compressive
loading at the function of the anterior and
posterior segments of the dental arch and
microlateral displacement of the cuspid during
excursive movements. Both of these factors will
3

4. lead to stress concentration with resultant
abrasive activity there.
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. In addition
tensile and shear stresses are present during
protrusive mandibular movement. The incisal
ridges of upper anterior teeth will have these
same stresses during protrusive and sometimes at
the protrusive border location of the mandible.
II. STRESS BEARING AND STRESS
CONCENTRATION AREAS OF POSTERIOR
TEETH
a) Cusp tips, especially on the functional
side bear compressive stresses.
4

5. b) Marginal and crossing ridges bear
tremendous tensile and compressive stresses.
c) Axial angles bear tensile and shear
stresses on the non-functional side and
compressive and shear stresses on the functional
side.
d) The function between the clinical root and
the clinical crown 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.
III. WEAK AREAS IN THE TOOTH SHOULD BE
IDENTIFIED AND RECOGNIZED BEFORE ANY
RESTORATIVE ATTEMPT, in order to avoid
destructive loading they are:
5

6. a. Bi and trifurcation.
b. Cementum should be eliminated as a component
of a cavity wall. The junction between the
cementum and the dentin is always irregular so
the dentin surface should be smoothed flat after
cementum removal.
c. Thin dentin bridges in deep cavity preparation.
d. Subpulpal floors in RCT treated teeth. Any stress
concentation there may split the tooth
interceptally.
e. Cracks or crazing in enamel, and / or dentin both
should be treated passively in any restoration
design. They may act as shear lines leading to
further spread.
SOME APPLIED MECHANICAL PROPERTIES OF
TEETH
I. Although the following figures are averages, they
provide an idea about the principal mechanical
properties of tooth structure. It must be understood
6

7. that these figures can differ from one location on a
tooth to another, and from one tooth to another.
a. Compressive strength of enamel supported by
vital dentin is usually 36-42,000PSI.
b. Compressive strength of vital dentin is 40-
50,000PSI.
c. Modulus of resilience of enamel supported by
vital dentin is 60-80 inch-lbs / cubic inch.
d. Modulus of resilience of vital dentin is 100-140
inch-lbs /inch3.
e. Modulus of elasticity of enamel supported by
vital dentin under compression is 7,000,000 PSI.
f. Modulus of elasticity of vital dentin is
1,90,000PSI.
II. In general, when enamel loses its support of dentin,
it loses more than 85% of its strength properties.
III. Tensile strength of dentin is about 10% less than its
compressive strength.
7

8. IV. Tensile strength and compressive strength of enamel
are similar, as long as the enamel is supported by
vital dentin.
V. Shear strength of dentin is almost 60% less than its
compressive strength, and this is very critical in
restorative design.
VI. There is minimal shear strength for enamel when it
loses its dentin support.
VII. When the dentin loses it vitality, there is a drop of
almost 40-60% in its strength properties.
To best resist masticatory forces, use floors or planes
at right angles to the direction of loading to avoid shearing
stresses.
If possible walls of preparations should be parallel to
the direction of the loading forces, in order to minimize or
avoid shearing stresses.
Intracoronal and intraradicular cavity preparations can
be done in box, cone or inverted truncated cone shapes.
8

9. From the drawings, it is possible to deduce that the
inverted truncated cone shapes will have a higher resistance
to loading than the box shapes, and the box shapes will have
a higher resistance than the cone shapes. Therefore if
conditions and requirements allow, cavity preparations
should be prepared in an inverted truncated cone shape.
Definite floors, walls and surfaces with line and point
angles are essential to prevent micromovements of
restorations with concomitant shear stresses on remaining
tooth structure.
Increasing the bulk of a restorative material or leaving
sufficient bulk of tooth structure in critical areas is one of
the most practical ways of decreasing stresses per unit
volume.
Designing the outline form with minimal exposure of
the restoration surface to occlusal loading will definitely
minimize stresses and the possibility of mechanical failure in
the restoration.
A comparative evaluation of the mechanical properties
of the restorative material relative to that of the tooth
9

