FORCES ACTING ON DENTAL RESTORATIONS

FORCES ACTING ON RESTORATIONS
CONTENTS
 INTRODUCTION
 FORCE
 FORCE ON DENTAL STRUCTURE
 STRESS
 TYPES OF STRESS
 MECHANICAL PROPERTIES OF MATERIAL
 BIOMECHANICS FOR RESTORATIVE DENTISTRY
 STRESS ANALYSIS AND DESIGN OF DENTAL STRUCTURES
a) FINITE – ELEMENT STRESS ANALYSIS
b) PHOTOELASTICITY
 STRESS IN THE PERIODONTAL MEMBRANE
 STRESS PATTERNS OF TEETH
 ON ANTERIOR TEETH
 ON POSTERIOR TEETH
 OCCLUSAL CONSIDERATIONS IN RESTORING TEETH
 FORCES EXERTED DURING OCCLUSION / MASTICATION AND
THEIR RESOLUTION
 FORCES ACTING ON AMALGAM RESTORATIONS
 CLASS I
 CLASS II
 FORCES ACTING ON INLAY RESTORATION
 FORCES ACTING ON COMPOSITE RESTORATION
 FORCES ACTING ON POSTS
 FORCES ACTING ON A CAST METAL AND PORCELAIN
RESTORATIONS

FORCES ACTING ON RESTORATIONS
INTRODUCTION:
Design of any structure requires a means to predict the stress that will
develop in the structure under the anticipated applied loads. In many respects
the design of structures for the oral environment is among the most demanding
because of the complexity of the functional and parafunctional loads that must
be accommodated and because of esthetic and space limitations. In spite of
these special conditions however all dental tissues and structures follow the
same laws of physics as any other material or structure.
By necessity these studies involve the application of physics and
engineering to the oral cavity and its surrounding structures. All structural
analysis and design require knowledge of the forces that will be applied and
the mechanical properties of the materials that must withstand these forces.
Since most restorative materials must withstand forces in service either
during mastication or fabrication. Those mechanical properties are
important, quantities of force, stress, strain, strength hardness, and others
can help identify the properties of a material.
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FORCE
The general concept of force is gained through the muscular action of
pushing or pulling on an object. When there is a tendency to change the
position of rest as the motion of a mass, it is said that a force is applied.
 A force always has a direction and the direction is often characteristic of the
type of force.
 If the body to which the force is applied remains at rest, the force causes the
body to deform.
 Units of force are the pound or the kilogram or Newton.
FORCES ON DENTAL STRUCTURES :
One of the most important applications of physics in dentistry is in the
study of forces applied to teeth and dental restorations. There are numerous
reports in the dental literature that describe the measurement of biting forces on
teeth. The maximum forces reported have ranged form 200 to 2440 N (45 to
550 lb).
Numerous instruments have been used to make this measurements,
including strain gauges and telemetric devices small enough to be incorporated
into dental restorations.
NORMAL BITING FORCES :
Experiments conducted on adults have shown that the biting force
decreases form the molar region to the incisors. Studies have revealed that four
4

