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.

1

FORCE

The general concept of force is gained through the muscular action of

pushing or pulling on an object. When there is a tendency to change the

position of rest as the motion of a mass, it is said that a force is applied.

A force always has a direction and the direction is often characteristic of the

type of force.

If the body to which the force is applied remains at rest, the force causes the

body to deform.

Units of force are the pound or the kilogram or Newton.

FORCES ON DENTAL STRUCTURES :

One of the most important applications of physics in dentistry is in the

study of forces applied to teeth and dental restorations. There are numerous

reports in the dental literature that describe the measurement of biting forces on

teeth. The maximum forces reported have ranged form 200 to 2440 N (45 to

550 lb).

Numerous instruments have been used to make this measurements,

including strain gauges and telemetric devices small enough to be incorporated

into dental restorations.

NORMAL BITING FORCES :

Experiments conducted on adults have shown that the biting force

decreases form the molar region to the incisors. Studies have revealed that four

2

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.

3

The responses by the teeth to the forces depend on such factors as the shape

and length of the roots the characteristics of the fluid content of the

periodontal space, the composition and orientation of the periodontal fibres

and the extent of the alveolar bone.

The responses by the teeth will also depend on the consistency of the bolus

being chewed and the muscular forces being used to crush it. This will also

apply to parafunctional clenching and chewing with or without a foreign

body between the teeth. It is, therefore, difficult to assess what is a normal

response to a force on a tooth and what is potentially harmful. As a result

of these forces, a tooth can be displaced in one of six directions : - apically,

mesiodistally or buccolingually, and each one producing a rotation or a

translation.

The result is likely to be a combination of all directions leading to an

omnidirectional movement. The same principle of movement will apply to

the opposing tooth involved.

OMNIDIRECTIONAL AND UNIDIRECTIONAL RESPONSES :

These omnidirectional tilting and rotations of teeth will reach a limit

when an equal and opposite resistance is reached and the periodontal receptors

cause a reflex arrest of the muscle force. When the force is removed, the teeth

will recover their positions due to the elastic recovery of the compressed

periodontal tissues. This is referred to as “replacement” of the teeth.

4

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.

5

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.

ForceStress = ---------

Area Area over which the force acts is an important factor of consideration

especially in dental restorations in which areas over which the force applied

often are extremely small. Since stress at a constant force is inversely

proportional to the area, the smaller the area, the larger the stress. And vice

versa.

Technically, stress is the internal resistance of the body in terms of force per

unit area and is equal and opposite in direction to the force (external)

applied. This external force is also known as load.

TYPES OF STRESSES :

Depending upon the nature of the force, all stresses can be divided into 3

basic types which are recognized as ;

i. Tension

ii. Compression and

iii. Shear

6

1) Tension : Results in a body when it is subjected to 2 sets of forces that are

directed away from each other in the same straight line.

F

F

2) Compression : Results when the body is subjected to 2 sets of forces in the

same straight lien and directed to each other.

F

F

3) Shear : Is a result of 2 forces directly parallel to each other.

S F

F

Tensile Stress :

- Is caused by a load that tends to stretch as elongate a body.

- The molecules making up the body must resist being pulled apart.

Compressive Stress :

- Produced by a load that tends to compress the body.

- The molecules resist being forced more closely together.

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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.

8

Strain is deformation per unit length.

During loading, bonds are generally not compressed as easily as they are

stretched. Therefore, materials resist compression more readily and are said to

be stronger in compression than in tension. Materials have different properties

under different directions of loading. “It is important to determine what the

clinical direction of loading is before assessing the mechanical property of

interest”.

As loading continues, the structure is deformed. At first this

deformation (or strain) is completely reversible (Elastic strain). However,

increased loading finally produces some irreversible strain as well (plastic

strain), which causes permanent deformation. The point of onset of plastic

strain is called the elastic limit. Continuing plastic strain ultimately leads to

failure by fracture. The highest stress before fracture is the ultimate strength.

The total plastic tensile strain at fracture is called the elongation.

The slope of the linear portion of the stress strain curve is called the

modulus, modulus of elasticity, young’s modulus, or the stiffness of the

material.

Two of the most useful mechanical properties are the modulus of

elasticity and elastic limit. A restorative material generally should be very stiff

so that under load, its elastic deformation will be externally small. An

exception is Class V composite which should be less stiff to accommodate

9

tooth flexure. If the stress is well beyond the elastic limit, then the resulting

deformation is primarily plastic strain and at some point ultimately results in

failure.

Often it is convenient to determine the elastic limit in a relative manner

by comparing the onset of plastic deformation of different materials using

scratch or indentation tests, called hardness tests.

The energy that a material can absorb before the onset of any plastic

deformation is called its resilience, and is described as the area under the stress-

strain curve up to the elastic limit. The total energy absorbed to the point of

fracture is called the toughness and is related to the entire area under the stress

strain curve.

Time-dependent responses to stress or strain also occur. Deformation

with time in response to a constant stress is called creep (strain relaxation).

Deformation overtime in response to a constant strain is called stress

relaxation.

BIOMECHANICS FOR RESTORATIVE DENTISTRY :

Teeth are subjected to many forces during normal use. The interactions

between the applied forces, the shape and structure of teeth, the supporting

structures, and the mechanical properties of tooth components and restorative

materials are all included in the subject of biomechanics.

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Biomechanical Unit :

The standard biomechanical unit involves the

1. Restorative material

2. Tooth structure, and

3. Interface between the restoration and tooth

The importance of considering three structures in the biomechanical unit

is to detect stresses that may cause unwanted fractures or debonding. The

restorative material may be strong enough to resist fracture, but the interface or

tooth structure may not be.

STRESS TRANSFER :

Normal tooth structure transfers external biting loads through enamel

into dentin as compression. The concentrated external loads are distributed

over a large internal volume of tooth structure and the local stresses are lower.

During this process a small amount of dentin deformation may occur which

results in tooth flexure.

A restored tooth tends to transfer stress differently than an intact tooth.

Any force on the restoration produces compression, tension, or shear along the

tooth restoration interface. Once enamel is no longer continuous, its resistance

is much lower. Therefore, most restorations are designed to distribute stresses

onto sound dentin, rather than to enamel. The process of stress transfer to

dentin becomes more complicated when the amount of remaining dentin is thin

11

and the restoration must bridge a significant distance to seat onto thicker dentin

(Liners or bases).

TOOTH FLEXURE :

Tooth flexure has been described as either a lateral bending or an axial

bending of a tooth during occlusal loading. This flexure produces the maximal

strain in the cervical region, and the strain appears to be resolved in tension or

compression within local regions, causing the loss of bonded class V

restorations in preparations with no relative grooves. Moreover, one current

hypothesis is that tensile or compressive strains produce microfractures (called

ABFRACTIONS) in the thinnest region of enamel at the CEJ. Such fractures

predispose enamel to loss when subjects to tooth brush abrasion and/or

chemical erosion. This process may be key in the formation of Class V defects.

PRINCIPLES OF BIOMECHANICS :

Stress transfer and the resulting deformations of structures are

principally governed by :

1. The elastic limit of the materials

2. The ratio of the elastic moduli involved

3. Thickness of the structures

Materials with a high elastic modulus transfer stresses without much

strain. Lower modulus materials undergo dangerous strains where stresses are

concentrated, unless there is adequate thickness.

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STRESS ANALYSIS AND DESIGN OF DENTAL STRUCTURES

The mechanical properties of a material used in a dental restoration must be

able to withstand the stresses and strains caused by the repetitive forces of

mastication. The design of dental restoration is particularly important if the

best advantage of a material is to be taken. It is necessary to use designs

that do not result in stresses or strains that exceed the strength properties of

a material under clinical conditions.

Stresses in dental structures have been studied by such techniques as brittle

coatings, strain gauges, two and three-dimensional photoelasticity, and

finite element analysis. Stress analysis studies of inlays, crowns, bases

supporting restorations, fixed bridges, complete dentures, partial dentures,

and implants have been reported.

a) Two Dimensional Photoelasticity :

The procedure for two-dimensional models is to prepare a transparent

plastic or other isotropic model of the restoration or appliance. This model is

usually larger than the actual size. The material becomes axis atropic when

stressed, and so the behaviour of light is affected by the direction it takes.

As a result of the applied stress, the plastic model exhibits double

refraction because of its an isotropic structure. The light from the source

passes through a polarizer, which transmits light waves parallel to the

polarizing axis, or plane polarized light. The plane polarized light is converted

to circularly polarized light by a quarter wave plate, and this polarized beam is

split into two components travelling along the direction of principal stress in

the model. Depending on the state of stress in the model, the two beams travel

at different rates. After the light emerges form the model, it passes through a

second quarter – wave plate, which is crossed with respect to the first, and an

analyzer that is most frequently perpendicular to the polarizer. The

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interference pattern may be recorded photographically, which is the

isochromatic fringe pattern. These isochromatic fringes, or dark liens,

represent locations where the difference in the principal stresses is a constant.

The magnitude of the stress can be determined by identification of the order of

the isochromatic fringes.

The fringe order multiplied by a constant and divided by the thickness

of the model gives the value of the differences in the principal stresses. Areas

in the model where the fringer are close together are under higher stress

gradients than areas where there are fewer fringes, and areas containing fringes

of higher order are under higher stress than these having fringes of lower order.