10. structure will dictate the preparation and restorative design
i.e. if the restorative material is stronger than the tooth
structure, the design should be such that the restorative
material will support the tooth structure and vice versa if the
restorative material is weaker than tooth structure.
Junction between different parts of the preparation
especially those acting as fulcra, should be rounded in order
to minimize stress concentration in both tooth structure and
restorations and to prevent any such sharp components from
acting as shear lines for fracture failure.
1. RETENTION FORM
Retention form is defined as that form given to the
tooth preparation, especially its detailed anatomy and
general shape which enables the restoration, that it will
accommodate, to avoid being dislodged by masticatory
loading.
Principal means of retention:
Frictional retention depends on 4 factors:
10

11. a. The surface area of contact between tooth
structure and restorative material. Greater surface area
produces a greater frictional component of retention. It
is directly proportional to the length, width and depth
of the walls and surface involved in the preparation.
b. Opposing walls or surface involved:
More opposing walls or surfaces in a tooth preparation
produce greater frictional components of retention and
consequently, a more stable restoration within the
preparation.
c. Parallelism and non-parallelism
A higher degree of parallelism between opposing walls
produces greater frictional components of retention.
Higher convergence of the walls in the intracoronal
preparation and higher divergence of walls in the
extracoronal preparation produce greater locking ability
of the tooth preparation to restorative material,
irrespective of frictional retention.
11

12. d. Proximity
Bringing the restorative material closer to the tooth
structure during insertion will substantially increase the
frictional retention.
2. ELASTIC DEFORMATION OF DENTIN
Changing position of dentinal walls and floors
microscopically by using condensation energy within the
dentin proportional limit. Can add more gripping action by
the tooth on the restorative material. This occurs when the
dentin regains its original position while the restorative
material remains rigid, thereby completely obliterating any
remaining space in the cavity preparation.
CLASS I
All Class I cavity preparations will have a mortise
shape i.e. each wall and floor is in the form of a flat plane
meeting each other at definite line and point angles. This
form is commonly applied in various mechanical structures,
so its application here is understandable.
12

13. It is advantageous to have a mortise shape preparation
in an inverted cone shape to minimize shear stresses that
tend to separate the buccal and lingual cuspal elements i.e.
to prevent the splitting of the tooth. The box shaped mortise
is less advantageous and the cone shaped is the least
advantageous in this regard. So, whenever the anatomical
and cariological factors allow the cavity preparation should
be an inverted cone shape.
When a caries cone penetrates deeply into dentin,
removing undermined and decayed tooth structures can lead
to a conical cavity preparation, mechanically, two problems
can occur if restoration is inserted into such a 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
increased tendency for tooth splitting (A).
If the occlusal loading is applied eccentrically the
restoration will have tendency to rotate laterally, for there
would be no lateral locking walls in definite angulation with
a floor. Although these lateral movements are microscopic
13

14. they occur frequently enough to encourage microleakage
around the restoration, predisposing to a recurrence of
decay. These measurements can also lead to fracture of
marginal tooth structure and even to splitting of lateral
walls.
To solve these problems, flatten the pulpal end of the
cavity preparation. (However if accomplishing this at a deep
location incurs increased risk of involving the pulp chamber,
pulp horns, or recessional lines containing remnants of pulp
tissues) make the pulpal floor at more than one level (B) one
level will be the ideal depth level (1.5mm) and the others
will be the caries cone level dictated by the pulpal extent of
the decay. The shallow level creates the flat portion of the
pulpal floor at definite angles to the surrounding walls,
adequately resisting occlusal forces and laterally locking the
restoration, without impinging on pulp tissues.
The first level should be as pronounced and
circumferentially continuous as possible. At least it should
exist at two opposing locations in the cavity preparation in
order to fulfill its objectives. This level is sometimes called
14

15. “The ledge” and it can be circumferential, interrupted or
opposing.
CLASS II
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 occluding area of both the tooth in
question and the opposing contacting cuspal elements.
For the purpose of this discussion, one can classify
these loading situations and their induced stress patterns in
the following way:
A small cusp contacts the fossa away from the restored
proximal surface, in a proximo-occlusal restoration at centric
closure.
A) As shown in mesio-distal cross
section, due to the elasticity of the dentin, especially in
young teeth, a restoration will bend at the axio-pulpal line
angle (provided the proximal part of the restoration is self
retained). This creates tensile stresses at the isthmus
15

