forces acting on restorations

41
Introduction In order to ensure the success of the restoration placed in the oral cavity. The physico-mechanics of the forces acting on it has to be understood, by restoring the tooth form, we aim at maintaining the integrity and continuity of dental arch which is very important as far as mastication is concerned. Therefore, the basic aim of cavity preparation design should be to establish the best possible shape that can cope with the distribution of stresses in tooth structure and restoration without failure. for this one should understand the nature of forces acting on it and resistance to such forces. Both resistance and retention form is very important as far as success of restoration is concerned. Resistance form is defined as the architectural form given to a tooth preparation which enables both the restoration and the remaining tooth to resist structural failure from occlusal loading stresses. 1

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Page 1: Forces acting on restorations

Introduction

In order to ensure the success of the restoration placed

in the oral cavity. The physico-mechanics of the forces

acting on it has to be understood, by restoring the tooth

form, we aim at maintaining the integrity and continuity of

dental arch which is very important as far as mastication is

concerned.

Therefore, the basic aim of cavity preparation design

should be to establish the best possible shape that can cope

with the distribution of stresses in tooth structure and

restoration without failure. for this one should understand

the nature of forces acting on it and resistance to such

forces.

Both resistance and retention form is very important

as far as success of restoration is concerned.

Resistance form is defined as the architectural form

given to a tooth preparation which enables both the

restoration and the remaining tooth to resist structural failure

from occlusal loading stresses.

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Building a restoration is similar to building any

mechanical structure, in that the stress patterns of the

available foundation and the contemplated structure must be

predetermined.

Accordingly, the following items should be considered.

A STRESS PATTERNS OF TEETH

Every tooth has its own stress pattern, and every

location on a tooth has special stress patterns. Recognizing

them is vital prior to designing a restoration without failure

potential.

I. STRESS BEARING & STRESS CONCENTRATION

AREAS IN ANTERIOR TEETH

a. The function between the clinical crown and the

clinical root bears shear components of stress

together with tension on the loading side and

compression at the non-loading side, during

excursive mandibular movements.

b. The Incisal angles, especially if they are square,

are subject to tensile and shear stresses in normal

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occlusion massive compressive stresses will be

present in edge-to-edge occlusion, and if the

incisal angles are involved in a disclusive

mechanisms, these stresses are substantially

increased.

c. The axial angles and lingual marginal ridges will

bear concentrated shear stresses. In addition on

the loading side tensile stresses are present, and

on the non-loading side compressive stresses are

found.

d. The slopes of the cuspid will bear concentrated

stresses, especially if the cuspid is a protector for

the occlusion or part of a function during

mandibular excursions.

e. The distal surface of a cuspid exhibits a unique

stress pattern as a result of the anterior

components of force concentrating compressive

loading at the function of the anterior and

posterior segments of the dental arch and

microlateral displacement of the cuspid during

excursive movements. Both of these factors will

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lead to stress concentration with resultant

abrasive activity there.

f. The lingual concavity in upper anterior teeth

bears substantial compressive stresses during

centric occlusion in addition to tensile and shear

stresses during protrusive mandibular

movements.

g. The incisal edges of lower anterior teeth are

subjected to compressive stresses. In addition

tensile and shear stresses are present during

protrusive mandibular movement. The incisal

ridges of upper anterior teeth will have these

same stresses during protrusive and sometimes at

the protrusive border location of the mandible.

II. STRESS BEARING AND STRESS

CONCENTRATION AREAS OF POSTERIOR

TEETH

a) Cusp tips, especially on the functional

side bear compressive stresses.

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b) Marginal and crossing ridges bear

tremendous tensile and compressive stresses.

c) Axial angles bear tensile and shear

stresses on the non-functional side and

compressive and shear stresses on the functional

side.

d) The function between the clinical root and

the clinical crown during function (especially

lateral excursion) bears tremendous shear stresses,

in addition to compression on the occluding

contacting side and tension on the non-contacting

side.

e) Any occlusal, facial or lingual concavity

will exhibit compressive stress concentration.

Especially if it has an opposing cuspal element in

static or functional occlusal contact with it.