A two dimensional photoelastic model of a second molar with a gold

crown is analyzed. The elastic modulus of the plastics used to represent the

gold, dentin and bone had the same relative values as the actual materials. The

crown was luted to the tooth with dental stone, and a layer of silicone rubber,

simulating the periodontal membrane, separated the tooth from the bone. A

force of 266 N (60 lb) was applied 30 degrees to the axis of the tooth at a single

site on the mesial cusp, and the isochromatic fringes were photographed.

High stresses are apparent under the contact and in the bone at the tip of the

mesial root (seven fringes). Considerably lower stresses occurred in the bone

just under the distal root and at the crest of the ridge on the mesial side.

The effect of various configurations of the proximal margins was

studied by two-dimensional photoelasticity on the stress distribution in Class II

14

inlays. Light field isochromatic fringes for rounded shoulder and shoulderless

models under a 445 N load were analyzed. The load was applied at 3 other

locations :

i) At the groove in the restoration

ii) On the cusp

iii) At the junction of the restoration and the tooth

The maximum shear stress was determined at nine critical areas, tow in

the restoration, two in the tooth and five at the junction of the restoration and

the tooth.

The study showed that the chamfer and rounded type of preparations are

the optimum designs in proximo-occlusal posterior restorations, since they

demonstrated the lowest stress when loaded vertically. The maximum fringe

order for the rounded shoulder was 10 whereas that for the shoulderless

preparation was 17. It was also shown that rounding the axiogingival line

angle in the shoulder geometry reduced the stress concentration factor by upto

50%. The gingival area of the proximal shoulder was the area of high stress,

and extra retentive features such as pins or grooves should not be placed in this

area.

FINITE ELEMENT STRESS ANALYSIS :

The finite element is a newer method than photoelasticity and offers

considerable advantages. In this method a finite number of discrete structural

elements are interconnected at a finite number of points or nodal points. These

15

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

16

base. When the stresses induced in the amalgam by a zinc phosphate base of 2

mm are considered in relation to those induced by a dentin floor alone one can

see that replacement of dentin by zinc phosphate to a depth of 2 mm does not

result in any significant increase in the tensile stress induced in the amalgam.

The zinc oxide eugenol cement base unlike the zinc phosphate cement bar,

does not function as rigid material and induces a larger displacement.

In comparison with zinc phosphate cement base the zinc oxide eugenol

material does not have adequate mechanical properties to support a restoration.

Even thin layers (0.5 mm) of zinc oxide eugenol cement caused significant

changes in the stress induced n the amalgam. Therefore the study indicates that

the fracture of amalgam is influenced more by the modulus of elasticity

(Stiffness) of the base material than by the compressive strength of the base.

An ideal situation would be to have a cement base with a modulus of elasticity

equal to that of the restorative material. Also, a subsequent study found that

tensile and shear stresses occurring in the cement base were of sufficient

magnitude to exceed the strength of some cements.

The stress distribution in porcelain fused to metal and porcelain jacket

crowns was conducted using a finite element method. Design parameters of

rounding of shoulders, avoidance of sharp notches, minimum thickness of

metal copings, and minimum labial bulk of porcelain were incorporated into

the model of an upper central incisors.

17

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.

18

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.

19

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

20

are involved in a disclusive mechanisms, these stresses are

substantially increased.

iii) The axial angles and lingual marginal ridges will bear concentration

shear stresses. In addition, on the loading side, tensile stresses are

present, and on the nonloading side, compressive stresses are found.

iv) The slopes of the cuspid will bear concentrated stresses (3 types),

especially fi the cuspid is a protector for the occlusion or part of a

group function during mandibular excursions.

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v) The distal surface of a cuspid exhibits a unique stress pattern as a

result of the anterior components of force concentrating

compressive loading at the junction of the anterior and posterior

segments of the dental arch and microlateral displacement of the

cuspid during excursive movements. Both of these factors will lead

to tremendous stress concentration with resultant abrasive activity

there.

vi) The lingual concavity in upper anterior teeth bears substantial

compressive stresses during centric occlusion, in addition to tensile

and shear stresses during protrusive mandibular movements.

22

vii) The incisal edges of lower anterior teeth are subjected to

compressive stresses. In addition, tensile and shear stresses are

present during protrusive mandibular movement. The incisal ridges

of upper anterior teeth will have these same stresses during the mid-

protrusive and sometimes at the protrusive border location of the

mandible.

B) STRESS BEARING AND STRESS CONCENTRATION AREAS OF

POSTERIOR TEETH :

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

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ii) Marginal and crossing ridges bear tremendous tensile and compressive

stresses.

iii) Axial angles bear tensile and shear stresses on the non-functional side,

and compressive and shear stresses on the functional side.

iv) The junction between the clinical root and the clinical crown during

function (especially lateral excursion) bears tremendous shear stresses,

in addition to compression on the occluding contacting side and tension

on the non-contracting side.

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

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

functional occlusal contact with it.

C) WEAK AREAS OF TOOTH :

Weak areas in the tooth should be identified and recognized before any

restorative attempt, in order to avoid destructive loading. They are ;

i) Bifurcation and trifurcation area.

ii) Cementum should be eliminated as a component of a cavity wall. The

junction between the cementum of the dentin is always irregular, so the

dentin surface should be smoothed flat after cementum removal.

25

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.

26

SOME APPLIED MECHANICAL PROPERTIES OF TEETH:

1. Although the following figures are averages, they provide an idea about the

principal mechanical properties of tooth structure. It must be understood

that these figures can differ from one location on a tooth to another and

from one tooth to another.

a) Compressive strength of enamel supported by vital dentin is usually 36-

42,000 psi.

b) Compressive strength of vital dentin is 40-50,000 psi.

c) Modulus of resilience of enamel supported by vital dentin is 60-80 inch

– lbs/cubic inch.

d) Modulus of resilience of vital dentin is 100-140 inch – lbs/cubic inch.

e) Modulus of elasticity of enamel supported by vital dentin under

compression is 7,000,000 psi.

f) Modulus of elasticity of vital dentin is 1,900,000 psi.

2. In general, when enamel loses its support of dentin, it loses more than 85%

of its strength properties.

3. Tensile strength of dentin is about 10% less than its compressive strength.

4. Tensile strength and compressive strength of enamel are similar, as long as

the enamel is supported by vital dentin.

5. Shear strength of dentin is almost 60% less than its compressive strength,

and this is very critical in restorative design.

6. There is minimal shear strength for enamel when it loses its dentin support.

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7. When the dentin loses its vitality, there is a drop of almost 40-60% in its

strength properties.

VALE EXPERIMENT :

The original experiment involved preparation of occlusoproximal

cavities with different crossing dimensions at the marginal and crossing ridges

with a standard depth. The teeth were then subjected to measured occlusal

loads. The load that split the tooth was recorded and compared to the control,

which was the load that split a round tooth. Later, the same experiment was

repeated by several investigators using more sophisticated equipment than that

used by vale. The results were consistent.

A summary of their findings brought to the closest round figures is as follows :

i) By crossing one marginal ridge at ¼ the intercuspal distance, there is

almost 10% loss of a tooth’s resistance to splitting.

ii) By crossing two marginal ridges at ¼ the intercuspal distance, there is

almost 15% loss of a tooth’s resistance to splitting.

iii) By crossing one marginal ridge at 1/3 the intercuspal distance, there is

almost 30% loss of a tooth’s resistance to splitting.

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% loss of a tooth’s resistance to splitting.

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vi) By crossing two marginal ridges at ½ the intercuspal distance, there is

almost 45% loss of a tooth’s resistance to splitting.

vii) By crossing a crossing ridge at ¼ the intercuspal distance, there is

almost 20% loss of a tooth’s resistance to splitting.

viii) By crossing a crossing ridge at 1/3 the intercuspal distance, there is

almost 35% loss of a tooth’s resistance to splitting.

ix) By crossing a crossing ridge at ½ the intercuspal distance, there is

almost 45% loss of a tooth’s resistance to splitting.

OBTAINING RESISTANCE FORM FOR TOOTH STRUCTURES :

1) To best resist masticatory forces, use floors or planes at right angles to the

direction of loading to avoid shearing stresses.

2) If possible, walls of preparations should be parallel to the direction of the

loading forces, in order to minimize or avoid shearing stresses.

3) Intracoronal and intraradicular cavity preparations can be done in box, or

cone or inverted truncated cone shapes.

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Thus from the above figures, it is possible to deduce that the inverted

truncated cone shapes will have a higher resistance to loading than the box

shapes, and the box shapes will have a higher resistance than the cone shapes.

Therefore, if conditions and requirements allow, cavity preparations should be

prepared in an inverted truncated cone shape.

4) Definite floors, walls and surfaces with line and point angles are essential to

prevent micromovements of restorations, with concomitant shear stresses

on remaining tooth structures.

5) Increasing the bulk of a restorative material or leaving sufficient bulk of

tooth structure in critical areas is one of the most practical ways of

decreasing stresses per unit volume.

Load – A Load A

10 stress units/mm3

1 stress unit / mm3

6) Designing the outline form with minimal exposure of the restoration surface

to occlusal loading will definitely minimize stresses and the possibility of

mechanical fracture in the restoration.

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7) Junctions between different parts of the preparation, especially those acting

as fulcra, should be rounded in order to minimize stress concentration in

both tooth structure and restorations and to prevent any such sharp

components from acting as shear lines for fracture future.

8) Retentive features must leave sufficient bulk of tooth structure to resist

stresses resulting from displacing forces.