16. portion of the restoration, shear stresses at the junction of
the main bulk of the proximal part of the restoration and
its self retained parts, and compressive stresses in the
underlying dentin.
B) A large cusp contacts the fossa
adjacent to the restored proximal surface in a proximo-
occlusal restoration at centric closure, either in the early
stages of moving out of centric or at the late stages of
moving toward it.
As the diagram shows, the large cusps will tend to
separate the proximal part of the restoration from the
occlusal part. This creates tensile stress at the isthmus
portion of the restoration even if the proximal portion is self
retained. This loading situation will deliver compressive
forces in the remaining tooth structure, apical to the
restorations.
C) Occluding cuspal elements contact
facial and lingual tooth structure surrounding a proximo-
occlusal or proximo-occluso-proximal restoration during
centric and excursion movements.
16

17. As shown in this bucco-lingual cross section
concentrated shear stresses will occur at the junction of the
surrounding tooth structure and corresponding floors, with a
tendency toward fracture failure there. This loading situation
can be unilateral or bilateral, depending on the direction of
mandibular movement, occluding surface morphology, stage
of movement, and degree of intercuspation. It is most
deleterious to tooth structure, especially on the biting side if
there is interference during lateral excursion.
D) Occluding cuspal elements contact
facial and lingual parts of the restoration, surrounded by
tooth structure during centric and excursive movements.
As shown in this bucco-lingual cross section this
arrangement will induce tensile and compressive stresses in
the restoration which will be transmitted to the surrounding
tooth structure.
E) Occluding cuspal elements contact
facial or lingual parts of the restoration, completely
replacing facial or lingual tooth structure during centric
and excursive movements.
17

18. As shown in the bucco-lingual cross section the stress
pattern will be similar to No. 2 with tensile stresses induced
at the junction of the occlusal and facial or lingual part of
the restoration in both occluding situation.
F) Occluding cuspal elements contact a
restorations marginal ridges or part of a marginal ridge
during centric and excursive movements.
As shown, in this mesio-distal cross section (assuming
the restoration is locked occlusally), there will be
concentrated tensile stresses at the junction of the marginal
ridge and the rest of the restoration.
G) Cuspal elements occlude or disclude
via the facial or lingual groove of a restoration.
Assuming, that the restoration is locked occlusally,
there will be tensile stresses at the junction of the occlusal
and facial or lingual parts of the restoration at full
intercuspation (A) and to and from that position (B)
H) Cusps and crossing ridges are part of
the restoration in centric and excursive movement.
18

19. - Both will be subjected to
compressive stresses during such positions and
movements. Besides, tensile stresses could concentrate at
their junction with the main restoration, especially during
contacting excursive movement.
I) Axial portions of the restoration
during centric occlusion and excursive movement
contacts.
Whenever these portions are in contact with opposing
occlusal surfaces, there will be induced compressive and
shear stresses whenever they are not reciprocating, the axial
surfaces will be stressed in a slight tensile and shear pattern
at their junction with the main bulk of the restoration.
J) Restoration is not in occluding
contact or is in premature contact during centric occlusion
or excursive movement of the mandible.
The first situation is not conducive to function, in so
far as the restoration will not be involved with direct loading
from the opposing occluding teeth. After a period of time
however the tooth will supraerupt, rotate or tilt, establishing
19

20. contact with the opposing cuspal elements. Usually this
newly acquired location will not be the most favourable
position for the restoration, tooth or the remainder of the
gnatho-stomatic system either mechanically or biologically.
It is safer to build the restoration to predetermined
contacting areas with opposing teeth which will lead to
predictable physiologic stress patterns in the tooth structure
and the restoration. Conversely, any portion of the
restoration occluding prematurely will tremendously
exaggerate the same types of stresses normally induced in
that area of the restoration. Besides additional shear
components of stress could be precipitated there. This could
lead to localized or generalized gnatho-stomatic disturbances
with eventual mechanical or biological features.
Needless to say, pre-existing premature contacting
areas should be eliminated before restorative treatment. This
is done primarily because cavity preparation increases the
susceptibility of remaining tooth structure to fracture failure,
besides, the restoration should be built to the predetermined
occlusal position, even if the preexisting tooth structures
were not.
20