patients developed biting forces on the first and second molars that varied form
390 to 800 N (88 to 198 lb), with the average being 565 N (127 lb). The
average force on the bicuspids, cupids and incisors was 288, 208 and 155 N
(65, 47 and 35 lb) respectively.
In a similar investigations of the biting forces in children, 783 boys and
girls were studied. Children form 6 to 17 years of age were included, and it
was observed that there was an increase in force form 235 to 494 N (53 to 111
lb) as age increased, with the average yearly increase being in the order of 22.2
N (5 lb).
The average biting forces in persons with normal and modified
occlusion were measured. Data indicate that the when the bite was raised
0.5 mm, the measured forces were generally higher, approaching twice the
values obtained with normal occlusion. This observation may be explained by
the fact that the force on teeth are determined by muscular effort, and this effort
is controlled by the nervous system. Thus some force – regulating mechanism
was operating and it probably exists in case of malocclusion. The maximum
force measured will depend on the type of food.
FORCES ACTING ON THE TEETH :
FORCES AND RESPONSES :
 The forces which act on the teeth and cause them to move within their
periodontal tissues vary in magnitude, duration, frequency and direction.
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 The responses by the teeth to the forces depend on such factors as the shape
and length of the roots the characteristics of the fluid content of the
periodontal space, the composition and orientation of the periodontal fibres
and the extent of the alveolar bone.
 The responses by the teeth will also depend on the consistency of the bolus
being chewed and the muscular forces being used to crush it. This will also
apply to parafunctional clenching and chewing with or without a foreign
body between the teeth. It is, therefore, difficult to assess what is a normal
response to a force on a tooth and what is potentially harmful. As a result
of these forces, a tooth can be displaced in one of six directions : - apically,
mesiodistally or buccolingually, and each one producing a rotation or a
translation.
 The result is likely to be a combination of all directions leading to an
omnidirectional movement. The same principle of movement will apply to
the opposing tooth involved.
OMNIDIRECTIONAL AND UNIDIRECTIONAL RESPONSES :
These omnidirectional tilting and rotations of teeth will reach a limit
when an equal and opposite resistance is reached and the periodontal receptors
cause a reflex arrest of the muscle force. When the force is removed, the teeth
will recover their positions due to the elastic recovery of the compressed
periodontal tissues. This is referred to as “replacement” of the teeth.
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This phenomenon may be modified by 3 factors ;
i) Alveolar bone support
ii) Adjacent teeth support
iii) Horizontal muscle activity on both buccal and lingual surfaces of the
teeth.
 These 3 variable factors may lead to an unidirectional movement of a tooth
or teeth when they will become repositioned. Teeth will continue to move
unidirectionally until positions of stability are reached. The opposing
forces are then equal to the moving forces. Thus, maxillary incisors with
poor periodontal support and incompetent lips will drift forwards. This
forward drift will continue until the teeth are shortened or are prevented
from moving further by an appliance and by treatment of the periodontal
breakdown.
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STRESS
• When a force acts on a body, tending to produce deformation, a resistance
is developed to this external force application.
• Stress is the internal reaction to the external force.
• Both the applied force and stress are distributed over a given area of the
body, and so the stress in a structure is designated as the force per unit area.
Force
Stress = ---------
Area
• Area over which the force acts is an important factor of consideration
especially in dental restorations in which areas over which the force applied
often are extremely small. Since stress at a constant force is inversely
proportional to the area, the smaller the area, the larger the stress. And vice
versa.
• Technically, stress is the internal resistance of the body in terms of force per
unit area and is equal and opposite in direction to the force (external)
applied. This external force is also known as load.
TYPES OF STRESSES :
Depending upon the nature of the force, all stresses can be divided into 3
basic types which are recognized as ;
i. Tension
ii. Compression and
iii. Shear
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1) Tension : Results in a body when it is subjected to 2 sets of forces that are
directed away from each other in the same straight line.
F
F
2) Compression : Results when the body is subjected to 2 sets of forces in the
same straight lien and directed to each other.
F
F
3) Shear : Is a result of 2 forces directly parallel to each other.
S F
F
Tensile Stress :
- Is caused by a load that tends to stretch as elongate a body.
- The molecules making up the body must resist being pulled apart.
Compressive Stress :
- Produced by a load that tends to compress the body.
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- The molecules resist being forced more closely together.
Shear Stress :
- A stress that tends to resist a twisting motion, or a sliding of one portion of
a body over another.
- The molecules resist sliding of one body past another.
- A force applied to a dental restoration may be resolved in the structures as a
combination of compressive, tensile and shear stresses.
Complex Stresses :
Whenever force is applied over a body, complex as multiple stresses are
produced. They may be a combination of tensile, shear or compressive stress.
These multiple stresses are called complex stresses.
MECHANICAL PROPERTIES OF A MATERIAL :
The mechanical properties of a material describe its response to loading.
It is common to simply describe the external load in terms of a single
dimension (direction) as compression, tension, or shear combination of these
can produce Torsion (Twisting) or Flexion (transverse bending).
When a load is applied, the structure undergoes deformation as it bonds
are compressed, stretched, or sheared. The load deformation characteristics are
only useful information if the absolute size and geometry of the structure
involved are known. Therefore, it is typical to normalize load and deformation
as stress and strain.
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 Stress is load per unit cross sectional area.
 Strain is deformation per unit length.
During loading, bonds are generally not compressed as easily as they are
stretched. Therefore, materials resist compression more readily and are said to
be stronger in compression than in tension. Materials have different properties
under different directions of loading. “It is important to determine what the
clinical direction of loading is before assessing the mechanical property of
interest”.
As loading continues, the structure is deformed. At first this
deformation (or strain) is completely reversible (Elastic strain). However,
increased loading finally produces some irreversible strain as well (plastic
strain), which causes permanent deformation. The point of onset of plastic
strain is called the elastic limit. Continuing plastic strain ultimately leads to
failure by fracture. The highest stress before fracture is the ultimate strength.
The total plastic tensile strain at fracture is called the elongation.
The slope of the linear portion of the stress strain curve is called the
modulus, modulus of elasticity, young’s modulus, or the stiffness of the
material.
Two of the most useful mechanical properties are the modulus of
elasticity and elastic limit. A restorative material generally should be very stiff
so that under load, its elastic deformation will be externally small. An
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exception is Class V composite which should be less stiff to accommodate
tooth flexure. If the stress is well beyond the elastic limit, then the resulting
deformation is primarily plastic strain and at some point ultimately results in
failure.
Often it is convenient to determine the elastic limit in a relative manner
by comparing the onset of plastic deformation of different materials using
scratch or indentation tests, called hardness tests.
The energy that a material can absorb before the onset of any plastic
deformation is called its resilience, and is described as the area under the stress-
strain curve up to the elastic limit. The total energy absorbed to the point of
fracture is called the toughness and is related to the entire area under the stress
strain curve.
Time-dependent responses to stress or strain also occur. Deformation
with time in response to a constant stress is called creep (strain relaxation).
Deformation overtime in response to a constant strain is called stress
relaxation.
BIOMECHANICS FOR RESTORATIVE DENTISTRY :
Teeth are subjected to many forces during normal use. The interactions
between the applied forces, the shape and structure of teeth, the supporting
structures, and the mechanical properties of tooth components and restorative
materials are all included in the subject of biomechanics.
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Biomechanical Unit :
The standard biomechanical unit involves the
1. Restorative material
2. Tooth structure, and
3. Interface between the restoration and tooth
The importance of considering three structures in the biomechanical unit
is to detect stresses that may cause unwanted fractures or debonding. The
restorative material may be strong enough to resist fracture, but the interface or
tooth structure may not be.
STRESS TRANSFER :
Normal tooth structure transfers external biting loads through enamel
into dentin as compression. The concentrated external loads are distributed
over a large internal volume of tooth structure and the local stresses are lower.
During this process a small amount of dentin deformation may occur which
results in tooth flexure.
A restored tooth tends to transfer stress differently than an intact tooth.
Any force on the restoration produces compression, tension, or shear along the
tooth restoration interface. Once enamel is no longer continuous, its resistance
is much lower. Therefore, most restorations are designed to distribute stresses
onto sound dentin, rather than to enamel. The process of stress transfer to
dentin becomes more complicated when the amount of remaining dentin is thin
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and the restoration must bridge a significant distance to seat onto thicker dentin
(Liners or bases).
TOOTH FLEXURE :
Tooth flexure has been described as either a lateral bending or an axial
bending of a tooth during occlusal loading. This flexure produces the maximal
strain in the cervical region, and the strain appears to be resolved in tension or
compression within local regions, causing the loss of bonded class V
restorations in preparations with no relative grooves. Moreover, one current
hypothesis is that tensile or compressive strains produce microfractures (called
ABFRACTIONS) in the thinnest region of enamel at the CEJ. Such fractures
predispose enamel to loss when subjects to tooth brush abrasion and/or
chemical erosion. This process may be key in the formation of Class V defects.
PRINCIPLES OF BIOMECHANICS :
Stress transfer and the resulting deformations of structures are
principally governed by :
1. The elastic limit of the materials
2. The ratio of the elastic moduli involved
3. Thickness of the structures
Materials with a high elastic modulus transfer stresses without much
strain. Lower modulus materials undergo dangerous strains where stresses are
concentrated, unless there is adequate thickness.
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STRESS ANALYSIS AND DESIGN OF DENTAL STRUCTURES
 The mechanical properties of a material used in a dental restoration must be
able to withstand the stresses and strains caused by the repetitive forces of
mastication. The design of dental restoration is particularly important if the
best advantage of a material is to be taken. It is necessary to use designs
that do not result in stresses or strains that exceed the strength properties of
a material under clinical conditions.
 Stresses in dental structures have been studied by such techniques as brittle
coatings, strain gauges, two and three-dimensional photoelasticity, and
finite element analysis. Stress analysis studies of inlays, crowns, bases
supporting restorations, fixed bridges, complete dentures, partial dentures,
and implants have been reported.
a) Two Dimensional Photoelasticity :
The procedure for two-dimensional models is to prepare a transparent
plastic or other isotropic model of the restoration or appliance. This model is
usually larger than the actual size. The material becomes axis atropic when
stressed, and so the behaviour of light is affected by the direction it takes.
As a result of the applied stress, the plastic model exhibits double
refraction because of its an isotropic structure. The light from the source
passes through a polarizer, which transmits light waves parallel to the
polarizing axis, or plane polarized light. The plane polarized light is converted
to circularly polarized light by a quarter wave plate, and this polarized beam is
split into two components travelling along the direction of principal stress in
the model. Depending on the state of stress in the model, the two beams travel
at different rates. After the light emerges form the model, it passes through a
second quarter – wave plate, which is crossed with respect to the first, and an
analyzer that is most frequently perpendicular to the polarizer. The
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interference pattern may be recorded photographically, which is the
isochromatic fringe pattern. These isochromatic fringes, or dark liens,
represent locations where the difference in the principal stresses is a constant.
The magnitude of the stress can be determined by identification of the order of
the isochromatic fringes.
The fringe order multiplied by a constant and divided by the thickness
of the model gives the value of the differences in the principal stresses. Areas
in the model where the fringer are close together are under higher stress
gradients than areas where there are fewer fringes, and areas containing fringes
of higher order are under higher stress than these having fringes of lower order.
A two dimensional photoelastic model of a second molar with a gold
crown is analyzed. The elastic modulus of the plastics used to represent the
gold, dentin and bone had the same relative values as the actual materials. The
crown was luted to the tooth with dental stone, and a layer of silicone rubber,
simulating the periodontal membrane, separated the tooth from the bone. A
force of 266 N (60 lb) was applied 30 degrees to the axis of the tooth at a single
site on the mesial cusp, and the isochromatic fringes were photographed.
High stresses are apparent under the contact and in the bone at the tip of the
mesial root (seven fringes). Considerably lower stresses occurred in the bone
just under the distal root and at the crest of the ridge on the mesial side.
The effect of various configurations of the proximal margins was
studied by two-dimensional photoelasticity on the stress distribution in Class II
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inlays. Light field isochromatic fringes for rounded shoulder and shoulderless
models under a 445 N load were analyzed. The load was applied at 3 other
locations :
i) At the groove in the restoration
ii) On the cusp
iii) At the junction of the restoration and the tooth
The maximum shear stress was determined at nine critical areas, tow in
the restoration, two in the tooth and five at the junction of the restoration and
the tooth.
The study showed that the chamfer and rounded type of preparations are
the optimum designs in proximo-occlusal posterior restorations, since they
demonstrated the lowest stress when loaded vertically. The maximum fringe
order for the rounded shoulder was 10 whereas that for the shoulderless
preparation was 17. It was also shown that rounding the axiogingival line
angle in the shoulder geometry reduced the stress concentration factor by upto
50%. The gingival area of the proximal shoulder was the area of high stress,
and extra retentive features such as pins or grooves should not be placed in this
area.
FINITE ELEMENT STRESS ANALYSIS :
The finite element is a newer method than photoelasticity and offers
considerable advantages. In this method a finite number of discrete structural
elements are interconnected at a finite number of points or nodal points. These
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finite elements are formed when the original structure is divided into a number
of appropriately shaped sections, with the sections retaining the actual
properties of the real materials.
The information needed to calculate the stresses and displacement in the
model is
1) The total number of nodal points and elements.
2) A numbering system for identifying each nodal point and element.
3) The elastic modulus and Poisson’s ratio for the materials associated with
each element.
4) The coordinates of each nodal point
5) The type of boundary constraints
6) The evaluation of the forces applied to the external nodes.
A first molar with an amalgam restoration was idealized by an
axisymmetrical model and analyzed by the two-dimensional finite element
method. The model is divided into a number of triangles. The smaller
triangles are located in areas of greater interest. The ability of various types
and thickness of cement bass to support the amalgam was studied. The plots of
maximum tensile stress start at the centre of the cavity and extend toward the
cavity wall.
The stress induced in the amalgam restoration was from four to five
times higher when the amalgam was supported by 2 mm Zinc Oxide – Eugenol
cement base, as compared with an equal thickness of zinc phosphate cement
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base. When the stresses induced in the amalgam by a zinc phosphate base of 2
mm are considered in relation to those induced by a dentin floor alone one can
see that replacement of dentin by zinc phosphate to a depth of 2 mm does not
result in any significant increase in the tensile stress induced in the amalgam.
The zinc oxide eugenol cement base unlike the zinc phosphate cement bar,
does not function as rigid material and induces a larger displacement.
In comparison with zinc phosphate cement base the zinc oxide eugenol
material does not have adequate mechanical properties to support a restoration.
Even thin layers (0.5 mm) of zinc oxide eugenol cement caused significant
changes in the stress induced n the amalgam. Therefore the study indicates that
the fracture of amalgam is influenced more by the modulus of elasticity
(Stiffness) of the base material than by the compressive strength of the base.
An ideal situation would be to have a cement base with a modulus of elasticity
equal to that of the restorative material. Also, a subsequent study found that
tensile and shear stresses occurring in the cement base were of sufficient
magnitude to exceed the strength of some cements.
The stress distribution in porcelain fused to metal and porcelain jacket
crowns was conducted using a finite element method. Design parameters of
rounding of shoulders, avoidance of sharp notches, minimum thickness of
metal copings, and minimum labial bulk of porcelain were incorporated into
the model of an upper central incisors.
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A load of 444 N was applied at the incisal third of the lingual surface
and at the middle third of the lingual surface.
 Vertical loading and loading 30 degrees to the vertical were used.
 Since fracture is probably initiated by tensile failure at the periphery, the
tensile stress at the boundary is of special importance.
 With vertical loading at the incisal third, the highest tensile stresses were
found tat the labial third and on the lingual surface near the load, decreasing
toward the incisal edge. Low stresses wee observed at the margin and on
the lingual surface below the load. The surface stress was nearly the same
whether a gold or Ni-Cr base alloy was used; the use of Ni-Cr caused a
slight decrease in surface stress.
 When the direction of the loading was changed to 30 degrees from the
vertical, high tensile stresses were observed near the lingual margin that
would be of sufficient magnitude to fracture the cement in this area.
STRESS IN THE PERIODONTAL MEMBRANES :
Although limited measurements have been made on the periodontal
membrane of animals, the actual stress in the membrane has not been
determined experimentally. However, the stress to be expected has been
calculated. In one case, it was assumed that the periodontal membrane was
incompressible, whereas in another it was assumed to be approximately that of
water. In both cases the root of the tooth was assumed to be a cone and the
elastic modulus of the membrane was taken as 1.45 MN/m2
.
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When the force was applied to the center o the tooth axis, the stress
distribution was uniform with respect to the longitudinal axis of the tooth and
the pressure was greatest at the apex.
If the loading was transverse, the maximum stress occurred near the
apical third of the root on the same side as the compression force.
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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.
A) STRESS BEARING AND STRESS CONCENTRATION AREAS IN
ANTERIOR TEETH :
i) The junction between the clinical crown and 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.
ii) The incisal angles, especially if they are square, are subject t tensile
and shear stress in normal occlusion. Massive compressive stresses
will be present in edge-to-edge occlusion, and if the incisal angles
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are involved in a disclusive mechanisms, these stresses are
substantially increased.
iii) The axial angles and lingual marginal ridges will bear concentration
shear stresses. In addition, on the loading side, tensile stresses are
present, and on the nonloading side, compressive stresses are found.
iv) The slopes of the cuspid will bear concentrated stresses (3 types),
especially fi the cuspid is a protector for the occlusion or part of a
group function during mandibular excursions.
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v) 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 junction of the anterior and posterior
segments of the dental arch and microlateral displacement of the
cuspid during excursive movements. Both of these factors will lead
to tremendous stress concentration with resultant abrasive activity
there.
vi) 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.
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vii) 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 the mid-
protrusive and sometimes at the protrusive border location of the
mandible.
B) STRESS BEARING AND STRESS CONCENTRATION AREAS OF
POSTERIOR TEETH :
i) Cusp tips, especially on the functional side, bear compressive stresses.
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ii) Marginal and crossing ridges bear tremendous tensile and compressive
stresses.
iii) Axial angles bear tensile and shear stresses on the non-functional side,
and compressive and shear stresses on the functional side.
iv) The junction 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-contracting side.
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v) 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.
C) WEAK AREAS OF TOOTH :
Weak areas in the tooth should be identified and recognized before any
restorative attempt, in order to avoid destructive loading. They are ;
i) Bifurcation and trifurcation area.
ii) Cementum should be eliminated as a component of a cavity wall. The
junction between the cementum of the dentin is always irregular, so the
dentin surface should be smoothed flat after cementum removal.
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iii) Thin dentin bridges in deep cavity preparations.
iv) Subpulpal floors in root canal treated teeth. Any stress concentration
there may split the tooth interceptally.
v) Cracks or crazing in enamel and/or dentin. Both should be treated
passively in any restorative design. They may act as shear lines leading
to further spread.
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SOME APPLIED MECHANICAL PROPERTIES OF TEETH:
1. Although the following figures are averages, they provide an idea about the
principal mechanical properties of tooth structure. It must be understood
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-
42,000 psi.
b) Compressive strength of vital dentin is 40-50,000 psi.
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/cubic inch.
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 1,900,000 psi.
2. In general, when enamel loses its support of dentin, it loses more than 85%
of its strength properties.
3. Tensile strength of dentin is about 10% less than its compressive strength.
4. Tensile strength and compressive strength of enamel are similar, as long as
the enamel is supported by vital dentin.
5. Shear strength of dentin is almost 60% less than its compressive strength,
and this is very critical in restorative design.
6. There is minimal shear strength for enamel when it loses its dentin support.
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7. When the dentin loses its vitality, there is a drop of almost 40-60% in its
strength properties.
VALE EXPERIMENT :
The original experiment involved preparation of occlusoproximal
cavities with different crossing dimensions at the marginal and crossing ridges
with a standard depth. The teeth were then subjected to measured occlusal
loads. The load that split the tooth was recorded and compared to the control,
which was the load that split a round tooth. Later, the same experiment was
repeated by several investigators using more sophisticated equipment than that
used by vale. The results were consistent.
A summary of their findings brought to the closest round figures is as follows :
i) By crossing one marginal ridge at ¼ the intercuspal distance, there is
almost 10% loss of a tooth’s resistance to splitting.
ii) By crossing two marginal ridges at ¼ the intercuspal distance, there is
iii) By crossing one marginal ridge at 1/3 the intercuspal distance, there is
iv) By crossing two marginal ridges by 1/3 the intercuspal distance, there is
v) By crossing one marginal ridge at ½ the intercuspal distance, there is
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vi) By crossing two marginal ridges at ½ the intercuspal distance, there is
vii) By crossing a crossing ridge at ¼ the intercuspal distance, there is
viii) By crossing a crossing ridge at 1/3 the intercuspal distance, there is
ix) By crossing a crossing ridge at ½ the intercuspal distance, there is
OBTAINING RESISTANCE FORM FOR TOOTH STRUCTURES :
1) To best resist masticatory forces, use floors or planes at right angles to the
direction of loading to avoid shearing stresses.
2) If possible, walls of preparations should be parallel to the direction of the
loading forces, in order to minimize or avoid shearing stresses.
3) Intracoronal and intraradicular cavity preparations can be done in box, or
cone or inverted truncated cone shapes.
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Thus from the above figures, 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.
4) Definite floors, walls and surfaces with line and point angles are essential to
prevent micromovements of restorations, with concomitant shear stresses
on remaining tooth structures.
5) 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.
Load – A Load A
10 stress units/mm3
1 stress unit / mm3
6) Designing the outline form with minimal exposure of the restoration surface
to occlusal loading will definitely minimize stresses and the possibility of
mechanical fracture in the restoration.
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7) Junctions 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 future.
8) Retentive features must leave sufficient bulk of tooth structure to resist
stresses resulting from displacing forces.
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OCCLUSAL CONSIDERATIONS IN RESTORING TEETH
The way we occlude teeth affects the periodontium, the
temporomandibular joints, throat muscles, tongue, cheeks, lips, nerves and so
son. The occlusion of the restored teeth should hence establish healthy
relations between the dentition and rest of the stomatognathic system. A
clinician must have adequate knowledge about the principles of occlusion,
which enables him to diagnose cases that need modifications / alteration of
occlusion with or without the use of various restorative materials. Before
initiating any restorative care, thorough occlusal examination should be carried
out.
The kind of occlusion, a patient should have, must be justified by the
principles of physiology.
The occlusion affects almost every part of stomatognathic system,
mainly :
a) The pulp of the tooth is a very sensitive organ. IT reacts immediately to
abnormal occlusal forces. Hence, occlusion should not be detrimental to
pulp.
b) The proximal relations of the occlusion should prevent food impaction
between teeth.
c) The cusp-fossa relationship should be such that the adequate forces exerted
during functional movement, aids in optimum health of the periodontium.
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Each tooth should be restored following the principles of occlusion, so as to
achieve optimum functions of the neuromusculature joints and the
supporting structures of the teeth. New restoration should not introduce any
premature contacts and cuspal interference’s.
POSTERIOR RESTORATIONS :
All posterior restorations should be planned keeping in mind the basic
principles of occlusion.
 Prior to cutting a tooth, its opposing occlusal surface should be examined.
Malpositioned opposing supporting cusps and ridges should be recontoured
in order to achieve optimal occlusal contacts in the restored tooth.
 Use articulating paper to register the centric holding spots and excursive
contacts so that these marked areas can either be excluded form the outline
form or properly restored.
 Plunger cusps and over erupted teeth should be reduced, removing all the
cuspal interference’s so as to improve the plane of occlusion and decrease
the chances of fracture of new restoration as a result of occlusal forces.
 When carving for occlusion, attempt to establish stable centric contacts of
cusps with opposing surfaces that are perpendicular to occlusal forces
should be made.
 Occlusal contacts located on a cuspal incline or ridge slope are undesirable
because these create a deflective force on the tooth and hence should be
adjusted until the resulting contact is stable.
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i) AMALGAM RESTORATIONS :
 Sufficient bulk of amalgam is mandatory when restoring a cavity with
amalgam so as to withstand the load of occlusion.
 Adequate thickness of amalgam should be provided at the marginal ridges
in order to support the opposing supporting cusps.
 Amalgam restorations are carved following the cuspal inclines.
 In case of large restorations, where there are no cuspal planes to guide
carving, the operator should follow a cautious approach :
• Buccal and lingual cusp tips should be placed in lines joining those of
adjacent teeth.
• The level of central fossa and the marginal ridge should be carved
similar to that of adjacent teeth.
• The bucco-lingual width of the occlusal surface is kept narrower than
the original buccolingual width of the tooth.
• In case both the working cusps on more than 2 cusps are restored,
preferably the occlusal table is kept narrowed.
This narrower occlusal table leads to :
• Reduction of force : When the occlusal table is made narrower, lesser force
is applied over the same to undergo masticatory functions. Force is
transmitted to all structures underlying the occlusal table, which include the
restoration, the tooth structure and the periodontium.
• Reduction of the effect of force : The direction in which the applied force is
transmitted is governed by muscular activities and the area on which the
36