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OCCLUSAL CONSIDERATIONS IN RESTORING TEETH

The way we occlude teeth affects the periodontium, the

temporomandibular joints, throat muscles, tongue, cheeks, lips, nerves and so

son. The occlusion of the restored teeth should hence establish healthy

relations between the dentition and rest of the stomatognathic system. A

clinician must have adequate knowledge about the principles of occlusion,

which enables him to diagnose cases that need modifications / alteration of

occlusion with or without the use of various restorative materials. Before

initiating any restorative care, thorough occlusal examination should be carried

out.

The kind of occlusion, a patient should have, must be justified by the

principles of physiology.

The occlusion affects almost every part of stomatognathic system,

mainly :

a) The pulp of the tooth is a very sensitive organ. IT reacts immediately to

abnormal occlusal forces. Hence, occlusion should not be detrimental to

pulp.

b) The proximal relations of the occlusion should prevent food impaction

between teeth.

c) The cusp-fossa relationship should be such that the adequate forces exerted

during functional movement, aids in optimum health of the periodontium.

32

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.

33

i) AMALGAM RESTORATIONS :

Sufficient bulk of amalgam is mandatory when restoring a cavity with

amalgam so as to withstand the load of occlusion.

Adequate thickness of amalgam should be provided at the marginal ridges

in order to support the opposing supporting cusps.

Amalgam restorations are carved following the cuspal inclines.

In case of large restorations, where there are no cuspal planes to guide

carving, the operator should follow a cautious approach :

Buccal and lingual cusp tips should be placed in lines joining those of

adjacent teeth.

The level of central fossa and the marginal ridge should be carved

similar to that of adjacent teeth.

The bucco-lingual width of the occlusal surface is kept narrower than

the original buccolingual width of the tooth.

In case both the working cusps on more than 2 cusps are restored,

preferably the occlusal table is kept narrowed.

This narrower occlusal table leads to :

Reduction of force : When the occlusal table is made narrower, lesser force

is applied over the same to undergo masticatory functions. Force is

transmitted to all structures underlying the occlusal table, which include the

restoration, the tooth structure and the periodontium.

Reduction of the effect of force : The direction in which the applied force is

transmitted is governed by muscular activities and the area on which the

34

force is applied. However, the effect of the force may be modified by

altering the surface at which the force is applied, thus altering the direction

of resolved components.

Reduction of torque : The tendency to rotate may be reduced by altering the

point of application of the force relative to the fulcrum. The point of

application of the force may be altered by modifying the occlusal table

which indirectly depends upon the design of the cavity and the restoration.

ii) CAST METAL RESTORATIONS :

Similar to amalgam restorations, before preparations of any tooth, evaluate

the occlusal contacts of the teeth in centric occlusion and in excursive

movements. As part of this evaluation decide if the existing occlusal

relationships can be improved with the cast metal restorations.

The cuspal interferences are depicted in mandibular working movements,

non working movements and protrusive movements.

The opposing occlusal surfaces should be examined an he malpositioned

cusps, plunger cusps and over erupted teeth should be recontoured.

Premature contacts or cuspal interferences from the teeth opposing the

required restoration should be removed.

The remaining tooth structure and the length of clinical crown dictates us to

choose the type of cast restoration.

35

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

36

likely to occur. The opposing interproximal surfaces of restorations must

be hard in order not to flow, flatten, wear or become abraded with use.

Relationship between tooth wear and restorative materials :

Occlusal forces lead to wear of enamel. The wear is, however, very slow if

occlusal forces are appropriately transmitted to underlying bony tissues.

The pattern of wear varies individually depending upon various factors.

Non-uniform ear of opposing teeth is quite common when one teeth is

restored with a restorative material whose wear resistance is different as

compared to that of enamel. Very rarely, the wear resistance of a

restorative material equal the wear resistance of enamel.

At present, no restorative material is available which wears at the same rate

as enamel or as enamel and dentin at later stages.

Differential wear can result in localization of occlusal loads with

subsequent failure of restorative materials or development of deflective

contacts with mandibular repositioning and an effect on a distant tooth.

Hypothetically, if two restorative materials, which wear at a slower rate

than the natural teeth, are placed so as to oppose each other in a dentition

undergoing wear, the restorations will produce occlusal interferences at a

later stage.

Non-wearing materials opposing each other can lead to natural teeth wear

during contact in lateral and protrusive movements.

Conversely, if the materials wear faster than the teeth, the opposing cusp

might over erupt into the worn material. IN lateral excursion, this cusp

37

might then come in contact with an opposing cusp and if weakened by

previous caries can lead to fracture.

Compensation for Occlusal Wear :

Occlusal interferences can develop through differential wear patterns and

unmatched compensatory mechanisms.

The clinician can shape and regulate the form of occlusal surfaces of teeth

and restorations so that he can determine surfaces of teeth and restorations,

which contact during activities such as mastication, swallowing and

bruxism.

The advantage of this approach are : -

The direction of stresses through the strongest portions of the restorations

an the remaining tooth structure can be arranged.

The effect of occlusal interferences developing from differential wear can

be minimized.

It is possible to maintain the partially restored dentition by means of

periodic adjustment.

Since wear defects are not repaired automatically, the dentist should replace

and maintain the configuration of teeth in accordance with the functional

activities.

38

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.

39

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

resolving force reacts perpendicular to the surface with an equal and opposite

force. If the surface is tilted at an angle to the horizontal, it still reacts at right

angle to the surface.

F

Surface F

Surface R

Thus, the reaction force no longer opposes the applied force in direction

nor is equal to its magnitude. Hence the forces are not in equilibrium when

applied on inclined planes.

The equilibrium can be maintained if more than one force is exerted on

tooth or the forces are resolved in both directions.

Forces acting on inclined planes.

40

AB is a tangent drawn at inclined plane or the contact between 2 cusps.

Angle ‘’ 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 ‘’ 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.

41

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 :

42

When mandible moves form lateral to centric occlusion, the resultant of

forces acting is not vertical but inclined medially.

When tough food is compressed or all cusps are in intimate contact at the 3

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

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

greater than Hab and the net resultant is Habc. So the horizontal component

is along the direction of c.

By using triangle of vector addition, the resultant is represented by Rabc.

The resultant Rabc is a thrust inclined palatally on the maxillary teeth and

buccally on the mandibular teeth, whose horizontal component is Habc.

MECHANICAL FUNCTIONS OF THE MARGINAL RIDGES :

Role of Marginal Ridges :

The marginal ridges play an important role in withstanding and dissipating

the occlusal stresses.

The correct form of marginal ridge compatible with the adjacent tooth and

also with its own surrounding is important during carving of posterior

restorations.

The absence of marginal ridge, or marginal ridge with improper height can

lead to altered dissipation of forces subsequently damaging the underlying

periodontium.

43

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

44

occur slight tilting of the tooth B. This will further deteriorate the

resolution of forces and lead to further food impaction.

3. A Marginal Ridge with a wider occlusal embrasure.

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

forces 1 and 2 act on adjacent teeth. The force 2 will put pressure on tooth

A and force 1 will put pressure on tooth B. This will lead to drifting of both

the teeth. The vertical components of forces will wedge the food is between

the two teeth.

Similar effect is seen when one marginal ridge is higher than other.

4. No occlusal embrassure

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

concentrated than horizontal components. Though there will to be any vertical

impaction of food, the continuous impact of higher concentration of vertical

component of forces may lead to changes in alveolar bone after sometime.

VERTICAL LOADS AND DISTRIBUTION OF STRESSES :

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

i) Parallel to the long axis and

ii) Perpendicular to the long axis

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

distribution depends upon various factors.

a) If the cross – section of that area is constant, stress distribution is

practically uniform.

b) If there is variation in cross-section (such areas are normally termed as

prisms); here stress varies form point to point, being inversely

proportional to area.

c) If change of cross-section area is abrupt; greater concentration of stress

occurs at that point.

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

This haring stress increases to a maximum at 45o and then decreases to zero

at 90o. Therefore, materials that are weaker in shear than in compression or

tension replace in planes at 45o to the axis.

The modulus of elasticity of the material is an important property and

should be taken care of. If a cavity is restored with gold inlay or porcelain,

46

the modulus of elasticity varies between the tooth and the restorative

material. With the vertical force exerting on both, the compression will be

the same for the restoration and the tooth, but since gold/porcelain is much

stiffer, they will be highly stressed, since S = dE.

S (Stress) = S (Unit strain) x E (Modulus of elasticity)

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

resolution is known as beam. Beam can be supported form both the ends

(simple beam) and may be supported form one end (cantilever beam).

Example of simple beam : MOD preparation

Example of Cantilever beam : MO / DO preparation

The retention of the restoration depends upon these beams, although the

strength and the deflection of the material also play part.

Moment of Force = Force x Perpendicular Distance

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

the restoration out of the cavity.

Gingival retention with a moment equal to F x L is required to counteract

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

Where l is the depth of the axial wall.

If we take depth of gingival wall (d) into account, then R and d will be in

the same direction, so their moment of force is zero. Therefore, the depth of

the gingival wall does not take part in retention.

47

In MOD Preparation :

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

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

the midpoint is :

F / 2 x L / 2 = FL / 4

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

the ends) then the moment of force :

FL 1 FL ----- x ----- = ----- 4 2 8

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

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

By equation R x l = FL / 8

So R = FL / 8 l

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

positive sign is used.

If we take depth of gingival wall (d) into account, then R and d will be in

the same direction, so their moment of force remains zero.

48

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.

49

Force (F) is applied at incline plane perpendicular to the tangent of the

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

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

by the horizontal component, which depends upon the height of the axial

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

APPLICATION OF STRESSES AND THEIR DISTRIBUTION IN

INDIVIDUAL RESTORATIONS :

1) Class I restoration

a) If restored with amalgam

It is recommended to converge the side walls occlusally and to keep the

floor flat.