21. Amalgam is least resistant to tensile stress and more
resistant to compressive stress. Tooth structure particularly
when interrupted by a cavity preparation, is least resistant to
shear stress. Therefore Class II cavity preparations for
amalgam restorations should be designed to resist cyclic
loading while minimizing tensile loading in the amalgam and
shear loading in the remaining tooth structure (Fig. 13).
Design features for the protection of the mechanical
integrity of the restoration.
1. ISTHMUS:
In the Isthmus, i.e. the junction between the occlusal
part of a restoration and the proximal potentially deleterious
tensile stresses occur under any type of loading.
Most mathematical, mechanical and photoelastic
analyses of these stresses reveal three things:
1. The fulcrum of binding occurs at the axio-pulpal line
angle.
2. Stresses increase closer to the surface of a restoration,
away from that fulcrum and
21

22. 3. Tensile stresses predominate at the marginal ridge area of
a Class II restoration.
Materials tend to fail, therefore, starting from the
surface, near the marginal ridge, and proceeding internally,
toward the axio-pulpal line angle.
These problems may be solved by applying common
engineering principles.
1) A theoretical solution might be to increase amalgam bulk at the
axio-pulpal line angle. Thereby placing the surface stresses away
from the fulcrum (fig). However this actually results in increased
stresses within the restorative material and a deepened cavity
preparation, dangerously lose to pulp anatomy. Therefore such a
solution, in and of itself is actually unacceptable.
2) Another solution might be to bring the axio-pulpal line angle
closer to the surface, in an effort to reduce tensile stresses
occurring near the marginal ridge. However, this too is
unacceptable in that consequent diminished bulk of amalgam
would no longer adequately resist compressive forces.
3) A combination of the two solutions i.e. increasing
amalgam bulk near the marginal ridge, while bringing
22

23. the axio-pulpal line angle away from the stress
concentration area and closer to the surface, can be
achieved simply by slanting the axial wall towards the
pulpal floor.
1. The obtuse-pulpal line angle thereby created not only
provides greater amalgam bulk in the marginal ridge
area of the restoration but also reduces tensile stresses
per unit area by bringing this critical area of the
preparation closer to the surface of the restoration.
2. If the axio-pulpal line angle is rounded, structural
projections or sharp junctions that may concentrate
stresses at the isthmus would be avoided as well as
increase the amalgam bulk at the fulcrum.
3. By slanting the axial wall, bulk is improved by
increased depth rather than increased width.
Increasing the width at the isthmus portion only
increases the surface area receiving deleterious
occluding stresses.
23

24. 4. The pulpal and gingival floors at the isthmus should
be perfectly flat in order to resist forces at the most
advantageous angulation.
5. The fifth design feature is that every part of the
preparation (occlusal, facial, lingual or proximal)
should be self retentive. If every part of the
restoration is locked in both structure-independently
from other parts, there will be minimum stresses at the
junction of one part with another i.e. the isthmi. This
can be achieved in amalgam preparation by retentive
grooves internal boxes, and undercuts.
6. One should avoid, as much as possible placing or
leaving any surface discontinues such as carved
developmental grooves, scratches etc. at these critical
areas in the restoration. These can precipitate and
accentuate stresses leading to fatigue failure.
Finally by checking occlusion to eliminate
prematurities in the restoration, immediate overloading and
failure can be avoided.
24

25. MARGINS
Frail, feather-edged margins of amalgam, which will
occur when the cavo-surface angles of preparations are
beveled, will fracture easily. Occluding forces will cause
amalgam at the bevel to bend with maximum tensile stress,
occurring as a result of elastic deformation of the tooth
structure beneath the bevel. Marginal excess of amalgam will
similarly fracture, leaving a ditch around the restoration that
will enhance recurrence of decay. So, for the margins of
these preparations, four design features should be observed:
1. Create butt joint amalgam tooth structure at the
margins.
2. Leave no frail enamel at the cavo-surface margins.
3. Remove flashes of amalgam on tooth surface adjacent
to amalgam margins.
4. The interface between amalgam and tooth structure
should not be at an occluding contact area with
opposing teeth either in centric or excursive
mandibular movements.
25