force is applied. However, the effect of the force may be modified by
altering the surface at which the force is applied, thus altering the direction
of resolved components.
• Reduction of torque : The tendency to rotate may be reduced by altering the
point of application of the force relative to the fulcrum. The point of
application of the force may be altered by modifying the occlusal table
which indirectly depends upon the design of the cavity and the restoration.
ii) CAST METAL RESTORATIONS :
 Similar to amalgam restorations, before preparations of any tooth, evaluate
the occlusal contacts of the teeth in centric occlusion and in excursive
movements. As part of this evaluation decide if the existing occlusal
relationships can be improved with the cast metal restorations.
 The cuspal interferences are depicted in mandibular working movements,
non working movements and protrusive movements.
 The opposing occlusal surfaces should be examined an he malpositioned
cusps, plunger cusps and over erupted teeth should be recontoured.
 Premature contacts or cuspal interferences from the teeth opposing the
required restoration should be removed.
 The remaining tooth structure and the length of clinical crown dictates us to
choose the type of cast restoration.
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ANTERIOR RESTORATIONS :
 The resin composites and the glass ionomer cements are mainly used in
anterior restorations. Though these teeth do not come under direct
occlusion, however, they do take part in various movements of the
mandible. The restoration should be carved and finished, maintaining the
contacts and the cervical curvature of these restorations. The lingual area is
carved to maintain the anatomy of cingulum and the lingual marginal
ridges. Patient is asked to protrude and the interferences are checked and
removed.
 Similarly, the relationship of lips with the labial surfaces of restored teeth
are checked and the over-contouring, if any, is removed.
 The gingival extension of the material is taken care of to maintain the
gingival health.
Role of Contact Areas :
 Good restorative dental procedures must reproduce the proper contact areas.
Restorations with contact areas which are flat, open, improperly placed,
rough or poorly polished will lead to failure.
 A slight frictional movement of teeth always occurs between the
interproximal surfaces of teeth during physiologic movement; and with
time, the contact point becomes broad resulting in a wider contact area. IF
the teeth remained in contact with each other merely by contact points, they
would eventually be forced out of the dental arch in either a buccal or
lingual direction. Whereas with a wider contact between teeth, this is not
38