III. WEAK AREAS IN THE TOOTH SHOULD BE

IDENTIFIED AND RECOGNIZED BEFORE ANY

RESTORATIVE ATTEMPT, in order to avoid

destructive loading they are:

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a. Bi and trifurcation.

b. Cementum should be eliminated as a component

of a cavity wall. The junction between the

cementum and the dentin is always irregular so

the dentin surface should be smoothed flat after

cementum removal.

c. Thin dentin bridges in deep cavity preparation.

d. Subpulpal floors in RCT treated teeth. Any stress

concentation there may split the tooth

interceptally.

e. Cracks or crazing in enamel, and / or dentin both

should be treated passively in any restoration

design. They may act as shear lines leading to

further spread.

SOME APPLIED MECHANICAL PROPERTIES OF

TEETH

I. Although the following figures are averages, they

provide an idea about the principal mechanical

properties of tooth structure. It must be understood

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that these figures can differ from one location on a

tooth to another, and from one tooth to another.

a. Compressive strength of enamel supported by

vital dentin is usually 36-42,000PSI.

b. Compressive strength of vital dentin is 40-

50,000PSI.

c. Modulus of resilience of enamel supported by

vital dentin is 60-80 inch-lbs / cubic inch.

d. Modulus of resilience of vital dentin is 100-140

inch-lbs /inch3.

e. Modulus of elasticity of enamel supported by

vital dentin under compression is 7,000,000 PSI.

f. Modulus of elasticity of vital dentin is

1,90,000PSI.

II. In general, when enamel loses its support of dentin,

it loses more than 85% of its strength properties.

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

compressive strength.

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IV. Tensile strength and compressive strength of enamel

are similar, as long as the enamel is supported by

vital dentin.

V. Shear strength of dentin is almost 60% less than its

compressive strength, and this is very critical in

restorative design.

VI. There is minimal shear strength for enamel when it

loses its dentin support.

VII. When the dentin loses it vitality, there is a drop of

almost 40-60% in its strength properties.

To best resist masticatory forces, use floors or planes

at right angles to the direction of loading to avoid shearing

stresses.

If possible walls of preparations should be parallel to

the direction of the loading forces, in order to minimize or

avoid shearing stresses.

Intracoronal and intraradicular cavity preparations can

be done in box, cone or inverted truncated cone shapes.

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From the drawings, it is possible to deduce that the

inverted truncated cone shapes will have a higher resistance

to loading than the box shapes, and the box shapes will have

a higher resistance than the cone shapes. Therefore if

conditions and requirements allow, cavity preparations

should be prepared in an inverted truncated cone shape.

Definite floors, walls and surfaces with line and point

angles are essential to prevent micromovements of

restorations with concomitant shear stresses on remaining

tooth structure.

Increasing the bulk of a restorative material or leaving

sufficient bulk of tooth structure in critical areas is one of

the most practical ways of decreasing stresses per unit

volume.

Designing the outline form with minimal exposure of

the restoration surface to occlusal loading will definitely

minimize stresses and the possibility of mechanical failure in

the restoration.

A comparative evaluation of the mechanical properties

of the restorative material relative to that of the tooth

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structure will dictate the preparation and restorative design

i.e. if the restorative material is stronger than the tooth

structure, the design should be such that the restorative

material will support the tooth structure and vice versa if the

restorative material is weaker than tooth structure.

Junction between different parts of the preparation

especially those acting as fulcra, should be rounded in order

to minimize stress concentration in both tooth structure and

restorations and to prevent any such sharp components from

acting as shear lines for fracture failure.

1. RETENTION FORM

Retention form is defined as that form given to the

tooth preparation, especially its detailed anatomy and

general shape which enables the restoration, that it will

accommodate, to avoid being dislodged by masticatory

loading.

Principal means of retention:

Frictional retention depends on 4 factors:

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a. The surface area of contact between tooth

structure and restorative material. Greater surface area

produces a greater frictional component of retention. It

is directly proportional to the length, width and depth

of the walls and surface involved in the preparation.

b. Opposing walls or surface involved:

More opposing walls or surfaces in a tooth preparation

produce greater frictional components of retention and

consequently, a more stable restoration within the

preparation.

c. Parallelism and non-parallelism

A higher degree of parallelism between opposing walls

produces greater frictional components of retention.