26. Cusps and axial angles:
The following are the design features for these parts of
a restoration:
a. Amalgam bulk in all three dimensions should be atleast
1-5mm.
b. Each portion of the amalgam should be completely
immobilized with retention modes.
c. Amalgam should be seated on a flat floor or table in
these areas.
d. Amalgam replacing cusps or axial angles should have a
bulky connection to the main part of the restoration
with similar design features as for the isthmus areas.
RETENTION FORM
In order to design a cavity preparation that will hold a
restorative material, it is necessary to know the possible
displacements that can happen to such a restoration, the
forces that can cause them, and the fulcrum of these
movements. There are such displacements for a Class II
proximo-occlusal restoration.
26

27. A) PROXIMAL DISPLACEMENT OF THE ENTIRE
RESTORATION
In analyzing the obliquely applied force ‘A’ into a vertical
component ‘v’ and a horizontal component ‘H’ it can be seen that ‘V’
will try to seat the restoration further into the tooth, but ‘H’ will tend to
rotate the restoration proximally around axis ‘X’ at the gingival cavo-
surface margin. To prevent such displacement self-retaining facial and
lingual grooves proximally 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 short arm
proximally. When the long arm is loaded by vertical force
‘V’, ‘H’ will seat the restoration more into the tooth. This is
due to elasticity of the dentin, especially in young teeth
wherein the pulpal floor will change location from position 1
to position 2. However, since the metallic restorations are
more rigid than the dentin, the short arm of the L will more
proximally, the fulcrum of this restoration is the axio-pulpal
line angle. In order to prevent such a displacement, proximal
27

28. self-retention in the form of facial and lingual grooves are
required.
C) LATERAL ROTATION OF THE RESTORATION
AROUND HEMISPHERICAL FLOORS (PULPAL
AND GINGIVAL)
A) 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. This
can definitely increase microleakage and initiate mechanical
and biological failure of the restoration and surrounding
tooth structure. Therefore, proper locking of the restoration
into the tooth should be exercised to minimize these hazards.
To repeat, every part of the cavity preparation should
be self retaining, if possible i.e. independent in its retention
from the rest of the cavity. This minimizes shear
concentration areas at the junctions of different parts of the
restorations with less failure to be expected as a result.
28

29. CLASS V
Class V restorations confined to one surface and not
subjected to direct loading may be thought of as free of any
mechanical problems. However, as the mandible moves in
lateral excursion, the lingual slopes of the buccal and lingual
cusps of maxillary teeth lead to the buccal slopes of the
buccal and lingual cusp of mandibular teeth. Assume that we
have a facial Class V restoration in the lower molar tooth.
As the tooth is firmly seated in bone, the tooth structure of
the crown can move from position 1 to position 2, making a
V-shape opening at the margin, together with a facial
component of force during the restoration facially.
Although this opening and the facial component of the
force are very minute and may not displace the restoration
completely, their repetition, thousands of times per day can
create marginal failure and eventually, facial protrusion of
the restoration.
The same thin can happen for a lingual restoration in
lower teeth and a facial or lingual one in upper teeth.
29

30. To minimize the effects of these displacing forces,
grooved occlusal and gingival walls are essential for any
Class V cavity preparation for amalgam, in addition to
definite surrounding walls, line and point angles.
FORCES ACTING ON CAVITY PREPARATION FOR
DIRECT TOOTH COLOURED MATERIALS
For any proximal restoration in anterior teeth there are
two possible displacing forces.
The first ‘H’ is a horizontal displacing or rotating the
restoration in a labio-proximo-lingual or linguo-proximo-
lateral direction. It has its fulcrum almost parallel to long
axis of the tooth being loaded.
The second is a vertical forces displacing or rotating
the restoration proximally and having a fulcrum at the
gingival margin of the preparation.
The mechanical picture can be summarized as follows:
1. With normal overbite and overjet during centric
closure of the mandible, mainly the horizontal forces
will be in action, these forces would try to move it
30