likely to occur. The opposing interproximal surfaces of restorations must
be hard in order not to flow, flatten, wear or become abraded with use.
Relationship between tooth wear and restorative materials :
 Occlusal forces lead to wear of enamel. The wear is, however, very slow if
occlusal forces are appropriately transmitted to underlying bony tissues.
 The pattern of wear varies individually depending upon various factors.
Non-uniform ear of opposing teeth is quite common when one teeth is
restored with a restorative material whose wear resistance is different as
compared to that of enamel. Very rarely, the wear resistance of a
restorative material equal the wear resistance of enamel.
 At present, no restorative material is available which wears at the same rate
as enamel or as enamel and dentin at later stages.
 Differential wear can result in localization of occlusal loads with
subsequent failure of restorative materials or development of deflective
contacts with mandibular repositioning and an effect on a distant tooth.
 Hypothetically, if two restorative materials, which wear at a slower rate
than the natural teeth, are placed so as to oppose each other in a dentition
undergoing wear, the restorations will produce occlusal interferences at a
later stage.
 Non-wearing materials opposing each other can lead to natural teeth wear
during contact in lateral and protrusive movements.
 Conversely, if the materials wear faster than the teeth, the opposing cusp
might over erupt into the worn material. IN lateral excursion, this cusp
39

might then come in contact with an opposing cusp and if weakened by
previous caries can lead to fracture.
Compensation for Occlusal Wear :
 Occlusal interferences can develop through differential wear patterns and
unmatched compensatory mechanisms.
 The clinician can shape and regulate the form of occlusal surfaces of teeth
and restorations so that he can determine surfaces of teeth and restorations,
which contact during activities such as mastication, swallowing and
bruxism.
The advantage of this approach are : -
• The direction of stresses through the strongest portions of the restorations
an the remaining tooth structure can be arranged.
• The effect of occlusal interferences developing from differential wear can
be minimized.
• It is possible to maintain the partially restored dentition by means of
periodic adjustment.
 Since wear defects are not repaired automatically, the dentist should replace
and maintain the configuration of teeth in accordance with the functional
activities.
40

FORCES EXERTED DURING OCCLUSION / MASTICATION AND
THEIR RESOLUTION
Various types of forces are exerted on teeth during movement of
mandible and also during mastication. Since the tooth surfaces are curved or at
an incline, these forces are not only vertical but other types of forces may also
be exerting n these surfaces. The tooth, in turn, counteracts these forces with
the help of periodontal membrane and alveolar bone.
If the surfaces are flat and perpendicular to the force of mastication, only
vertical forces would take part. But in curved surfaces, other forces are also set
up and the resulting forces might not be exerted along the long axis of the
tooth.
This phenomenon can be understood by studying the resolution of forces
on inclined planes. The cuspal planes are taken as inclined planes.
41

When a force acts perpendicular to a fixed horizontal surface, the
resolving force reacts perpendicular to the surface with an equal and opposite
force. If the surface is tilted at an angle to the horizontal, it still reacts at right
angle to the surface.
F
Surface F
Surface R
Thus, the reaction force no longer opposes the applied force in direction
nor is equal to its magnitude. Hence the forces are not in equilibrium when
applied on inclined planes.
The equilibrium can be maintained if more than one force is exerted on
tooth or the forces are resolved in both directions.
Forces acting on inclined planes.
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AB is a tangent drawn at inclined plane or the contact between 2 cusps.
Angle ‘α’ represents the angle made with the horizontal AC by the tangent AB
of the cuspal contact. M is the force of mastication and N is the resolving
force. M is perpendicular to the horizontal AC and N is perpendicular to the
incline plane, tangent AB, H is the horizontal component of the resolving force,
which maintains the equilibrium.
As the angle ‘α’ decreases, i.e. incline plane decreases, N and H become
shorter and finally merge with M i.e. equal to zero.
The effect of friction between cusps also plays an important role.
Friction is the resistance to a sliding motion of one body over another and the
coefficient of friction is the force of friction over normal force.
Many a times, two or more inclined surfaces with slopes facing each
other of one tooth contact the buccal and lingual cusps of the opposing tooth or
the buccal and lingual cusps and marginal ridges. This condition accounts for
the proper balance in occlusion and in case the contact is not normal, it may
account for displacements of the restoration or the fracture of the teeth. The
effect so produced is termed as wedging effect.
The horizontal components of the normal force are responsible for this
wedging effect. These horizontal components set up by inclines are equal and
opposite and tend to push the inclined surfaces apart.
43

FORCES ACTING ON THE TOOTH :
A) In centric occlusion
a, b, c are forces which acts at 3 contact points.
 Rab is the resultant of forces a and b. Rab and c are the 2 adjacent sides of
the parallelogram passing through a given point as shown. The resultant is
represented by diagonal passing through the same point i.e. Vabc.
 Hc is the horizontal component of force c. Hab the horizontal component of
force a and b and Hc should be equal for achieving equilibrium that is why
Rabc and Vabc are equal.
B) During Chewing :
44