Higher convergence of the walls in the intracoronal

preparation and higher divergence of walls in the

extracoronal preparation produce greater locking ability

of the tooth preparation to restorative material,

irrespective of frictional retention.

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d. Proximity

Bringing the restorative material closer to the tooth

structure during insertion will substantially increase the

frictional retention.

2. ELASTIC DEFORMATION OF DENTIN

Changing position of dentinal walls and floors

microscopically by using condensation energy within the

dentin proportional limit. Can add more gripping action by

the tooth on the restorative material. This occurs when the

dentin regains its original position while the restorative

material remains rigid, thereby completely obliterating any

remaining space in the cavity preparation.

CLASS I

All Class I cavity preparations will have a mortise

shape i.e. each wall and floor is in the form of a flat plane

meeting each other at definite line and point angles. This

form is commonly applied in various mechanical structures,

so its application here is understandable.

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It is advantageous to have a mortise shape preparation

in an inverted cone shape to minimize shear stresses that

tend to separate the buccal and lingual cuspal elements i.e.

to prevent the splitting of the tooth. The box shaped mortise

is less advantageous and the cone shaped is the least

advantageous in this regard. So, whenever the anatomical

and cariological factors allow the cavity preparation should

be an inverted cone shape.

When a caries cone penetrates deeply into dentin,

removing undermined and decayed tooth structures can lead

to a conical cavity preparation, mechanically, two problems

can occur if restoration is inserted into such a cavity

preparation.

If the occlusal loading is applied centrically the

restoration may act as a wedge, concentrating forces at the

pulpal floor and leading to dentin bridge cracking and

increased tendency for tooth splitting (A).

If the occlusal loading is applied eccentrically the

restoration will have tendency to rotate laterally, for there

would be no lateral locking walls in definite angulation with

a floor. Although these lateral movements are microscopic

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they occur frequently enough to encourage microleakage

around the restoration, predisposing to a recurrence of

decay. These measurements can also lead to fracture of

marginal tooth structure and even to splitting of lateral

walls.

To solve these problems, flatten the pulpal end of the

cavity preparation. (However if accomplishing this at a deep

location incurs increased risk of involving the pulp chamber,

pulp horns, or recessional lines containing remnants of pulp

tissues) make the pulpal floor at more than one level (B) one

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

will be the caries cone level dictated by the pulpal extent of

the decay. The shallow level creates the flat portion of the

pulpal floor at definite angles to the surrounding walls,

adequately resisting occlusal forces and laterally locking the

restoration, without impinging on pulp tissues.

The first level should be as pronounced and

circumferentially continuous as possible. At least it should

exist at two opposing locations in the cavity preparation in

order to fulfill its objectives. This level is sometimes called

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“The ledge” and it can be circumferential, interrupted or

opposing.

CLASS II

During centric and excursive movements of the

mandible both the restoration and tooth structure are

periodically loaded both separately and jointly. This brings

about different stress patterns, depending upon the actual

morphology of the occluding area of both the tooth in

question and the opposing contacting cuspal elements.

For the purpose of this discussion, one can classify

these loading situations and their induced stress patterns in

the following way:

A small cusp contacts the fossa away from the restored

proximal surface, in a proximo-occlusal restoration at centric

closure.

A) As shown in mesio-distal cross

section, due to the elasticity of the dentin, especially in

young teeth, a restoration will bend at the axio-pulpal line

angle (provided the proximal part of the restoration is self

retained). This creates tensile stresses at the isthmus

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portion of the restoration, shear stresses at the junction of

the main bulk of the proximal part of the restoration and

its self retained parts, and compressive stresses in the

underlying dentin.

B) A large cusp contacts the fossa

adjacent to the restored proximal surface in a proximo-

occlusal restoration at centric closure, either in the early

stages of moving out of centric or at the late stages of

moving toward it.