31. linguo-proximo-laterally (for the upper restoration)
and labio-proximo-lingually (for lower).
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. The results of this loading are rotational
forces as well as forces rotating the restoration laterally and
proximally (for upper) or lingually and proximally (for the
lower).
2. If anterior teeth meet in edge to edge fashion i.e. there
will be vertical displacing forces with very limited
horizontal components.
3. If the upper and lower anterior teeth meeth such that
the lowers are labial to the uppers in centric occlusion
(Angle’s Class III), the horizontal loading will tend to
rotate or displace restorations labio-proximo-lingually
(for uppers) and linguo-proximo-labially (lowers).
4. In occlusions, with deep anterior overbite and normal
or no overjet, the horizontal type of loading will be
31

32. greatly exaggerated. The vertical displacement
although present will be minimal in comparison.
5. In occlusions with anterior open bite or severe overjet
or any other condition that creates a no contact
situation between upper and lower anterior teeth during
centric occlusion and excursive movements of the
mandible, proximal restorations will not be loaded
directly either vertically or horizontally.
It should be understood that none of these loading
forces work separately. They work together and
simultaneously. It should be mentioned here that a
restoration replacing part or all of the incisal ridges of an
anterior tooth will have the same pattern of loading as
mentioned in (1)-(6) but with increased intensity. Loss of
incisal angle of a tooth i.e. conversion from a Class III to a
Class IV represents a major complication in the mechanical
problems of anterior tooth restoration. This loss will lead to
definite direct loading of the restoration, definite vertical
loading with its sequelae, and the placement of margins on
the incisal ridge. This further exposes the restoration to the
maximal loading possible in anterior teeth.
32

33. 6. In cases when the proximal restoration of an anterior
tooth is a part of a mutually protective occlusion i.e.
an incisor and the adjacent cuspid are involved in an
anterior lateral disocclusion mechanism, the teeth and
the restoration will be part of that disocclusion
mechanism with excessive horizontal and vertical
loading forces.
Ideally, a restoration made of tooth colored materials
should not be loaded directly i.e. there should be intervening
tooth structure between the occluding tooth and the
restoration. This situation can only be achieved by force
intact walls surrounding the restoration, unfortunately, this
is usually not the case that is why the clinical performance
of tooth colored materials diffuses from one situation to
another, sometimes dramatically.
CAST PREPARATIONS
Cast restorations are usually used for compound or
complex tooth involvement. The possible loading and
displacing forces, their fulcra, and their effects on
restorations, together with their effect on remaining tooth
structure, have been fully described in the discussions of the
33

34. different cavity preparations for amalgam and in the general
principles of preparation design. The formability of casting
materials enables us to use myriad retention and resistance
means that are impossible to use with any other materials.
INLAY RESTORATIONS
Here are 3 illustrations representing the different
design of a proximal box cavity
(A) (B) (C)
Fig.(A) shows walls parallel to each other where
rotational force is applied by means of a bar in a counter
clockwise direction. The tendency for ‘x’ to use occlusally
on area xy is resisted by dentin lying withing the area xyz.
In Fig. (B) same rotational force finds no resistance to
point ‘x’ rotating might out of the cavity because of too
great divergence of buccal and lingual walls.
In Fig. (C) shows gingivo-occlusal divergence of 5°
from vertical plane where the rotational force finds
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35. resistance to point x by the bulk of dentin contained in area
xyz.
Therefore, while parallelism of the walls offers
maximum rotational resistance from clinical standpoint, a
slight divergence of 2° to 5° from parallelism will furnish
necessary resistance to bucco-lingual torque displacement.
This figure shows a proximal view of a MOD inlay not
quite seated in the cavity.
The width of inlay is ‘N’ at about its vertical center.
Contact is assumed to have been made between the
inlay and the walls of the cavity. After the contact, the inlay
is further forced downwards – an amount ‘dh’.
The walls of the cavity make an angle θ with the
vertical wall of the restoration. Assuming that the tooth
structure is prevented from deformation, the total shortening
per unit width of gold is:
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36. Σg = dh tanθ
W
And the unit stress becomes
S = Eg dh tanθ
W
Eg = modulus of elasticity of gold.
The unit stress is not perpendicular to the cavity wall
but is parallel to ‘W’ and may be resolved into two
components.
F Cosθ - perpendicular to cavity wall.
And F Sinθ - parallel to the wall.
Assuming the coefficient of friction is ‘µ’ between
gold of the inlay and tooth structure than µF Cosθ becomes
the frictional force between gold and tooth structure which
prevents the movement of the two with respect to each other.
The component FSinθ parallel to the cavity wall tends to
push the inlay back to the cavity wall.
Thus the total force of frictional retention tending to
hold the inlay in its cavity is:
P = µ FCosθ - F Sinθ
36