In case the floor is not kept flat, the forces will rotate the restoration on

both the sides. And also, since the remaining dentin will be less at the

centre and if the restoration is deep, the forces might fracture the tooth.

b) Cast restorations :

Movement / rotation of the cast restoration is easy, if the pulpal floor is

not kept flat.

Since the walls are diverging occlusally, the chances of rotation are

much more.

c) Composites or GIC

These adhesive materials counteracts such rotational forces.

50

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

51

should be as close to the incisal edge as possible and still be in dentin to

reduce the stress in this lingual lock.

In maxillary teeth, force of mastication ahs labial component, which

provides the seating effect on the restoration. In case the labial enamel is

not intact, the chances of dislodgment of the restoration will increase. In

mandibular teeth, the component of the masticatory force is from the labial

to the lingual so chances of dislodgement of restoration are more.

4) Class V restorations :

Analysis indicates that physical forces putting on occlusal surfaces could

result in displacement of the restoration.

During occlusion, the vertical stresses on the teeth led to transverse stresses

and this component of stresses tends to rotate the cervical restoration.

The mandibular teeth bend more than maxillary teeth.

A gap is evident on the cervical / occlusal wall of the cavity and if the depth

of these walls is less, the restoration may come out.

52

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

53

microscopic, they occur frequently enough to encourage microleakage

around the restoration, predisposing to a recurrence of decay. These

movements can also lead to fracture of marginal tooth structure, and even to

splitting of lateral walls.

To solve these problems, flatten the pulpal end of the cavity preparation.

However, if accomplishing this at a deep location incurs increased risk of

involving the pulp chamber, pulp horns, or recessional lines containing

remnants of pulp tissues, make the pulpal floor at more than one level. One

level will be the ideal depth level (1.5 mm) and the others will be the caries

cone(s) level(s), dictated by the pulpal extent of the decay. The shallow level

creates the flat portion of the pulpal floor at definite angles to the surrounding

walls, adequately resisting occlusal forces and laterally locking the restoration,

without impinging on pulp tissues. Reiterating, the other level(s) is (are) only

necessitated by the caries extent., creating one or more concavities or cones in

the pulpal floor.

C. When a cavity wall comes in contact with a marginal ridge, the wall should

be divergent pulpo-occlusally, making an obtuse angle with the pulpal floor.

This design allows for maximum bulk of tooth structure supporting the

marginal ridge and avoids undermining of the marginal ridge, creating more

mechanical and biological problems.

D. If cariogenic conditions do not dictate otherwise, the width of the cavity

should be limited to ¼ to 1/5 the intercuspal distance (not less than 1.5 mm).

This minimizes loss of tooth structure in this critical cross-section of the tooth.

This width will also facilitate easy carving of the restoration, and minimize the

possibilities of occlusal interference’s.

E. All cavosurface angles should be right angled to create a butt-joint with the

marginal amalgam. This configuration allows marginal amalgam to withstand

stresses with the least possibility of failure.

54

F. All line and point angles, or any junction between different details in the

cavity preparation, should be rounded but definite. This design has all the

advantages of the mortise shape, while avoiding stress concentration in the

tooth structure and restorative materials that may occur from sharp angulations.

G.

Occluding forces will tend to move marginal amalgam and tooth

structures from position #1 to position #2. As vital tooth structures are more

deformable than set amalgam, the displacement will not be equal thereby

creating a gap between them. This places the marginal amalgam under

intolerable tensile loading which may lead to amalgam failure if the amalgam is

in thin cross sections, i.e. acute angled marginal amalgam will fracture. B, If

marginal amalgam is right angled, it can be stand induced stresses from

occlusal loading with less possibility of failure, even if the stresses are tensile

in nature.

CLASS II AMALGAM RESTORATION

By definition Class II cavity preparation is proximal preparations of

molars and premolars.

Resistance Form :

The fundamental concept of resistance form is based on the reaction

within the restoration and the remaining tooth structure to occlusal loading.

The objective of a cavity preparation design is to establish the best

possible configuration that can cope with the distribution and magnitude

of the stresses in both structure and the restoration without failure. To

55

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.

56

Concentrated shear stresses will occur at the junction of the surrounding

tooth structure and corresponding floors, with a tendency towards failure

there. This loading situation can be unilateral or bilateral, depending on the

mandibular movement it is the most deleterious to tooth structure especially

on the orbiting side if there is interference during lateral excursion.

4) Occluding facial elements contact facial and lingual parts of the restoration

surrounded by tooth structure, during centric and excursive movements.

This arrangement will induce tensile and compressive stresses in the

restoration which will be transmitted to the surrounding tooth structure.

5) Occluding cuspal elements contact facial or lingual parts of the restoration

completely replacing facial or lingual tooth structure during centric or

excursive movements.

The tensile stresses will be induced at the junction of the occlusal and facial

and/or lingual part of the restoration in both occluding situations.

6) Occluding cuspal elements contact a restoration’s marginal ridge(s) or part

of a marginal ridge during centric or excursive movements (assuming the

restoration is locked occlusally), there will be concentrated tensile stresses

at the (junction of the occlusal and facial or lingual parts of the restoration

at full intercuspation and to end from that position) at the junction of the

marginal ridge and the rest of the restoration. This will be true if its an area

of advance contact during mandibular closure.

7) Cuspal elements occlude or disclude via the facial or lingual groove of a

restoration. There will be tensile stresses at the junction of the occlusal and

facial or lingual parts of the restoration at full intercuspation, and to and

from that position.

8) Cusps and crossing ridges are part of the restoration in centric and excursive

movement.

Both will be subjected to compressive stresses during such positions and

movement. Besides tensile stresses could concentrate at their junction with

the main restoration, specially during contacting excursive movement.

57

9) Axial portions of the restoration during centric occlusion and excursive

movement contacts:

Whenever these portions are in contact with opposing occlusal surfaces,

there will be induced compressive and shear stresses when they are not

reciprocating (one side not in contact with occluding surfaces while other

axial portion). The axial surfaces will be stressed in a slight tensile and

shear pattern at their junction with the main bulk of the restoration.

10)Restoration is not in occluding contact or is in premature contact during

centric occlusion or excursive movement of the mandible. The first

situation is not conducive to function, insofar as the restoration will not be

involved with direct loading from the opposing occluding teeth. After a

period of time, however, the tooth will supraerupt, rotate, and or tilt,

establishing contact with the opposing cuspal elements. Usually, this newly

acquired location will not be the most favorable position for the restoration,

tooth, or the remainder of the gnatho stomatic system, either mechanically

or biologically. It is safer to build the restoration to predetermined

contacting areas with opposing teeth which will lead to predictable

physiologic stress patterns in the tooth structure and restoration.

Conversely, any portion of the restoration occluding prematurely will

tremendously exaggerate the same types of stresses normally induced in

that area of the restoration. Besides, additional shear components of stress

could be precipitated there. This, too, could lead to localized or generalized

gnatho stomatic disturbances, with eventual mechanical and biological

failures.

Needless to say, pre-existing premature contacting areas should be

eliminated before restorative treatment. This is done for many reasons, but

primarily, because cavity preparation increases the susceptibility of

remaining tooth structure to fracture failure. Besides, the restoration should

be built to the predetermined occlusal position, even fi the preexisting tooth

structures were not.

58

Several factors must be accommodated in designing Class II preparations

for amalgam. Occlusal loading is dynamic and cyclic in nature, which is a

far more destructive type of loading than static loading. Amalgam is least

resistant to tensile stress and most resistant to compressive stress. Tooth

structure, particularly when interrupted by a cavity preparation, is least

resistant to shear stress. Therefore, Class II cavity preparations for

amalgam restorations should be designed to resist cyclic loading while

minimizing tensile loading in the amalgam and shear loading in the

remaining tooth stricture.

B. Design features for the protection of the mechanical integrity of the

restoration :

1. Isthmus :

In the isthmus, i.e. the junction between the occlusal part of a restoration

and the proximal, facial or lingual parts, potentially deleterious tensile stresses

occur under any type of loading.

Most mathematical, mechanical and photoelastic analyses of these stresses

reveal three things :

1) The fulcrum of bending occurs at the axio-pulpal line angle

2) Stresses increase closer to the surface of a restoration, away from that

fulcrum, and

3) Tensile stresses concentrate at the marginal ridge area of a Class II

restoration.

Materials tend to fail, therefore, starting from the surface, near the marginal

ridge, and proceeding internally, toward the axio-pulpal line angle.

These problems may be solved by applying common engineering

principles. A theoretical solution might be :

1) to increase amalgam bulk at the axio-pulpal line angle, thereby, placing the

surface stresses away from the fulcrum. However, its actually results in

increased stresses within the restorative material and a deepened cavity

59

preparation, dangerously close to pulp anatomy. Therefore, such a solution,

in and of itself, is wholly unacceptable.

2) Another solution might be to bring the axio-pulpal line angle closer to the

surface, in an effort to reduce tensile stresses occurring near the marginal

ridge. However, this, too, is unacceptable in that the consequent diminished

bulk of amalgam would no longer adequately resist compressive forces.