 When mandible moves form lateral to centric occlusion, the resultant of
forces acting is not vertical but inclined medially.
 When tough food is compressed or all cusps are in intimate contact at the 3
points, the forces a and b are decreased and c is increased with resultant
changes in horizontal and vertical components. Since during chewing, Hc is
greater than Hab and the net resultant is Habc. So the horizontal component
is along the direction of c.
 By using triangle of vector addition, the resultant is represented by Rabc.
 The resultant Rabc is a thrust inclined palatally on the maxillary teeth and
buccally on the mandibular teeth, whose horizontal component is Habc.
MECHANICAL FUNCTIONS OF THE MARGINAL RIDGES :
Role of Marginal Ridges :
 The marginal ridges play an important role in withstanding and dissipating
the occlusal stresses.
 The correct form of marginal ridge compatible with the adjacent tooth and
also with its own surrounding is important during carving of posterior
restorations.
 The absence of marginal ridge, or marginal ridge with improper height can
lead to altered dissipation of forces subsequently damaging the underlying
periodontium.
45

1. Normal Marginal Ridge :
 Forces 1 and 2 act on marginal ridges of teeth A and B respectively. The
horizontal component of 1, H1 and the horizontal component of 2, H2,
counteract each other. The vertical component V1 and V2 are resolved
normally by the underlying tissues.
2. No marginal ridge
 Tooth B has no marginal ridge. Force 1 and 2 are acting on tooth a and B
respectively. The horizontal component of 2, H2 is missing in the tooth B,
because force 2 is mainly directed towards tooth A.
 Horizontal component H2 will drift the tooth A apart and the vertical
component V1 and V2 of both the forces 1 and 2 will help the food impact
vertically. The vertical force V2 will be more than required, there may
46

occur slight tilting of the tooth B. This will further deteriorate the
resolution of forces and lead to further food impaction.
3. A Marginal Ridge with a wider occlusal embrasure.
 Inspite of putting optimal pressure on marginal ridges of tooth A and B, the
forces 1 and 2 act on adjacent teeth. The force 2 will put pressure on tooth
A and force 1 will put pressure on tooth B. This will lead to drifting of both
the teeth. The vertical components of forces will wedge the food is between
the two teeth.
 Similar effect is seen when one marginal ridge is higher than other.
4. No occlusal embrassure
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In totality, the vertical component of forces 1 and 2 will be more
concentrated than horizontal components. Though there will to be any vertical
impaction 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.
i) Parallel to the long axis and
ii) Perpendicular to the long axis
 The force or the load is applied at different areas at a time and the stress
distribution depends upon various factors.
a) If the cross – section of that area is constant, stress distribution is
practically uniform.
b) If there is variation in cross-section (such areas are normally termed as
prisms); here stress varies form point to point, being inversely
proportional to area.
c) 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 haring stress increases to a maximum at 45o
and then decreases to zero
at 90o
. Therefore, materials that are weaker in shear than in compression or
tension replace in planes at 45o
to the axis.
 The modulus of elasticity of the material is an important property and
should be taken care of. If a cavity is restored with gold inlay or porcelain,
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the modulus of elasticity varies between the tooth and the restorative
material. With the vertical force exerting on both, the compression will be
the same for the restoration and the tooth, but since gold/porcelain is much
stiffer, they will be highly stressed, since S = dE.
S (Stress) = S (Unit strain) x E (Modulus of elasticity)
 When the force is applied perpendicular to the prism axis, the resultant
resolution is known as beam. Beam can be supported form both the ends
(simple beam) and may be supported form one end (cantilever beam).
Example of simple beam : MOD preparation
Example of Cantilever beam : MO / DO preparation
The retention of the restoration depends upon these beams, although the
strength and the deflection of the material also play part.
Moment of Force = Force x Perpendicular Distance
 The bonding 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 x L is required to counteract
this moment. The total retentive force (R) is equal to F x L / l
Where l is the depth of the axial wall.
 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 is zero. Therefore, the depth
of the gingival wall does not take part in retention.
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In MOD Preparation :
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 midpoint is :
F / 2 x L / 2 = FL / 4
If this moment of force is divided into two (because it is actually acting on both
the ends) then the moment of force :
FL 1 FL
----- x ----- = -----
4 2 8
Since the beam forces 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 x l = FL / 8
So R = FL / 8 l
The negative sign is used only in vector form and in magnitude only
positive sign is used.
 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.
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 It is presumed in MOD preparations that the length of the axial wall (l) is
kept equal on booth the ends. If there is marked discrepancy between the
two ends, the end result may not be the same as is described earlier.
Therefore, preferably the length of the two axial walls should be the same.
In Cervical / Gingival Restorations :
 It has been established that certain forces act on the cervical reign, which
could destabilize the restoration and even lead to cracks at the cemento-
enamel junction.
 The forces acting on inclined planes of the occluding cusps consequently
lead to transverse stresses. These transverse stresses try to bend the tooth
gingivo-occlusally. Since the teeth are firmly held in alveolar socket, these
rotations are minimum and counteracted.
 In cases where a cavity is cut on the cervical surfaces, depending upon the
height of the axial wall, the deflective force is increased. If the restorative
materials are not adhesive in nature, a gap can be created at the cervical
surface of the restoration on buccal side and occlusal surface on the lingual
side.
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 Force (F) is applied at incline plane perpendicular 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).
APPLICATION OF STRESSES AND THEIR DISTRIBUTION IN
INDIVIDUAL RESTORATIONS :
1) Class I restoration
a) If restored with amalgam
 It is recommended to converge the side walls occlusally and to keep the
floor flat.
 In case the floor is not kept flat, the forces will rotate the restoration on
both the sides. And also, since the remaining dentin will be less at the
centre and if the restoration is deep, the forces might fracture the tooth.
b) Cast restorations :
 Movement / rotation of the cast restoration is easy, if the pulpal floor is
not kept flat.
 Since the walls are diverging occlusally, the chances of rotation are
much more.
c) Composites or GIC
 These adhesive materials counteracts such rotational forces.
52

2) Class II restoration
 Stresses which tend to rotate the restoration, mostly act on marginal
ridges.
 Stresses also is more at axiopulpal line angle, hence, this axiopulpal line
angle should be well rounded, thereby decreasing stress concentration
and increasing the bulk of the material at this point.
 In MOD restorations, bending of the occlusal portion is caused by the
difference between the total masticatory force and the support given by
the pulpal floor of the cavity.
 Gingival retention and rounding of the axiopulpal line angles are
required as in MO and DO cavity.
 In cases where the opposing cusps occlude in such a way that one
contact point is on a proximoocclusal restoration while the other is on
tooth structure, there is a tendency to wedge the two apart. To prevent
this wedging, the occlusal lock is used even though occlusal surface is
not involved by caries.
3) Class III and Class IV Restorations :
 Since these lesions are not in direct contact with opposing teeth, only
transverse stresses play part in dislodging / rotating the restoration
 In such restorations, there is tendency to rotate about an axial approximately
parallel to the long axis of the tooth. As incisal retention cannot be made
due to thin labiolingual size, so lingual lock, is placed on lingual surface. It
53

should be as close to the incisal edge as possible and still be in dentin to
reduce the stress in this lingual lock.
 In maxillary teeth, force of mastication ahs labial component, which
provides the seating effect on the restoration. In case the labial enamel is
not intact, the chances of dislodgment of the restoration will increase. In
mandibular teeth, the component of the masticatory force is from the labial
to the lingual so chances of dislodgement of restoration are more.
4) Class V restorations :
 Analysis indicates that physical forces putting on occlusal surfaces could
result in displacement of the restoration.
 During occlusion, the vertical stresses on the teeth led to transverse stresses
and this component of stresses tends to rotate the cervical restoration.
 The mandibular teeth bend more than maxillary teeth.
 A gap is evident on the cervical / occlusal wall of the cavity and if the depth
of these walls is less, the restoration may come out.
54

FORCES ACTING ON AMALGAM RESTORATION
CLASS – I :
By definition, Class I cavity preparations are placed in pit and fissure
lesions that occur in one more of the following locations :
A. Occlusal surfaces of molars and premolars
B. Occlusal 2/3 of the buccal and lingual surfaces of molars
C. Lingual surfaces of the upper anterior teeth (usually the central and lateral
incisors)
D. Any other usually located pit or fissure involved with decay.
Mechanical problems in Class I restoration and their solutions.
A. 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.
- The seat of the restoration is placed at a distinct right angle to the direction
of stresses.
- It is advantageous to have a mortise shape preparation in an inverted cone
shape to minimize shear stresses that tend to seperate the buccal and lingual
cuspid elements i.e. to prevent the splitting of the tooth. So whenever the
anatomical and cariological factors allow, the cavity preparation should be
an inverted cone shape.
B. When a caries cone penetrates deeply into dentin, removing undermined and
decayed tooth structures can lead to a conical (hemispherical in cross-section)
cavity preparation. Mechanically, two problems can occur if a restoration is
inserted into such a cavity preparation.
1. 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.
2. 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
55

microscopic, they occur frequently enough to encourage microleakage
around the restoration, predisposing to a recurrence of decay. These
movements 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. One
level will be the ideal depth level (1.5 mm) and the others will be the caries
cone(s) level(s), 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. Reiterating, the other level(s) is (are) only
necessitated by the caries extent., creating one or more concavities or cones in
the pulpal floor.
C. When a cavity wall comes in contact with a marginal ridge, the wall should
be divergent pulpo-occlusally, making an obtuse angle with the pulpal floor.
This design allows for maximum bulk of tooth structure supporting the
marginal ridge and avoids undermining of the marginal ridge, creating more
mechanical and biological problems.
D. If cariogenic conditions do not dictate otherwise, the width of the cavity
should be limited to ¼ to 1/5 the intercuspal distance (not less than 1.5 mm).
This minimizes loss of tooth structure in this critical cross-section of the tooth.
This width will also facilitate easy carving of the restoration, and minimize the
possibilities of occlusal interference’s.
E. All cavosurface angles should be right angled to create a butt-joint with the
marginal amalgam. This configuration allows marginal amalgam to withstand
stresses with the least possibility of failure.
56