As the diagram shows, the large cusps will tend to

separate the proximal part of the restoration from the

occlusal part. This creates tensile stress at the isthmus

portion of the restoration even if the proximal portion is self

retained. This loading situation will deliver compressive

forces in the remaining tooth structure, apical to the

restorations.

C) Occluding cuspal elements contact

facial and lingual tooth structure surrounding a proximo-

occlusal or proximo-occluso-proximal restoration during

centric and excursion movements.

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As shown in this bucco-lingual cross section

concentrated shear stresses will occur at the junction of the

surrounding tooth structure and corresponding floors, with a

tendency toward fracture failure there. This loading situation

can be unilateral or bilateral, depending on the direction of

mandibular movement, occluding surface morphology, stage

of movement, and degree of intercuspation. It is most

deleterious to tooth structure, especially on the biting side if

there is interference during lateral excursion.

D) Occluding cuspal elements contact

facial and lingual parts of the restoration, surrounded by

tooth structure during centric and excursive movements.

As shown in this bucco-lingual cross section this

arrangement will induce tensile and compressive stresses in

the restoration which will be transmitted to the surrounding

tooth structure.

E) Occluding cuspal elements contact

facial or lingual parts of the restoration, completely

replacing facial or lingual tooth structure during centric

and excursive movements.

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As shown in the bucco-lingual cross section the stress

pattern will be similar to No. 2 with tensile stresses induced

at the junction of the occlusal and facial or lingual part of

the restoration in both occluding situation.

F) Occluding cuspal elements contact a

restorations marginal ridges or part of a marginal ridge

during centric and excursive movements.

As shown, in this mesio-distal cross section (assuming

the restoration is locked occlusally), there will be

concentrated tensile stresses at the junction of the marginal

ridge and the rest of the restoration.

G) Cuspal elements occlude or disclude

via the facial or lingual groove of a restoration.

Assuming, that the restoration is locked occlusally,

there will be tensile stresses at the junction of the occlusal

and facial or lingual parts of the restoration at full

intercuspation (A) and to and from that position (B)

H) Cusps and crossing ridges are part of

the restoration in centric and excursive movement.

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- Both will be subjected to

compressive stresses during such positions and

movements. Besides, tensile stresses could concentrate at

their junction with the main restoration, especially during

contacting excursive movement.

I) Axial portions of the restoration

during centric occlusion and excursive movement

contacts.

Whenever these portions are in contact with opposing

occlusal surfaces, there will be induced compressive and

shear stresses whenever they are not reciprocating, the axial

surfaces will be stressed in a slight tensile and shear pattern

at their junction with the main bulk of the restoration.

J) Restoration is not in occluding

contact or is in premature contact during centric occlusion

or excursive movement of the mandible.

The first situation is not conducive to function, in so

far as the restoration will not be involved with direct loading

from the opposing occluding teeth. After a period of time

however the tooth will supraerupt, rotate or tilt, establishing

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contact with the opposing cuspal elements. Usually this

newly acquired location will not be the most favourable

position for the restoration, tooth or the remainder of the

gnatho-stomatic system either mechanically or biologically.

It is safer to build the restoration to predetermined

contacting areas with opposing teeth which will lead to

predictable physiologic stress patterns in the tooth structure

and the restoration. Conversely, any portion of the

restoration occluding prematurely will tremendously

exaggerate the same types of stresses normally induced in

that area of the restoration. Besides additional shear

components of stress could be precipitated there. This could

lead to localized or generalized gnatho-stomatic disturbances

with eventual mechanical or biological features.

Needless to say, pre-existing premature contacting

areas should be eliminated before restorative treatment. This

is done primarily because cavity preparation increases the

susceptibility of remaining tooth structure to fracture failure,

besides, the restoration should be built to the predetermined

occlusal position, even if the preexisting tooth structures

were not.

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Amalgam is least resistant to tensile stress and more

resistant to compressive stress. Tooth structure particularly

when interrupted by a cavity preparation, is least resistant to

shear stress. Therefore Class II cavity preparations for

amalgam restorations should be designed to resist cyclic

loading while minimizing tensile loading in the amalgam and

shear loading in the remaining tooth structure (Fig. 13).