3) A combination of the two solutions i.e. increasing amalgam bulk near the

marginal ridge, while bringing the axio-pulpal line angle away from that

stress concentration area and closer to the surface, can be achieved simply

by slanting the axial wall toward the pulpal floor.

a) The obtuse axial pulpal line angle thereby created not only provides

greater amalgam bulk in the marginal ridge area of the restoration, but

also reduces tensile stresses per unit area by bringing this critical area of

the preparation closer to the surface of the restoration. Furthermore, this

design feature improves accessibility to the proximal facial and proximal

lingual parts of the cavity during preparation procedures. This is the

first design feature.

b) Secondly, if the axio pulpal line angle is rounded, structural projections

or sharp junctions that may concentrate stresses at the isthmus would be

avoided. This second feature will also improve the visibility for the

facial and lingual gingivo-axial corners of the preparation proximally, as

well as increase the amalgam bulk at the fulcrum.

c) Thirdly, by slanting the axial wall, bulk is improved by increased depth

rather than increased width. Increasing the width at the isthmus portion

only increases the surface area receiving deleterious occluding stresses.

4) As a fourth design feature, the pulpal and gingival floors at the isthmus

should be perfectly flat in order to resist forces at the most advantageous

angulation.

5) The fifth design feature is that every part of the preparation (occlusal, facial,

lingual or proximal) should be self-retentive. If every part of the restoration

is locked in tooth structure independently from other parts, there will be

60

minimum stresses at the junction of one part with another, i.e. the isthmi.

This can be achieved in amalgam preparations by retentive grooves, internal

boxes, and undercuts.

6) Sixth, one should avoid, as much as possible, placing or leaving any surface

discontinuities, such as carved developmental grooves, scratches, etc at

these critical areas in the restoration. These can precipitate and accentuate

stresses leading to fatigue failure.

Finally, by checking occlusion to eliminate prematurities in the restoration,

immediate overloading and failure can be avoided.

2. Margins :

Amalgam has good compressive strength when it has sufficient bulk

(1.5 mm minimum). However, frail, feather edged margins of amalgam, which

will occur when the cavosurface angles of preparations are bevelled, will

fracture easily. Occluding forces will cause amalgam at the bevel to bend with

maximum tensile stresses, occurring as a result of elastic deformation of the

tooth structure beneath the bevel. Marginal excess of amalgam will similarly

fracture, leaving a ditch around the restoration that will enhance recurrence of

decay. So, for the margins of these p reparations, four design features should

be observed ; create butt joint amalgam tooth structure at the margins, leave no

frail enamel at the cavosurface margins, remove flashes of amalgam on tooth

surface adjacent to amalgam margins, and, as practically as possible, the

interface between amalgam and tooth structure should not be at an occluding

contact area with opposing teeth either in centric or excursive mandibular

movements.

3. Cuspal and Axial angles :

The following are the design features for these parts of a restoration.

a. Amalgam bulk in all three dimensions should be atleast 1.5 mm

b. Each portion of the amalgam should be completely immobilized with

retention modes.

c. Amalgam should be seated on a flat floor or table in these areas.

61

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.

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B. Proximal Displacement of the Proximal Portion :

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

arm of the L occlusally and the short arm proximally, when the long arm is

loaded by vertical force “V”, it will seat the restoration more into the tooth.

This is due to the elasticity of the dentin, especially in young teeth, wherein the

pulpal floor will change location from position 1 to position 2. However, since

metallic restorations are more rigid than the dentin, the short arm of the L will

move proximally, as shown in the figure. The fulcrum of this rotation is the

axio-pulpal line angle. In order to prevent such a displacement, proximal self-

retention in the form of facial, lingual and/or gingival grooves are required.

However, shear stresses will be induced at the junction between the amalgam

of the main restoration and that in the grooves. Therefore, it is to be

understood that these grooves are prepared only when there is complete

assurance that there will be sufficient dentinal bulk to accommodate them, and

that they will not impinge on the axial angle or on the pulp anatomy.

C. Lateral Rotation of the Restoration Around Hemispherical Floors (Pulpal

and Gingival)

63

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.

64

FORCES ACTING ON INLAY RESTORATION

The cavity should have such retention form that the restorations will be

firmly held in place, the cavity should also have resistance form that the

restoration will withstand the stress without being dislodged.

An understanding of the materials used in constructing an inlay, together

with a knowledge of correct manipulation is also an important factor in the

success or failure of an inlay (inlay is not only a part of mechanical structure

replacing lost teeth, but it is also intimately related to the vital tissues, it is the

medium through which mechanical and physical forces are translated into

physiological functions and biological reactions in living tissues.

The other preparation features that will help solve the mechanical

problems of cast restorations are as follows :

All the line and point angles should be definite, but not angular, so they

can be easily reproduced in a casting and to avoid stress concentration in the

casting and the tooth structure. The roundness must be substantial for Class V

materials.

The axial wall should slant toward the pulpal floor, as part of the taper.

This, together with rounding of the axio-pulpal line angle, can reduce stresses

at the isthmus area.

Reduction of tooth structure should follow the original anatomy of the

tooth, to create even reduction, with minimum tooth involvement, and even

physiologic distribution of forces applied on the restoration and remaining

tooth structure.

Maximum reduction should be at the occluding surfaces, especially the

parts of the tooth surfaces that are in contact during static and dynamic

relations of the mandible. An average of 1 mm should be cleared for metallic

65

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).

66

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.

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Axioproximal Walls (Facial / Lingual) :

Compressive forces resulting from vertical pressure have an important

bearing on the retention of the inlay. This bears on the relationship of the

buccal and lingual proximal walls. Now whether they should flare

axioproximally or be parallel to each other (that is the part of the wall lying

within the dentin).

There are 3 different relationships of wall A to wall B in the

gingivoocclusal direction.

1) The walls are parallel to each other.

2) Walls are widely divergent.

3) Divergence not exceeding 5o from the vertical plane.

When forces are applied at an angle other than right angle, force is

resolved in 2 ways, one of which reacts in its effectiveness at right angle to the

surface. This force is not opposite in direction, nor is it equally magnitude to

the original force. The tendency in a tooth is for the cusp of the opposing tooth

to slide down the inclined plane or for an inlay to be pushed out of the cavity in

a horizontal plane.

When a vertical force is applied to a proximal extension the filling is

rotated occlusoproximally out of its cavity. The rotation point of fulcrum being

gingival marginal wall. These forces are always effective unless counteracted

by an opposing movement. This can be achieved by properly prepared occlusal

lock, by proper preparation of gingival wall, pulpal wall and lastly by the

proper contour and contact point.

Slice :

Slice preparation provides external support of weakened tooth or areas

subjected to high stresses during function. It increases the resistance and

retention form by exposing a larger amount of tooth structure to the frictional

grasp of the restoration.

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Occlusal Dove Tail : Tensile stresses developed by this is one of the strongest

means of resisting the displacement of an inlay. Clinical precaution demands

that by lingual inclined planes which extend into the isthmus of the occlusal

block be on sound cusps with a sufficient amount of supporting dentin. If these

are lacking, there is likelihood, of fracture of one or both the cusps whenever

inlay is subjected to horizontal forces.

Now the buccal and lingual axial walls, instead of flaring from the axial

line angles to the cavosurface margin in a continuous plane, are now changed

into two narrower but parallel planes and two smaller diverging planes. It is

evident that in this type of preparation, it is possible to retain the retentive form

of the preparation, even if the walls diverge in a continuous plane, when stress

is applied to the occlusal surface, the reaction of the opposite forces will tend to

dislodge the filling. So retention in this type of preparation is by placing a

gingival groove in the gingival wall and by adding an occlusal lock. Hence

effort is made to parallel at least part of the buccal and lingual proximal walls

that lie in dentin.

Second method of resisting horizontal displacing forces is by the proper

preparation of gingival walls. The properly prepared gingival groove assist in

preventing the lateral displacement of an inlay. But because of the inherent

weakness of the gingival groove the possible fracture to this wall of the tooth

structure between the groove an the cavosurface angle, so many operators

prefer the inward beveling of the gingival wall, forming an acute angle between

the axial and gingival walls.

Pulpal Wall : Third method of obtaining opposing movements to horizontal

displacing force is by establishing resistance into pulpal wall. The pulp wall

which is flat offers no resistance to horizontal displacement when it is prepared

with two inclined planes it will prevent the lateral displacement of the inlay.

69

Another modification is placement of grooves parallel to the long axis of

the tooth at the axial angles. Such preparation resist horizontal displacement of

the inlay. This will also resist rotary displacement because of the frictional

resistance of the dentin at this point of the cavity.

In addition to increased mechanical retention resulting from slight

modification of cavity preparations, it is essential that suitable gold alloys be

used and casting made of such alloys be properly heat treated in order that their

maximum physical properties are made available.

Axiopulpal Line Angle :

This line angle is slightly rounded to dissipate the stresses.

Gingival Bevel :

30-45o to have sliding lap fit joint, cement tooth interface.

Certain forces collectively act on a cemented restoration mainly in the same

direction as the path of withdrawal.

Some of the factors pertaining to these forces are :

1) Magnitude of the dislodging forces : Forces that tend to remove a cemented

restoration along its path of withdrawal are small compared to those that

tend to tilt it. Generally exceptionally sticky food stuffs act as a pulling

force. The quantum of vertical and oblique forces also tend to dislodge the

restoration. The magnitude of the dislodging forces depends on the

stickiness of food, occluding and lateral movement forces of the jaws and

the surface area and texture of restoration being pulled.

2) Stress Concentration : Stresses are not uniform throughout the cement but

are concentrated around the junction of the axial and occlusal surfaces (axio

pulpal line angle). This may explain the retentive failure of the cast

restoration. The strength of the cement is less than the induced stresses.

70

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

71

forces with very limited horizontal components. This loading will continue

during all centric closures and excursion movements of the mandible.