37. This shows that as the value of θ increases greatly, the
inlay will bounce out of the cavity and this angle is known
as the “Critical Angle” θc.
In a proximo-occlusal restoration one of the proximal
wall is absent and opposite retentive stresses are developed
only on the buccal and the lingual surfaces whereas the
gripping power has been lost in proximal direction, due to
which there is a force tending to push the restoration out
through the absent wall. This displacing force can be
counteracted by the retaining stresses present in the buccal
and lingual walls and can be supplemented clinically by
placing a gingival groove in the gingival wall. The occlusal
dovetail lock also resists lateral displacement of the key by
the additional tensile stresses developed within the lock.
With the help of the diagram it is seen that by
increasing the angulation to 35°-45° of the gingival bevel the
resistance to displacement is offered by that portion of
dentin which comes in the path of the arc formed by radius
FE and FF with P as the rotation center. Keeping the gingival
bevel at 15° won’t serve any purpose as the filling may be
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38. rotated out of the cavity because no resistance to
displacement is offered by either the axial wall CG or DG.
Where the buccal and the lingual walls instead of
flaring from the axial line angle to the cavo-surface margin
in a continuous plane are changed into two narrow-and two
smaller diverging planes even with such a modification of
the buccal and lingual proximal walls, it is possible to retain
the retentive stresses of a preparation since the
supplementary diverging planes are mostly line angles
leaving the balance of the wall in the elastic dentin.
If the proximal walls diverge excessively occlusally
from the gingival wall. The reacting stresses in the dentin
since all forces react as displacing stresses. Hence, such a
divergence is not acceptable and every effort should be made
to approach parallelism not exceeding 2°-5° gingivo-
occlusally.
BEVELLING
Bevelling plays a very important role in reducing the
stresses on the remaining tooth structure, thus maintaining
the integrity of both the tooth and the restoration.
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39. The lower surface bevel helps to seal and protect the
margin resulting in a strong enamel margin with an 140°-
150° angulation. This leaves 30°-40° marginal metal on the
inlay. The marginal gold alloy is too thin and weak if its
angle is less than 30°, conversely the metal at the margins is
too bulky and difficult to burnish, if its angle is greater than
40°.
In small teeth such as premolar, the joining of mesial
and distal cavity across the occlusal surface results in a
considerable weakening of the tooth and an occlusal stress is
liable to produce a vertical fracture.
When it is thought that there is risk of this occurring
the occlusal bevel should be increased so as to extend
beyond the summit of the cusps. Occlusal stresses will then
be taken entirely by the inlay and transmitted to the flat
floor, splitting strains thus being much reduced.
CONCLUSION
Tooth is a engineering marvel, which can withstand
forces because of resiliency of dentin.
39

40. Increased amount of dentin increases the retention of a
restoration and better resistance to forces.
An intact tooth can best withstand forces but when lost
due to caries, has to be replaced by a restorative material.
There are various forces that can act on these restorations
hence based on sound principles these restorations should be
placed so as to prevent their dislodgement and increase the
resistance of tooth as well as restoration to forces.
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41. FORCES ACTING ON RESTORATIONS
CONTENTS
 Introduction
 Retention and Resistance forms in general
 Forces acting on Class I, Class II, Class V
 Direct tooth colored restorations
 Cast restorations i.e. Inlay
 Conclusion
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