Force acting on restoration

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
 Conclusions

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.
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).
2

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 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.
 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
3

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.
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.
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
4

• 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
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.
5

Compressive Stress :
- Produced by a load that tends to compress the body.
- 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.
 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
6

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 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.
Biomechanical Unit :
The standard biomechanical unit involves the
1. Restorative material
2. Tooth structure, and
3. Interface between the restoration and tooth
7

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 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
8

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.
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 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
9

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 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
10

interconnected at a finite number of points or nodal points. These 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 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
11

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.
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
.
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.
12

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 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.
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.
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
13

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.
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.
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.
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.
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.
14

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.
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 almost 15%
iii) By crossing one marginal ridge at 1/3 the intercuspal distance, there is almost
30% loss of a tooth’s resistance to splitting.
15

iv) By crossing two marginal ridges by 1/3 the intercuspal distance, there is almost
35% loss of a tooth’s resistance to splitting.
v) By crossing one marginal ridge at ½ the intercuspal distance, there is almost 40%
vi) By crossing two marginal ridges at ½ the intercuspal distance, there is almost 45%
vii) By crossing a crossing ridge at ¼ the intercuspal distance, there is almost 20%
viii) By crossing a crossing ridge at 1/3 the intercuspal distance, there is almost 35%
ix) By crossing a crossing ridge at ½ the intercuspal distance, there is almost 45%
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.
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
16

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.
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.
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.
17

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.
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.
i) AMALGAM RESTORATIONS :
 Sufficient bulk of amalgam is mandatory when restoring a cavity with amalgam so as
to withstand the load of occlusion.
18

 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 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.
19

 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.
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 likely to occur. The opposing interproximal surfaces of
restorations must be hard in order not to flow, flatten, wear or become abraded with
use.
20

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 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 : -
21

• 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.
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.
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.
22

Forces acting on inclined planes.
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.
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 :
 When mandible moves form lateral to centric occlusion, the resultant of forces acting
is not vertical but inclined medially.
23

 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.
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 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.
24

 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
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, 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.
25

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.
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.
26

 If we take depth of gingival wall (d) into account, then R and d will be in the same
direction, so their moment of force remains zero.
 It is presumed in MOD preparations that the length of the axial wall (l) is kept equal
on 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.
 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 :
27

 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.
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 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 :
28

 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.
29

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 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
30

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.
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.
31

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 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.
Concentrated shear stresses will occur at the junction of the surrounding tooth
structure and corresponding floors, with a tendency towards failure there. This
32

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.
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.
33

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.
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.
34

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 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.
35

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 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
36

b. Each portion of the amalgam should be completely immobilized with retention
modes.
c. Amalgam should be seated on a flat floor or table in these areas.
d. Amalgam replacing cusps or axial angles should have a bulky connection to the main
part of the restoration with similar design features as for the isthmus areas.
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.
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
37

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)
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.
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 :
38

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 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).
39

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.
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).
40

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.
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
41

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.
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
42

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.
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 forces with very limited
horizontal components. This loading will continue during all centric closures and
43

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 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
44

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.
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.
45

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.
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.
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.
46

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 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 :
47

Force acting on restoration

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to Force acting on restoration

Similar to Force acting on restoration (20)

More from Indian dental academy

More from Indian dental academy (20)

Recently uploaded

Recently uploaded (20)

Force acting on restoration