However, if the incisal angle is intact (Class III), these displacing forces

will be minimal.

3. If the upper and lower anterior teeth meet such that the lowers are labial to

the uppers in centric occlusion (Angle’s Class III), there will be the same

type of loading conditions mentioned in (1) except the horizontal loading

will tend to rotate or displace restorations labio proximo lingually (for

uppers) and linguo-proximo labially (for lowers). During excursive

movements, if teeth are in contact and there is a possibility of retrusive

mandibular movements, the loading will be much less than that described in

(1), with its horizontal displacement capability exactly the reverse to that

described in (1).

4. In occlusions with deep anterior overbite and normal or no overjet, the

horizontal type of loading will be greatly exaggerated. The vertical

displacement, although present, will be minimal by comparison.

5. In occlusion with anterior open bite or severe overjet, or any other condition

that creates a no-contact situation between upper and lower anterior teeth

during centric occlusion and excursive movements of the mandible,

proximal restorations will not be loaded directly either vertically or

horizontally.

6. In cases when the proximal restoration of an anterior tooth is a part of a

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

involved in an anterior lateral disclusion mechanism, the teeth and

restoration will be part of that disclusion mechanism with excessive

horizontal and vertical loading forces. This situation should be properly

diagnosed, so that the tooth preparation can be designed and prepared

accordingly. It should be understood that none of these loading forces work

separately. They work together and simultaneously. However, they may

differ in magnitude at different stages of mandibular movement. It should

be mentioned here that a restoration replacing part or all of the incisal ridges

72

of an anterior tooth will have the same pattern of loading as mentioned in

(1) – (6), but with increased intensity. Loss of the incisal angle of a tooth,

i.e. conversion from a Class III to a Class IV represents a major

complication in the mechanical problems of anterior tooth restorations.

This loss will lead to definite direct loading of the restoration, loss of the

incisal wall which would normally accommodate one of the two possible

main internal retentive modes for the restoration, definite vertical loading

with its sequelae, and the placement of margins on the incisal ridge. This

further exposes the restoration to the maximal loading possible in anterior

teeth, and it is with the minimal tooth structure to be used for resistance

and/or retention against such loading.

The structure of anterior teeth themselves, have a comparatively different

stress pattern, as a result of occlusal loading from that of posterior teeth.

The unique shape as well as the mechanical structure and function of these

teeth is very important to comprehend before designing a cavity and/or

tooth preparation for a direct tooth colored restoration. The following is a

summary of these unique features :

a) Anterior teeth have their maximal bulk gingivally. They taper incisally

with the least bulk at the incisal ridge. So resistance to stress fractures

will be maximum at the gingival end and decrease incisally.

b) Forces are directed horizontally and vertically on anterior teeth as

mentioned with the force analyses on restorations for these teeth. These

forces accumulate maximal shear stresses at the junction of the clinical

root with the clinical crown and maximum tensile stresses at the incisal

ridges, especially their corners (incisal angles).

c) The labial enamel plate is much thicker than the lingual or proximal

ones, with maximal thickness of enamel usually at the incisal ridge.

d) The incisors may be involved in a disclusion mechanism of the

mandible with loading similar to that of the cuspid, but to a much lesser

extent.

73

e) Cervical portions of anterior teeth when they are affected with a Class V

lesion or cavity preparation will have a stress pattern similar to posterior

teeth, and the stress pattern is governed by the same factors as in

posterior teeth. In addition, the deeper the overbite is, the more induced

the stresses are at these cervical areas.

f) AS mentioned previously, loss of an axial angle, incisal angle, or tooth

structure at the neck of the tooth will dramatically reduce that tooth’s

ability to resist loading without fracture failure.

Ideally, a restoration made of tooth colored materials should not be

loaded directly, i.e. there should be intervening tooth structure between the

occluding tooth and the restoration. This situation can only be achieved by

four intact walls surrounding the restoration. Unfortunately, this is usually not

the case. That is why the clinical performance of tooth colored materials

differs from one situation to another, sometimes dramatically.

Anterior teeth have their maximal bulk gingivally. They taper incisally

with the lest bulk at the incisal ridge. So resistance to stress fractures will be

maximum at the gingival end and decrease incisally.

Forces are directed horizontally and vertically on anterior teeth as

mentioned with the force analyses on restorations for these teeth. These forces

accumulate maximal shear stresses at the junction of the clinical root with the

clinical crown and maximum tensile stresses at the incisal ridges, especially

their corners (incisal angles).

The labial enamel plate is much thicker than the lingual or proximal

ones, with maximal thickness of enamel usually at the incisal ridge. The

incisor may be involved in a disclusion mechanism of the mandible with

loading similar to that of the cuspid, but to a much lesser extent.

74

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.

75

FORCES ACTING ON POSTS

An endodontically treated tooth has been structurally compromised by

caries and its removal, prior restorations, and finally, endodontic preparation

and filling.

It should be emphasized again that posts are only used for retaining the

restorative material in the remaining tooth structures, and by no means will

they reinforce or improve the strengths of these tooth structures.

Because the retention of posts is accomplished by various means, it

might be expected that different stresses are associated with post installation.

With posts retained by the cement alone, the main potential for installation

induced stresses is the build up of hydrostatic back pressure. This potential

with parallel – sided post is circumvented by means of longitudinal vents along

the posts, which provide an outlet for the pressure. Tapered post are self-

venting, and consequently there is no pressure build up.

Endodontic posts provide a protection function through their ability to

distribute the forces of mastication to the remaining tooth structure. How well

this protection is achieved depends upon post design, embedment depth and

diameter.

MECHANICAL ASPECTS OF POST-RETAINED RESTORATIONS

AND FOUNDATIONS :

A. Stressing Capabilities of Posts :

The following features and factors of posts and the involved tooth will

govern the stress patter induced in the surrounding tooth structures due to the

use of posts as retentive means :

1. Type of Posts :

Parallel sided posts will have the tendency to evenly distribute the forces

it receives at and around its cavity end onto the root canal walls, if these forces

are applied parallel (a) to the post axis (vertical occlusal loading. IF the forces

76

applied are at a right angle (b) or oblique (c) to the post axis, the induced

stresses in the root canal walls will be unevenly distributed, i.e. there is a great

possibility of stress concentration due to uneven thickness of the root canal

walls around the post (root taper) while the post remains the same diameter.

This leads to a thin sectioned wall at the very apical end of the post.

On the contrary, taper sided posts and combination type posts will

concentrate stresses due to apical loading (a) in the root canal walls resulting

from its wedge shape. Lateral loading on and around cavity ends of the post,

however, will induce evenly distributed stresses in the root canal walls for the

taper of the post will correspond with the root and root canal taper, leading to

an even thickness of walls occlusoapically.

2. Method of Inserting root canal posts :

During insertion of a post into the root canals, highly threaded posts can

induce ten times the amount and extent of stresses as smooth sided posts.

Serrated surface posts will induce about one and a half to two times the stresses

that are induced by smooth surfaced posts. This can be explained by the

cemented technique utilized by the serrated and smooth surfaced posts.

3. Bulk of dentin in root canal walls :

Naturally, the bulkier that the dentin surrounding a root canal post is, the

less will be the induced stresses per unit volume during the post insertion and

functional use of the post retained restoration. It has been estimated that a

minimum of 2 mm of dentinal root canal wall should surround a post, so that

the stresses induced there will not lead to dentinal failure in the form of cracks

and gross fracture.

4. Length of clinical root involved with the root canal post :

Although the tooth to receive a root canal post should be non-vital and

endodontically treated, the clinical crown portion of the tooth is much more

dehydrated than the clinical root portion as the dentin portion of the root still

77

receives some fluids from the adjacent periodontal ligament. The more

dehydration that there is, the less will be the modulus of resilience and

elasticity of the dentin, and consequently the less will be the dentin’s ability to

absorb and resist stresses without failure.

5. Ferrule or embracing features of the restoration :

Post-core and dowel coping foundations for endodontically treated teeth

will always induce stresses in the root canal walls and remaining tooth

structures which can only be counteracted by embracing the buccal and lingual

cuspal elements of the tooth and/or banding (circumferential embracing) the

tooth at its most apical part of the clinical crown (i.e. area of maximum

stresses). Such bracing is referred to as the Ferrule effect. The Ferrule feature

of the restoration should involve at least 2 mm of crown length to counteract

stresses induced by the post. Using less than 2 mm of crown tooth structure,

the counteracting Ferrule effect will be reduced. The closer this embracing

feature is to the junction between the clinical crown and the root, the more

effective it will be. This is the major protecting feature against induced stresses

in a restoration for endodontically treated teeth.

6. Lateral Locking Mechanisms for the post and restoration :

Because most premade posts are rounded in cross-section there is a great

tendency for the post and the restoration retained by the post to rotate under

torsional forces. This rotational tendency can induce unnecessary stresses in

remaining tooth structures. The presence of a method to lock the post and the

restoration against such rotation (e.g. a lateral pin, internal boxes, opposing

walls, etc) will drastically reduce the effect of torsional forces.

7. Presence of a pulp chamber with pronounced walls :

Walls of the pulp chamber, especially if they are opposing each other,

will increase the frictional retention of the foundation or restoration,

minimizing the retention demands on the pot and thereby minimizing stresses

in the root canal walls.

78

8. Presence of intact marginal or crossing ridges :

These ridges will act as a binder between the buccal and lingual cuspal

elements, resulting n better distribution and resistance of induced stresses.