Design features for the protection of the mechanical

integrity of the restoration.

1. ISTHMUS:

In the Isthmus, i.e. the junction between the occlusal

part of a restoration and the proximal potentially deleterious

tensile stresses occur under any type of loading.

Most mathematical, mechanical and photoelastic

analyses of these stresses reveal three things:

1. The fulcrum of binding occurs at the axio-pulpal line

angle.

2. Stresses increase closer to the surface of a restoration,

away from that fulcrum and

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3. Tensile stresses predominate at the marginal ridge area of

a Class II restoration.

Materials tend to fail, therefore, starting from the

surface, near the marginal ridge, and proceeding internally,

toward the axio-pulpal line angle.

These problems may be solved by applying common

engineering principles.

1) A theoretical solution might be to increase amalgam bulk at the

axio-pulpal line angle. Thereby placing the surface stresses away

from the fulcrum (fig). However this actually results in increased

stresses within the restorative material and a deepened cavity

preparation, dangerously lose to pulp anatomy. Therefore such a

solution, in and of itself is actually unacceptable.

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

closer to the surface, in an effort to reduce tensile stresses

occurring near the marginal ridge. However, this too is

unacceptable in that consequent diminished bulk of amalgam

would no longer adequately resist compressive forces.

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

amalgam bulk near the marginal ridge, while bringing

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the axio-pulpal line angle away from the stress

concentration area and closer to the surface, can be

achieved simply by slanting the axial wall towards the

pulpal floor .

1. The obtuse-pulpal line angle thereby created not only

provides greater amalgam bulk in the marginal ridge

area of the restoration but also reduces tensile stresses

per unit area by bringing this critical area of the

preparation closer to the surface of the restoration.

2. If the axio-pulpal line angle is rounded, structural

projections or sharp junctions that may concentrate

stresses at the isthmus would be avoided as well as

increase the amalgam bulk at the fulcrum.

3. By slanting the axial wall, bulk is improved by

increased depth rather than increased width.

Increasing the width at the isthmus portion only

increases the surface area receiving deleterious

occluding stresses.

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4. The pulpal and gingival floors at the isthmus should

be perfectly flat in order to resist forces at the most

advantageous angulation.

5. The fifth design feature is that every part of the

preparation (occlusal, facial, lingual or proximal)

should be self retentive. If every part of the

restoration is locked in both structure-independently

from other parts, there will be minimum stresses at the

junction of one part with another i.e. the isthmi. This

can be achieved in amalgam preparation by retentive

grooves internal boxes, and undercuts.

6. One should avoid, as much as possible placing or

leaving any surface discontinues such as carved

developmental grooves, scratches etc. at these critical

areas in the restoration. These can precipitate and

accentuate stresses leading to fatigue failure.

Finally by checking occlusion to eliminate

prematurities in the restoration, immediate overloading and

failure can be avoided.

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MARGINS

Frail, feather-edged margins of amalgam, which will

occur when the cavo-surface angles of preparations are

beveled, will fracture easily. Occluding forces will cause

amalgam at the bevel to bend with maximum tensile stress,

occurring as a result of elastic deformation of the tooth

structure beneath the bevel. Marginal excess of amalgam will

similarly fracture, leaving a ditch around the restoration that

will enhance recurrence of decay. So, for the margins of

these preparations, four design features should be observed:

1. Create butt joint amalgam tooth structure at the

margins.

2. Leave no frail enamel at the cavo-surface margins.

3. Remove flashes of amalgam on tooth surface adjacent

to amalgam margins.

4. The interface between amalgam and tooth structure

should not be at an occluding contact area with

opposing teeth either in centric or excursive

mandibular movements.

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Cusps and axial angles:

The following are the design features for these parts of

a restoration:

a. Amalgam bulk in all three dimensions should be atleast

1-5mm.

b. Each portion of the amalgam should be completely

immobilized with retention modes.

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

these areas.

d. Amalgam replacing cusps or axial angles should have a

bulky connection to the main part of the restoration

with similar design features as for the isthmus areas.