F. All line and point angles, or any junction between different details in the
cavity preparation, should be rounded but definite. This design has all the
advantages of the mortise shape, while avoiding stress concentration in the
tooth structure and restorative materials that may occur from sharp angulations.
G.
Occluding forces will tend to move marginal amalgam and tooth
structures from position #1 to position #2. As vital tooth structures are more
deformable than set amalgam, the displacement will not be equal thereby
creating a gap between them. This places the marginal amalgam under
intolerable tensile loading which may lead to amalgam failure if the amalgam is
in thin cross sections, i.e. acute angled marginal amalgam will fracture. B, If
marginal amalgam is right angled, it can be stand induced stresses from
occlusal loading with less possibility of failure, even if the stresses are tensile
in nature.
CLASS II AMALGAM RESTORATION
By definition Class II cavity preparation is proximal preparations of
molars and premolars.
Resistance Form :
The fundamental concept of resistance form is based on the reaction
within the restoration and the remaining tooth structure to occlusal loading.
The objective of a cavity preparation design is to establish the best
possible configuration that can cope with the distribution and magnitude
of the stresses in both structure and the restoration without failure. To
57

design such a configuration, one must first comprehend the nature of
loading and of resistance to such loading.
A. Occlusal Loading and Its Effect :
During centric and excursive movements of the mandible both
restoration and the tooth structure are periodically loaded both separately and
jointly. This brings about different stresses patterns depending on the actual
morphology of the occluding area of the both the tooth in question and
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.
1) A small cusp contacts the fossa away from the restored proximal surface, in
a proximo occlusal restoration at centric closure.
As shown due to the elasticity of the dentin, (in young teeth) a restoration
will bevel at the axio-pulpal line angle (provided the proximal part of the
restoration is self-retained). This creates tensile stresses at the isthmus
portion of the restoration, shear stresses at the junction of the main bulk of
the proximal part of the restoration and self retained parts and compressive
stresses in the underlying dentin.
2) A large cusp contacts the fossa adjacent to the restored proximal surface in
a proximo-occlusal restorations at centric closure, either in the early stages
of moving out of centric or at the late stages of moving toward it.
As shown, the large cusps will tend to separate the proximal part of the
restoration from the occlusal part. This crates tensile stresses at the isthmus
portion of the restoration even fi the proximal portion is self – retained.
This loading situation will deliver compressive forces in the remaining
tooth structure, apical to the restoration.
3) Occluding cuspal elements contact facial and lingual tooth structure
surrounding a proximo-occlusal or proximo-occluso-proximal restoration,
during centric and excursive movements.
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Concentrated shear stresses will occur at the junction of the surrounding
tooth structure and corresponding floors, with a tendency towards failure
there. This loading situation can be unilateral or bilateral, depending on the
mandibular movement it is the most deleterious to tooth structure especially
on the orbiting side if there is interference during lateral excursion.
4) Occluding facial elements contact facial and lingual parts of the restoration
surrounded by tooth structure, during centric and excursive movements.
This arrangement will induce tensile and compressive stresses in the
restoration which will be transmitted to the surrounding tooth structure.
5) Occluding cuspal elements contact facial or lingual parts of the restoration
completely replacing facial or lingual tooth structure during centric or
excursive movements.
The tensile stresses will be induced at the junction of the occlusal and facial
and/or lingual part of the restoration in both occluding situations.
6) Occluding cuspal elements contact a restoration’s marginal ridge(s) or part
of a marginal ridge during centric or excursive movements (assuming the
restoration is locked occlusally), there will be concentrated tensile stresses
at the (junction of the occlusal and facial or lingual parts of the restoration
at full intercuspation and to end from that position) at the junction of the
marginal ridge and the rest of the restoration. This will be true if its an area
of advance contact during mandibular closure.
7) Cuspal elements occlude or disclude via the facial or lingual groove of a
restoration. There will be tensile stresses at the junction of the occlusal and
facial or lingual parts of the restoration at full intercuspation, and to and
from that position.
8) Cusps and crossing ridges are part of the restoration in centric and excursive
movement.
Both will be subjected to compressive stresses during such positions and
movement. Besides tensile stresses could concentrate at their junction with
the main restoration, specially during contacting excursive movement.
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9) 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 when they are not
reciprocating (one side not in contact with occluding surfaces while other
axial portion). The axial surfaces will be stressed in a slight tensile and
shear pattern at their junction with the main bulk of the restoration.
10)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, insofar 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, and or tilt,
establishing contact with the opposing cuspal elements. Usually, this newly
acquired location will not be the most favorable 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 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, too, could lead to localized or generalized
gnatho stomatic disturbances, with eventual mechanical and biological
failures.
Needless to say, pre-existing premature contacting areas should be
eliminated before restorative treatment. This is done for many reasons, but
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 fi the preexisting tooth
structures were not.
60

Several factors must be accommodated in designing Class II preparations
for amalgam. Occlusal loading is dynamic and cyclic in nature, which is a
far more destructive type of loading than static loading. Amalgam is least
resistant to tensile stress and most 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 stricture.
B. 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, facial or lingual parts, 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 bending occurs at the axio-pulpal line angle
2) Stresses increase closer to the surface of a restoration, away from that
fulcrum, and
3) Tensile stresses concentrate 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. A theoretical solution might be :
1) to increase amalgam bulk at the axio-pulpal line angle, thereby, placing the
surface stresses away from the fulcrum. However, its actually results in
increased stresses within the restorative material and a deepened cavity
61

preparation, dangerously close to pulp anatomy. Therefore, such a solution,
in and of itself, is wholly 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 the 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 the axio-pulpal line angle away from that
stress concentration area and closer to the surface, can be achieved simply
by slanting the axial wall toward the pulpal floor.
a) The obtuse axial 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. Furthermore, this
design feature improves accessibility to the proximal facial and proximal
lingual parts of the cavity during preparation procedures. This is the
first design feature.
b) Secondly, if the axio pulpal line angle is rounded, structural projections
or sharp junctions that may concentrate stresses at the isthmus would be
avoided. This second feature will also improve the visibility for the
facial and lingual gingivo-axial corners of the preparation proximally, as
well as increase the amalgam bulk at the fulcrum.
c) Thirdly, 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.
4) As a fourth design feature, 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 tooth structure independently from other parts, there will be
62

minimum stresses at the junction of one part with another, i.e. the isthmi.
This can be achieved in amalgam preparations by retentive grooves, internal
boxes, and undercuts.
6) Sixth, one should avoid, as much as possible, placing or leaving any surface
discontinuities, 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.
2. Margins :
Amalgam has good compressive strength when it has sufficient bulk
(1.5 mm minimum). However, frail, feather edged margins of amalgam, which
will occur when the cavosurface angles of preparations are bevelled, will
fracture easily. Occluding forces will cause amalgam at the bevel to bend with
maximum tensile stresses, 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 p reparations, four design features should
be observed ; create butt joint amalgam tooth structure at the margins, leave no
frail enamel at the cavosurface margins, remove flashes of amalgam on tooth
surface adjacent to amalgam margins, and, as practically as possible, 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.
3. Cuspal 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 1.5 mm
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.
63

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.
C. Design features for the protection of the physiomechanical integrity of
remaining tooth structure :
In addition to design features in the restoration, there are also certain
design features in the tooth structure, which enhance resistance of the restored
tooth to deleterious stresses.
Retention from :
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 four such displacements for a Class II proximo-occlusal
restoration.
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 cavosurface margin. To prevent
such displacement, self-retaining facial and lingual grooves proximally are
necessary, in addition to an occlusal dovetail.
64