9. Proximity of the post to the root canal filling :

Root canal fillings should not be involved in the mechanical problems of

the posts. For this reason, there should be a space between the apical end of the

post and the occlusal end of the root canal filling. IF the post approximates the

root canal filling, forces can be transmitted to that filling, which mechanically

is made of very weak materials, and lead to profound straining. This can move

the post in an undesirable direction, and it may induce unnecessary stresses in

the remaining tooth structure. In addition, the direct or indirect loading of the

root canal filling may change its relationship to the surrounding walls and

apical anatomy, resulting in endodontic failure.

10. Presence of flat planes in the remaining tooth structures, at a right angle to

occluding forces :

Flat planes, in the form of tables, gingival floors and ledges, etc, which

will be able to receive and resist occluding forces before arriving to the post,

are the second major feature used to reduce induces stresses in the remaining

tooth structure. Besides partially protecting the post from direct loading, these

flat planes will protect a very weak subpulpal floor from being directly loaded.

11. Presence of lateral walls in the remaining tooth structure :

Extra or intracoronal axial walls, that will receive and resist laterally

applied forces on the restoration before they arrive at the post, will drastically

reduce stresses I the remaining weakened tooth structure, primarily in the root

canal walls.

12. The root post portion relative to the crown post portion :

The ideal ratio is to have the root portion of the post twice as long as the

crown portion, i.e. a ratio of 2:1. Less than that, especially less than a ratio of

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1:1, will definitely concentrate intolerable stresses on the lateral walls of the

root canal adjacent to the apical end of the post.

13. Hydraulic pressure during post cementation :

If there are no lateral vents in the post, or if the post diameter is very

close to that of the post channel diameter, the semi-liquid cement mix, during

the cementation of the posts, may exert tremendous amounts of hydraulic

pressure that exceed the elastic limit of the surrounding dentin or prevent

complete seating of the post.

14. Surface texture and shape of the root end of the post :

Greater post surface roughness and/or the presence of a chisel, wedge, or

irregular configuration on the root end of the post, increases the possibilities of

stress concentration on the root canal walls. The concentration of these stresses

will increase with increasing proximity of the pot to the involved root canal

anatomy.

15. The length of the post relative to the entire length of the root :

Generally, speaking the more that the root canal length is involved with

a post, the more evenly distributed and the better resisted the stresses will be in

the root canal walls. On the other hand, the apical one third of root canals

usually have a very limited thickness of dentin walls. By placing the tip of the

root pot there, with attendant possibilities of substantial stresses being

concentrated at that tip, catastrophic failures become inevitable. As a rule from

one half to two thirds of the root canal should encapsulate the post if the forces

transmitted by the post are to be adequately dissipated.

16. Shape of the post in cross section relative to the shape of the post channel :

A post should have a circumference that coincides with the post channel.

Differences, e.g. rounded post in an oval post channel, will concentrate stresses

at isolated locations in the root canal wall, possibly exceeding the local

breaking point of the dentin.

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17. Loose post in the post channel :

Unconfined movements of a post within a root canal can exaggerate

stresses in the root canal walls upto the fracture point of dentin.

18. Post ending apically at the junction of the clinical root with the clinical

crown.

This specific location is an area of appreciable stress concentration in

normal, sound teeth. With root canal therapy, the strength of the area is

decreased. If, in restoring a tooth, the apical end of the root canal post is

placed at this junction, a when the clinical crown is far longer than the

anatomical crown, three problems will be concentrated at these locations. Less

strength than normal (due to a decrease in bulk resulting from the post’s taper)

above normal stress concentration resulting from a reduced crown root ratio

and maximum stresses from the apical end of the post, as it is, in effect, the end

of a level. These stresses may approach the failure level of the dentin.

19. Central Slitting of Posts :

Length wise slitting of a post involving one half or more of its length

will make the post elastically collapsible in a lateral direction. If such a post is

a threaded type, and during threading into the root excessive stresses are

induced at the post dentin interface, instead of these stresses being consumed in

detrimental deformation of dentin, they may be consumed, in part, to partially

close the central slit. The rigidity of the two parts of the post at this area will

keep the post engaged in dentin for retention, and their elasticity will reduce the

stress concentration in that dentin.

20. Thread numbers and patterns :

Continuous threads from one end of a post to another create more

stresses than interrupted threading. The greater that the spacing is between

threads, the less will be the attendant stresses. The sharper that the threads are,

the less will be the stresses. Circumferentially interrupted threading creates

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less stresses than continuous threading. The wider and more frequent that the

interruptions are, the less will be the stresses. Interruptions (cross cuts) further

serve to facilitate escape of debris during post insertion. The more extended

that the threads are laterally, the more the surface interfacial contact with dentin

will be and consequently, the higher the stresses.

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FORCES ACTING ON A CAST METAL AND PORCELAIN

RESTORATIONS

BIOMECHANICAL PRINCIPLES OF PREPARATIONS:

The design and preparation of a tooth for a cast metal or porcelain

restoration are governed by :

1) Preservation of tooth structures.

2) Retention and resistance forms

3) Structural durability of the restoration

4) Marginal integrity

5) Preservation of the periodontium.

A restoration can meet its functional, biological and esthetic

requirements if it remains firmly attached to the tooth. Its capability for

retention and resistance must be great enough to withstand the dislodging

forces it will encounter in function. An estimate as to the prevailing occlusal

forces can be had by noting the degree of wear on the other teeth, firmness of

the opposing teeth, thickness of the supporting base and the bulk of masticatory

muscles.

RETENTION AND RESISTANCE :

If a restoration does not remain firmly attached to the tooth, it cannot meet

its functional, biological, and esthetic requirements.

Its capability for retention and resistance must be great enough to withstand

the dislodging forces it will encounter in function.

Some estimate of the prevailing occlusal forces in an individual patient can

be made by noting the degree of wear on the other teeth, the firmness of the

opposing teeth, the thickness of the supporting bone, and the bulk of the

masticatory muscles.

It is the geometric form that determines the orientation of the tooth-

restorations interfaces to the direction of forces encountered. This in turn

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determines whether the cement in a given area will be subjected to tension,

shear, or compression.

All cements exhibit their greatest strength under compression. They are

weakest under tension, with the value for shear strength lying in between.

Where a part of the restoration is pulled directly from the tooth, separation

is prevented only by the relatively weak tensile strength and adhesive

properties of the cement.

If the applied force is parallel with the cement film, movement at the

cement-tooth and cement-metal interfaces is more effectively impeded by

the minute projections of cement into the surface irregularities than when

the force is tensile in nature.

Movement within the cement film itself is resisted by its relatively greater

shear strength.

A force directed at an angle toward the restoration, has one component

parallel with and one component perpendicular to the joined surfaces. Thus

the cement is subjected to a combination of shear and compression, and

movement is resisted ore effectively than if the forces were purely tensile or

shear in nature.

A compressive force perpendicular to the cement film can produce no

movement of the restoration relative to the tooth unless it is great enough to

crush the cement or deform the structures. Such forces are seldom

encountered in function.

Retention and resistance can be maximized by shaping the preparation so

that as much of its surface as possible will experience compression and

shear when the restoration is subjected to an unseating force.

RETENTION : It is the ability of the preparation to impede removal of the restoration along

its path of insertion.

Under this condition, the cement bond subjected to tension and shear.

A restoration can experience withdrawing forces along its path of insertion

during mastication of sticky foods.

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There are 4 factors under the control of the operator during tooth

preparation which influence retention.

i) Degree of taper

ii) Total surface area of the cement film

iii) Area of cement under shear

iv) Roughness of the tooth surface

i) Degree of Taper : The more nearly parallel the opposing walls of a preparation, the greater

will be the retention. Thus retention decreases as taper increases.

However, in order to avoid undercuts and to allow complete seating of the

restoration during cementation, the walls must have some taper.

An overall taper or angle of convergence of 6 degrees is considered as

appropriate i.e. approximately 3 degrees being produced on each surface,

external or internal, by the sides of a tapered instrument.

ii) Total Surface Area of Cement Film : The greater the surface area of cement film or the of the preparation, the

greater the retention of the restoration.

The total surface are of preparation is influenced by the size of the tooth, the

extent of coverage by the restoration and features such as grooves and

boxes that are placed in the preparation.

iii) Area under shear : More important for retention than the total surface area is the area of cement

that will experience shearing rather than tensile stress when the restoration

is subjected to forces along the path of insertion.

To decrease the failure potential, it is essential to minimize tensile stress.

For the shear strength of the cement to be utilized, the preparation must

have opposing walls, i.e. two surfaces of the preparation in separate planes

must be nearly parallel with each other an the line of draw.

To obtain the greatest area of cement under shear, the direction in which a

restoration can be removed must be limited to essentially one path.

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Thus the addition of parallel sided grooves, limits the path of withdrawal to

one direction, thereby reducing the possibility of dislodgment.

The length and width of the preparation is an important factors in retention :

a long preparation as well as wider preparation has greater retention than

does a shorter or a narrower preparation.

iv) Surface Roughness : Adhesion of dental cements depends primarily on projections of the cement

into microscopic irregularities on the surfaces to be joined. Therefore

prepared tooth surface should not be highly polished.

RESISTANCE :

it is the ability of the preparation to present dislodgment of the restoration

by forces directed in an apical, oblique, or horizontal direction.

Where there is effective resistance, much of the cement film will be placed

under compression, although some parts will be still be subjected to tension

and shear.

If the cement film is disrupted by the restorations sliding or tipping on its

preparation, the smallest fraction of a millimeter, the restoration is doomed

through percolation of fluids, dissolution of the cement, and recurrent

caries.