RETENTION FORM

In order to design a cavity preparation that will hold a

restorative material, it is necessary to know the possible

displacements that can happen to such a restoration, the

forces that can cause them, and the fulcrum of these

movements. There are such displacements for a Class II

proximo-occlusal restoration.

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A) PROXIMAL DISPLACEMENT OF THE ENTIRE

RESTORATION

In analyzing the obliquely applied force ‘A’ into a vertical

component ‘v’ and a horizontal component ‘H’ it can be seen that ‘V’

will try to seat the restoration further into the tooth, but ‘H’ will tend to

rotate the restoration proximally around axis ‘X’ at the gingival cavo-

surface margin. To prevent such displacement self-retaining facial and

lingual grooves proximally are necessary, in addition to an occlusal

dovetail.

B) PROXIMAL DISPLACEMENT OF THE

PROXIMAL PORTION

If one were to consider the restoration as being L-

shaped with the long arm of the L occlusally and short arm

proximally. When the long arm is loaded by vertical force

‘V’, ‘H’ will seat the restoration more into the tooth. This is

due to elasticity of the dentin, especially in young teeth

wherein the pulpal floor will change location from position 1

to position 2. However, since the metallic restorations are

more rigid than the dentin, the short arm of the L will more

proximally, the fulcrum of this restoration is the axio-pulpal

line angle. In order to prevent such a displacement, proximal

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self-retention in the form of facial and lingual grooves are

required.

C) LATERAL ROTATION OF THE RESTORATION

AROUND HEMISPHERICAL FLOORS (PULPAL

AND GINGIVAL)

A) OCCLUSAL DISPLACEMENT

This can be prevented by directing occlusal loading to

seat the restoration and by inverted truncated cone shaping

of key parts of the preparation.

Although the magnitude of these four displacements is

minute, they are repeated thousands of times per day. This

can definitely increase microleakage and initiate mechanical

and biological failure of the restoration and surrounding

tooth structure. Therefore, proper locking of the restoration

into the tooth should be exercised to minimize these hazards.

To repeat, every part of the cavity preparation should

be self retaining, if possible i.e. independent in its retention

from the rest of the cavity. This minimizes shear

concentration areas at the junctions of different parts of the

restorations with less failure to be expected as a result.

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CLASS V

Class V restorations confined to one surface and not

subjected to direct loading may be thought of as free of any

mechanical problems. However, as the mandible moves in

lateral excursion, the lingual slopes of the buccal and lingual

cusps of maxillary teeth lead to the buccal slopes of the

buccal and lingual cusp of mandibular teeth. Assume that we

have a facial Class V restoration in the lower molar tooth.

As the tooth is firmly seated in bone, the tooth structure of

the crown can move from position 1 to position 2, making a

V-shape opening at the margin, together with a facial

component of force during the restoration facially.

Although this opening and the facial component of the

force are very minute and may not displace the restoration

completely, their repetition, thousands of times per day can

create marginal failure and eventually, facial protrusion of

the restoration.

The same thin can happen for a lingual restoration in

lower teeth and a facial or lingual one in upper teeth.

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To minimize the effects of these displacing forces,

grooved occlusal and gingival walls are essential for any

Class V cavity preparation for amalgam, in addition to

definite surrounding walls, line and point angles.

FORCES ACTING ON CAVITY PREPARATION FOR

DIRECT TOOTH COLOURED MATERIALS

For any proximal restoration in anterior teeth there are

two possible displacing forces.

The first ‘H’ is a horizontal displacing or rotating the

restoration in a labio-proximo-lingual or linguo-proximo-

lateral direction. It has its fulcrum almost parallel to long

axis of the tooth being loaded.

The second is a vertical forces displacing or rotating

the restoration proximally and having a fulcrum at the

gingival margin of the preparation.

The mechanical picture can be summarized as follows:

1. With normal overbite and overjet during centric

closure of the mandible, mainly the horizontal forces

will be in action, these forces would try to move it

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linguo-proximo-laterally (for the upper restoration)

and labio-proximo-lingually (for lower).