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.
This is due to the elasticity of the dentin, especially in young teeth, wherein the
pulpal floor will change location from position 1 to position 2. However, since
metallic restorations are more rigid than the dentin, the short arm of the L will
move proximally, as shown in the figure. The fulcrum of this rotation is the
axio-pulpal line angle. In order to prevent such a displacement, proximal self-
retention in the form of facial, lingual and/or gingival grooves are required.
However, shear stresses will be induced at the junction between the amalgam
of the main restoration and that in the grooves. Therefore, it is to be
understood that these grooves are prepared only when there is complete
assurance that there will be sufficient dentinal bulk to accommodate them, and
that they will not impinge on the axial angle or on the pulp anatomy.
C. Lateral Rotation of the Restoration Around Hemispherical Floors (Pulpal
and Gingival)
65

As in Class I cavity preparations this displacement can be prevented by
definite point and line angles, and ledges where indicated.
D. Occlusal displacement :
The 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
restoration, with less failure to be expected as a result.
66

FORCES ACTING ON INLAY RESTORATION
The cavity should have such retention form that the restorations will be
firmly held in place, the cavity should also have resistance form that the
restoration will withstand the stress without being dislodged.
An understanding of the materials used in constructing an inlay, together
with a knowledge of correct manipulation is also an important factor in the
success or failure of an inlay (inlay is not only a part of mechanical structure
replacing lost teeth, but it is also intimately related to the vital tissues, it is the
medium through which mechanical and physical forces are translated into
physiological functions and biological reactions in living tissues.
The other preparation features that will help solve the mechanical
problems of cast restorations are as follows :
All the line and point angles should be definite, but not angular, so they
can be easily reproduced in a casting and to avoid stress concentration in the
casting and the tooth structure. The roundness must be substantial for Class V
materials.
The axial wall should slant toward the pulpal floor, as part of the taper.
This, together with rounding of the axio-pulpal line angle, can reduce stresses
at the isthmus area.
Reduction of tooth structure should follow the original anatomy of the
tooth, to create even reduction, with minimum tooth involvement, and even
physiologic distribution of forces applied on the restoration and remaining
tooth structure.
Maximum reduction should be at the occluding surfaces, especially the
parts of the tooth surfaces that are in contact during static and dynamic
relations of the mandible. An average of 1 mm should be cleared for metallic
67

casting in the inclined planes of the cusps. This reduction should be 1.5 mm
for cast ceramics. The reduction of the occluding inclined planes should be cut
in a concave form, to accommodate maximal bulk of the casting where stresses
are at their maximum.
The internal parts of the cavity preparation should be mortised to
preserve the resistance and retention features of the preparation (and to assure
one path for the preparation). The internal boxed up portion should occupy the
maximum dimensions of the cavity preparation as practically as possible. This
will necessitate making the cavity wall in different planes. At least, the internal
planes are fixed in their angulation (almost right angle) with the adjacent floors
or walls.
Since the retention of an inlay and its resistance to displacement are
primarily mechanical problems, a group of the principles of retention is based
on understanding the forces of mastication and the analysis of strains which are
present in the restoration.
It has been stated that when a force is applied at right angles to a surface
its effectiveness with the direction of force and that is proportional to its
magnitude likewise, the opposing forces are equal and opposite in direction.
Another law states that if the force is applied at an angle to the surface other
than right angle, the magnitude of which depends n the angle of application ad
that the reacting force is neither equal nor opposite in direction.
Lateral or tangential forces may cause displacement of the restoration
unless adequate resistance and retention have been incorporated in the
preparation.
Frictional retention can be achieved by the action of dentin and enamel
walls grasping the restoration (intracoronal retention).
68

Now let us consider the forces applied at right angles to the flat surface
of a restoration.
Pulpal Floor and Gingival Seal :
1) A typical proximoocclusal cavity will have two such surfaces to vertical
forces – the pulpal and gingival walls.
If the forces are perpendicular to these surfaces the opposing forces are
equal and opposite, then there is no tendency to displace the filling. Floors
positioned perpendicular to these lines of force absorbs the stress over a
broad area of tooth.
2) It is only when the pulpal wall is flat and the two vertical walls are parallel
to each other that the maximum retention form is obtained.
While these above illustration refer to simple box type cavity preparation,
the same principles hold good when the force is applied at right angles to
the occlusal surfaces of proximo occlusal inlay.
3) In a tooth weakened by extensive caries, the resistance form is obtained by
extracoronal extension of the preparation in the form of extra long reverse
bevel in capped cusps or by partial or complete coverage of facial or lingual
surfaces.
4) If the dentin of the pulpal wall is compressed elastically under vertical
forces, if the compression is conical then the gingival portion of the filling
would rotate out of the cavity with the axiopulpal line angle acting as the
fulcrum.
Because of the added retention obtained by the pulpal extension and if
the diagonal force is applied to the casting which is ‘L’ shaped. It will have a
tendency to straighten out, so this causes the metal to move out laterally at the
gingival area. To resist this lateral spreading, at the gingival wall provision is
made for the depression of the wall and creating the gingival groove which
restores the retentive form to a certain extent.
69

Axioproximal Walls (Facial / Lingual) :
Compressive forces resulting from vertical pressure have an important
bearing on the retention of the inlay. This bears on the relationship of the
buccal and lingual proximal walls. Now whether they should flare
axioproximally or be parallel to each other (that is the part of the wall lying
within the dentin).
There are 3 different relationships of wall A to wall B in the
gingivoocclusal direction.
1) The walls are parallel to each other.
2) Walls are widely divergent.
3) Divergence not exceeding 5o
from the vertical plane.
When forces are applied at an angle other than right angle, force is
resolved in 2 ways, one of which reacts in its effectiveness at right angle to the
surface. This force is not opposite in direction, nor is it equally magnitude to
the original force. The tendency in a tooth is for the cusp of the opposing tooth
to slide down the inclined plane or for an inlay to be pushed out of the cavity in
a horizontal plane.
When a vertical force is applied to a proximal extension the filling is
rotated occlusoproximally out of its cavity. The rotation point of fulcrum being
gingival marginal wall. These forces are always effective unless counteracted
by an opposing movement. This can be achieved by properly prepared occlusal
lock, by proper preparation of gingival wall, pulpal wall and lastly by the
proper contour and contact point.
Slice :
Slice preparation provides external support of weakened tooth or areas
subjected to high stresses during function. It increases the resistance and
retention form by exposing a larger amount of tooth structure to the frictional
grasp of the restoration.
70

Occlusal Dove Tail : Tensile stresses developed by this is one of the strongest
means of resisting the displacement of an inlay. Clinical precaution demands
that by lingual inclined planes which extend into the isthmus of the occlusal
block be on sound cusps with a sufficient amount of supporting dentin. If these
are lacking, there is likelihood, of fracture of one or both the cusps whenever
inlay is subjected to horizontal forces.
Now the buccal and lingual axial walls, instead of flaring from the axial
line angles to the cavosurface margin in a continuous plane, are now changed
into two narrower but parallel planes and two smaller diverging planes. It is
evident that in this type of preparation, it is possible to retain the retentive form
of the preparation, even if the walls diverge in a continuous plane, when stress
is applied to the occlusal surface, the reaction of the opposite forces will tend to
dislodge the filling. So retention in this type of preparation is by placing a
gingival groove in the gingival wall and by adding an occlusal lock. Hence
effort is made to parallel at least part of the buccal and lingual proximal walls
that lie in dentin.
Second method of resisting horizontal displacing forces is by the proper
preparation of gingival walls. The properly prepared gingival groove assist in
preventing the lateral displacement of an inlay. But because of the inherent
weakness of the gingival groove the possible fracture to this wall of the tooth
structure between the groove an the cavosurface angle, so many operators
prefer the inward beveling of the gingival wall, forming an acute angle between
the axial and gingival walls.
Pulpal Wall : Third method of obtaining opposing movements to horizontal
displacing force is by establishing resistance into pulpal wall. The pulp wall
which is flat offers no resistance to horizontal displacement when it is prepared
with two inclined planes it will prevent the lateral displacement of the inlay.
71

Another modification is placement of grooves parallel to the long axis of
the tooth at the axial angles. Such preparation resist horizontal displacement of
the inlay. This will also resist rotary displacement because of the frictional
resistance of the dentin at this point of the cavity.
In addition to increased mechanical retention resulting from slight
modification of cavity preparations, it is essential that suitable gold alloys be
used and casting made of such alloys be properly heat treated in order that their
maximum physical properties are made available.
Axiopulpal Line Angle :
This line angle is slightly rounded to dissipate the stresses.
Gingival Bevel :
30-45o
to have sliding lap fit joint, cement tooth interface.
Certain forces collectively act on a cemented restoration mainly in the same
direction as the path of withdrawal.
Some of the factors pertaining to these forces are :
1) Magnitude of the dislodging forces : Forces that tend to remove a cemented
restoration along its path of withdrawal are small compared to those that
tend to tilt it. Generally exceptionally sticky food stuffs act as a pulling
force. The quantum of vertical and oblique forces also tend to dislodge the
restoration. The magnitude of the dislodging forces depends on the
stickiness of food, occluding and lateral movement forces of the jaws and
the surface area and texture of restoration being pulled.
2) Stress Concentration : Stresses are not uniform throughout the cement but
are concentrated around the junction of the axial and occlusal surfaces (axio
pulpal line angle). This may explain the retentive failure of the cast
restoration. The strength of the cement is less than the induced stresses.
72