Resistance to sliding and tipping must be designed into a preparation by

forming walls to block the anticipated movements. The more nearly

perpendicular it lies to the force, the greater is the resistance provided by

the supporting surface, because the cement will be compressed, and failures

are less likely to occur form compression than shear.

Leverage and Resistance :

The strongest forces encountered in function are apically directed and can

produce tension and shear in the cement film only through leverage.

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Leverage, probably the predominant factor in the dislodgment of cemented

reiterations, occurs when the line of action of a force passes outside the

supporting tooth structure, or when the structures flex.

If the line of action of force passes within the margin of a restoration, there

will be no tipping of restoration. The margin on all sides of the restoration

is supported by the preparation. The torque produced merely tends to seat

the crown further.

If the line of action of force passes outside the margins of restoration the

occlusal table of the restoration is wide, even a vertical force can pass

outside the supported margin and produce destructive torque. This can also

occur in crowns on tipped teeth.

A force applied to a cemented crown at an oblique angle can also produce a

line of action which will pass outside the supporting tooth structure.

Preparation Length and Resistance : The ability of a restoration to resist tipping depends not only on the

preparation, but also on the magnitude of the torque

If two crowns of unequal length on two preparations of equal length, are

subjected to identical forces, the longer crown is more likely to fail because

the force on it acts through a longer lever arm.

Resistance and Tooth Width : A wide preparation has greater retention than a narrower one of equal

height.

Taper and Resistance : The resisting area decreases as the preparation taper increases.

The walls of a short, wide preparation must be kept nearly parallel to

achieve adequate resistance form.

Rotation around a vertical axis : When a crown is subjected to an eccentric horizontal force, movements of

torque occur around a vertical as well as horizontal axis.

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It is possible for a full crown on a cylindrical preparation to rotate enough

to break the cement bond before may compressive resistance is

encountered.

Geometric forms such as grooves or “wings” increase resistance by

blocking rotation around a vertical axis.

Path of Insertion : The path of insertion for posterior full and partial veneer crown is usually

parallel with the long axis of the tooth.

A tipped tooth must be handled differently.

If the path of insertion on a tipped tooth parallels the long axis, the crown

will be prevented form seating by those parts of the adjacent teeth which

protrude into the path of insertion.

The correct path of insertion for such a tooth is perpendicular to the

occlusal plane.

All negative taper or undercut, must be eliminated or it will prevent the

restoration form seating.

Occlusal Reduction : It should reflect the geometric inclined planes underlying the morphology

of the finished crown.

Avoid creating steep planes with sharp angles, since these ca increase stress

and hinder complete seating of the casting. To diminish stress, round the

angels and avoid deep grooves in the centre of the occlusal surface.

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FORCES ACTING ON PARTIAL VENEER CROWN

Since vertical occlusal forces have horizontal resultants, displacing

forces have a tendency to tip or rotate a restoration, usually the tipping is in the

lingual direction and rotation occurs mesiolingually or distolingually.

Observe the proximal groove of a force ‘P’, directed lingually which is

applied at the incisal edge. It will have a tendency to tip the casting out of the

cavity, turning on the fulcrum ‘f’. The resistance to the displacement is offered

by a rib of ‘Aa, which lies in the axial groove ‘ab’ also by that part of the axial

wall lying lingually to the axial groove and encompasses area e and s.

It is observed that the lingual wall of the mesial groove does not furnish

any resistance to lingual displacement. Since the plane of this wall lies in the

tipping path of arc ‘C’ for this reason, the incisal edge is usually prepared in a

plane and not with a groove.

When force P is applied mesiolingually to the marginal ridge of the

upper central, the tendency is to rotate the restoration out of the mesiolabial

wall of the cavity, point ‘F’ acting as the centre of rotation. Obviously then the

resistance to this rotational displacement is furnished by the distoproximal

groove and that portion of the proximal surface lying within the arcs R1 and R2.

Analogous resting forces are present when acting forces are in a

distolingual direction, then the distolabial wall act as to point of rotation, the

mesio proximal wall and groove furnish the resistance to displacement.

There is a horizontal force ‘P’ applied distally to the incisal area. This

has a tendency to tip the casting mesially, rotating on point ‘F’. This displacing

force is resisted by the proximodistal groove and that portion of the

proximodistal surface lying between R1 and R2.

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The same general displacing forces are present in the posterior partial

crown as in the anterior partial crown, but the ability of the preparation to resist

displacement is more favourable than in the anterior teeth. The occlusal forces

may be occlusal, horizontal or any one component of force indicated by ‘P’.

Minimum problems exerts when the force is vertical, for the resistance

is equal and opposite. When the force tends to displace the casting lingually, it

does so along the paths R1, R2 and R3 with its rotation center at point F.

Resistance in this displacing force is furnished by the ribs of gold lying

within the axial groove and by that portion of the proximal and surface

extending lingually from the proximal groove and lying within the areas R1, R2

and R3. In addition, the occlusal surface lying to plane R2 – R3 offers resistance

when force ‘P’ is applied in mesiolingual direction, the tendency is to rotate the

casting mesiodistally with the rotation centre being point F, the mesiobuccal

wall. The resistance is this displacing force is developed by the rib of gold

lying into distoproximal groove and by the portion of the casting coming in

contact with the proximal surface lying between the areas R1 and R2.

Additional resistance to displacement is offered by the occlusal inclined planes

R3, R4 and R5.

When force P is applied in the distal direction, the tendency is to rotate

the casting occlusally, with its dislodgment along the areas R1 and R2 with F

serving as the point of fulcrum.

The resistance to this displacing force is furnished by the rib of gold

lying in the mesio proximal groove and also by the buccal and mesioproximal

walls lying within the areas R1 and R2.

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FULL COVERAGE CROWN

(ANTERIOR PORCELAIN JACKET CROWN) :

As mentioned earlier the anticipated forces place don the restoration

cannot be taken lightly.

Incisal Reduction : In keeping with the rule that planes are placed at right

angles to the applied forces, the incisal edges of the upper anterior teeth slopes

lingually whereas that of the lower teeth slopes labially. The incisal reduction

should be adequate to ensure clearance in protrusive movements of the

mandible and permit satisfactory esthetics and enhance optimal function.

-2-5o parallelism is desired with proximal reduction. After the removal

of enamel, the labial gingival termination is made at or just above the crest of

the labial gingiva. Another retention area is immediately below the cingulum.

More than any other restoration, porcelain jacket crown depends on its

tooth preparation. Tooth support is more critical to fracture resistance of the

restoration than is the bulk of porcelain. The crescent moon fracture seen on

the labial cervical region is a direct result of inadequate preparation length.

Incisal reduction recommended ranges from 1.5 to 2 mm. For esthetic

result, it is best to reduce the incisal edge by 2 mm to the level of depth

orientation grooves. Any greater reduction will increase the stress on the facial

surface which can result on the facial half moon fracture. The plane of the

reduced incisal surface should parallel with the former incisal surface and more

importantly perpendicular to the forces of mastication. Failure to create this

near incisolingual bevel (45oC) will produce excessive stress at the shoulder.

(Shoulder 0.5 – 0.7 mm) Plane of shoulder is perpendicular to long axis of the

tooth. If it is at obtuse angle possibilities of fracture at the cervical region is

more.

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The mesioaxial and distoaxial walls are more favourable for developing

parallelism to frictional resistance. The buccal and lingual surfaces due to their

natural contour, do not afford the same opportunity for paralleling walls. The

occlusal planes are reproduced at a lower level. These planes help

considerably to resist stability or displacement. When necessary and where

indicated, additional resistance form may be obtained by placing pins, grooves

or boxes in any available surface where the length of this surface is adequate.

If an occlusal force is directed ‘P’ buccally, the lingual portion of the

crown tends to be dislodged occlusally and buccally with the point of rotation

situation at ‘F’, this displacement is resisted by that lingual surface when it lies

outside the arc R1.

On the other hand, an occlusal force P2 directed lingually is likely to

dislodge the crown lingually, since the buccal wall of the preparation lies

within the tipping path of the arc H2.

When such a condition prevails either in the buccal or lingual wall,

resistance to displacement can be developed by placement of 2 proximal

grooves at G in the mesial and distal surface as shown in the figure and it is

evident that an occlusal force P directed mesially will not dislodge the crown,

since the distal wall of the preparation lies outside the tipping path of arc R1.

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CONCLUSION

Optimal functional capacity and stability of occlusal relationships are

major considerations in every phase of restorative dentistry.

The first phase objective of a cavity preparation design is to establish the

best possible configuration that can cope with the distribution and magnitude of

stresses in tooth structure and restoration without failure. To design such a

configuration one must first comprehend the nature of loading and resistance to

such loading.

Restoration not only mechanically replace the lost part but, acts as a

medium through which physical and mechanical forces are transmitted to the

tooth and investing tissues. Each tooth ahs its own stress patterns. A thorough

knowledge in dental materials is necessary to understand the physical

properties including their response to stress.

Before any restorative procedure, always check location of the tooth in

the arch and the patients occlusal relationship. The functional, non functional

cuspal elements should be noted bye examining the involved teeth during static

and functional mandibular movements The operator can then recognize the

nature of stresses that can be expected in the remaining tooth structure

especially the occluding ones.

From these information’s obtained during the patient evaluation, the

operator must envision the restoration replacing lost tooth structure being

subjected to functional loading and then try to plan the best tooth preparation to

both retain this restoration and make it resistant to these loads.

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