FORCES ACTING ON DIRECT TOOTH COLOURED
RESTORATIONS
For any proximal restoration in anterior teeth, there are two possible
displacing forces. The first is a horizontal force displacing or rotating the
restoration in a labio-proximo lingual or linguo proximo labial direction. It
has its fulcrum almost parallel to the long axis of the tooth being loaded. The
second is a vertical force displacing or rotating the restoration
proximally(sometimes facially or lingually). The latter has a loading
arrangement similar to occluso-proximal (occluso-facial or occluso-lingual)
restorations in posterior teeth. The amount of teeth depends upon the location,
extent and type of occluding contacts between the upper and lower teeth during
function.
The mechanical picture can be summarized as follows :
1. In anterior teeth with normal overbite and overjet during centric closure of
the mandible (from centric relation to centric occlusion), mainly the
horizontal forces will be in action. Those forces, if loading the proximal
restoration directly, would try to move it linguo-proximo labially (for the
upper restoration) and labio-proximo lingually (for the lower one). The
magnitude of the horizontal force component 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 angel. The results of this loading are
rotational forces (previously described), as well as forces rotating the
restoration labially and proximally (for the upper) or lingually and
proximally (for the lower).
2. If anterior teeth meet in edge-to-edge fashion at centric occlusion, loading
of the proximal restoration, involving incisal angles (Class IV) will be
similar to any Class II proximo-occlusal restorations, i.e. vertical displacing
73

forces with very limited horizontal components. This loading will continue
during all centric closures and excursion movements of the mandible.
However, if the incisal angle is intact (Class III), these displacing forces
will be minimal.
3. If the upper and lower anterior teeth meet such that the lowers are labial to
the uppers in centric occlusion (Angle’s Class III), there will be the same
type of loading conditions mentioned in (1) except the horizontal loading
will tend to rotate or displace restorations labio proximo lingually (for
uppers) and linguo-proximo labially (for lowers). During excursive
movements, if teeth are in contact and there is a possibility of retrusive
mandibular movements, the loading will be much less than that described in
(1), with its horizontal displacement capability exactly the reverse to that
described in (1).
4. In occlusions with deep anterior overbite and normal or no overjet, the
horizontal type of loading will be greatly exaggerated. The vertical
displacement, although present, will be minimal by comparison.
5. In occlusion 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.
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 disclusion mechanism, the teeth and
restoration will be part of that disclusion mechanism with excessive
horizontal and vertical loading forces. This situation should be properly
diagnosed, so that the tooth preparation can be designed and prepared
accordingly. It should be understood that none of these loading forces work
separately. They work together and simultaneously. However, they may
differ in magnitude at different stages of mandibular movement. It should
be mentioned here that a restoration replacing part or all of the incisal
74

ridges of an anterior tooth will have the same pattern of loading as
mentioned in (1) – (6), but with increased intensity. Loss of the 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
restorations. This loss will lead to definite direct loading of the restoration,
loss of the incisal wall which would normally accommodate one of the two
possible main internal retentive modes for 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, and it is with the minimal tooth structure to be used for
resistance and/or retention against such loading.
The structure of anterior teeth themselves, have a comparatively different
stress pattern, as a result of occlusal loading from that of posterior teeth.
The unique shape as well as the mechanical structure and function of these
teeth is very important to comprehend before designing a cavity and/or
tooth preparation for a direct tooth colored restoration. The following is a
summary of these unique features :
a) Anterior teeth have their maximal bulk gingivally. They taper incisally
with the least bulk at the incisal ridge. So resistance to stress fractures
will be maximum at the gingival end and decrease incisally.
b) Forces are directed horizontally and vertically on anterior teeth as
mentioned with the force analyses on restorations for these teeth. These
forces accumulate maximal shear stresses at the junction of the clinical
root with the clinical crown and maximum tensile stresses at the incisal
ridges, especially their corners (incisal angles).
c) The labial enamel plate is much thicker than the lingual or proximal
ones, with maximal thickness of enamel usually at the incisal ridge.
d) The incisors may be involved in a disclusion mechanism of the
mandible with loading similar to that of the cuspid, but to a much lesser
extent.
75

e) Cervical portions of anterior teeth when they are affected with a Class V
lesion or cavity preparation will have a stress pattern similar to posterior
teeth, and the stress pattern is governed by the same factors as in
posterior teeth. In addition, the deeper the overbite is, the more induced
the stresses are at these cervical areas.
f) AS mentioned previously, loss of an axial angle, incisal angle, or tooth
structure at the neck of the tooth will dramatically reduce that tooth’s
ability to resist loading without fracture failure.
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
four intact walls surrounding the restoration. Unfortunately, this is usually not
the case. That is why the clinical performance of tooth colored materials
differs from one situation to another, sometimes dramatically.
Anterior teeth have their maximal bulk gingivally. They taper incisally
with the lest bulk at the incisal ridge. So resistance to stress fractures will be
maximum at the gingival end and decrease incisally.
Forces are directed horizontally and vertically on anterior teeth as
mentioned with the force analyses on restorations for these teeth. These forces
accumulate maximal shear stresses at the junction of the clinical root with the
clinical crown and maximum tensile stresses at the incisal ridges, especially
their corners (incisal angles).
The labial enamel plate is much thicker than the lingual or proximal
ones, with maximal thickness of enamel usually at the incisal ridge. The
incisor may be involved in a disclusion mechanism of the mandible with
loading similar to that of the cuspid, but to a much lesser extent.
76

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 four
intact walls surrounding the restoration. Unfortunately, this is usually not the
case. That is why the clinical performance of tooth colored materials differs
form the situation to another, sometimes dramatically.
77

FORCES ACTING ON POSTS
An endodontically treated tooth has been structurally compromised by
caries and its removal, prior restorations, and finally, endodontic preparation
and filling.
It should be emphasized again that posts are only used for retaining the
restorative material in the remaining tooth structures, and by no means will
they reinforce or improve the strengths of these tooth structures.
Because the retention of posts is accomplished by various means, it
might be expected that different stresses are associated with post installation.
With posts retained by the cement alone, the main potential for installation
induced stresses is the build up of hydrostatic back pressure. This potential
with parallel – sided post is circumvented by means of longitudinal vents along
the posts, which provide an outlet for the pressure. Tapered post are self-
venting, and consequently there is no pressure build up.
Endodontic posts provide a protection function through their ability to
distribute the forces of mastication to the remaining tooth structure. How well
this protection is achieved depends upon post design, embedment depth and
diameter.
MECHANICAL ASPECTS OF POST-RETAINED RESTORATIONS
AND FOUNDATIONS :
A. Stressing Capabilities of Posts :
The following features and factors of posts and the involved tooth will
govern the stress patter induced in the surrounding tooth structures due to the
use of posts as retentive means :
1. Type of Posts :
Parallel sided posts will have the tendency to evenly distribute the forces
it receives at and around its cavity end onto the root canal walls, if these forces
are applied parallel (a) to the post axis (vertical occlusal loading. IF the forces
78

applied are at a right angle (b) or oblique (c) to the post axis, the induced
stresses in the root canal walls will be unevenly distributed, i.e. there is a great
possibility of stress concentration due to uneven thickness of the root canal
walls around the post (root taper) while the post remains the same diameter.
This leads to a thin sectioned wall at the very apical end of the post.
On the contrary, taper sided posts and combination type posts will
concentrate stresses due to apical loading (a) in the root canal walls resulting
from its wedge shape. Lateral loading on and around cavity ends of the post,
however, will induce evenly distributed stresses in the root canal walls for the
taper of the post will correspond with the root and root canal taper, leading to
an even thickness of walls occlusoapically.
2. Method of Inserting root canal posts :
During insertion of a post into the root canals, highly threaded posts can
induce ten times the amount and extent of stresses as smooth sided posts.
Serrated surface posts will induce about one and a half to two times the stresses
that are induced by smooth surfaced posts. This can be explained by the
cemented technique utilized by the serrated and smooth surfaced posts.
3. Bulk of dentin in root canal walls :
Naturally, the bulkier that the dentin surrounding a root canal post is, the
less will be the induced stresses per unit volume during the post insertion and
functional use of the post retained restoration. It has been estimated that a
minimum of 2 mm of dentinal root canal wall should surround a post, so that
the stresses induced there will not lead to dentinal failure in the form of cracks
and gross fracture.
4. Length of clinical root involved with the root canal post :
Although the tooth to receive a root canal post should be non-vital and
endodontically treated, the clinical crown portion of the tooth is much more
dehydrated than the clinical root portion as the dentin portion of the root still
79

FORCES ACTING ON DENTAL RESTORATIONS

FORCES ACTING ON DENTAL RESTORATIONS

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