In protrusive and lateral protrusive movements of the

mandible, directly loaded proximal restorations in anterior

teeth will be subjected to substantial horizontal as well as

vertical displacing forces especially in restorations replacing

the incisal angle. The results of this loading are rotational

forces as well as forces rotating the restoration laterally and

proximally (for upper) or lingually and proximally (for the

lower).

2. If anterior teeth meet in edge to edge fashion i.e. there

will be vertical displacing forces with very limited

horizontal components.

3. If the upper and lower anterior teeth meeth such that

the lowers are labial to the uppers in centric occlusion

(Angle’s Class III), the horizontal loading will tend to

rotate or displace restorations labio-proximo-lingually

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

4. In occlusions, with deep anterior overbite and normal

or no overjet, the horizontal type of loading will be

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greatly exaggerated. The vertical displacement

although present will be minimal in comparison.

5. In occlusions with anterior open bite or severe overjet

or any other condition that creates a no contact

situation between upper and lower anterior teeth during

centric occlusion and excursive movements of the

mandible, proximal restorations will not be loaded

directly either vertically or horizontally.

It should be understood that none of these loading

forces work separately. They work together and

simultaneously. It should be mentioned here that a

restoration replacing part or all of the incisal ridges of an

anterior tooth will have the same pattern of loading as

mentioned in (1)-(6) but with increased intensity. Loss of

incisal angle of a tooth i.e. conversion from a Class III to a

Class IV represents a major complication in the mechanical

problems of anterior tooth restoration. This loss will lead to

definite direct loading of the restoration, definite vertical

loading with its sequelae, and the placement of margins on

the incisal ridge. This further exposes the restoration to the

maximal loading possible in anterior teeth.

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6. In cases when the proximal restoration of an anterior

tooth is a part of a mutually protective occlusion i.e.

an incisor and the adjacent cuspid are involved in an

anterior lateral disocclusion mechanism, the teeth and

the restoration will be part of that disocclusion

mechanism with excessive horizontal and vertical

loading forces.

Ideally, a restoration made of tooth colored materials

should not be loaded directly i.e. there should be intervening

tooth structure between the occluding tooth and the

restoration. This situation can only be achieved by force

intact walls surrounding the restoration, unfortunately, this

is usually not the case that is why the clinical performance

of tooth colored materials diffuses from one situation to

another, sometimes dramatically.

CAST PREPARATIONS

Cast restorations are usually used for compound or

complex tooth involvement. The possible loading and

displacing forces, their fulcra, and their effects on

restorations, together with their effect on remaining tooth

structure, have been fully described in the discussions of the

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different cavity preparations for amalgam and in the general

principles of preparation design. The formability of casting

materials enables us to use myriad retention and resistance

means that are impossible to use with any other materials.

INLAY RESTORATIONS

Here are 3 illustrations representing the different

design of a proximal box cavity

(A) (B) (C)

Fig.(A) shows walls parallel to each other where

rotational force is applied by means of a bar in a counter

clockwise direction. The tendency for ‘x’ to use occlusally

on area xy is resisted by dentin lying withing the area xyz.

In Fig. (B) same rotational force finds no resistance to

point ‘x’ rotating might out of the cavity because of too

great divergence of buccal and lingual walls.

In Fig. (C) shows gingivo-occlusal divergence of 5°

from vertical plane where the rotational force finds

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resistance to point x by the bulk of dentin contained in area

xyz.

Therefore, while parallelism of the walls offers

maximum rotational resistance from clinical standpoint, a

slight divergence of 2° to 5° from parallelism will furnish

necessary resistance to bucco-lingual torque displacement.

This figure shows a proximal view of a MOD inlay not

quite seated in the cavity.

The width of inlay is ‘N’ at about its vertical center.

Contact is assumed to have been made between the

inlay and the walls of the cavity. After the contact, the inlay

is further forced downwards – an amount ‘dh’.

The walls of the cavity make an angle θ with the

vertical wall of the restoration. Assuming that the tooth

structure is prevented from deformation, the total shortening

per unit width of gold is:

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Σg = dh tanθ

W

And the unit stress becomes

S = Eg dh tanθ

W

Eg = modulus of elasticity of gold.

The unit stress is not perpendicular to the cavity wall

but is parallel to ‘W’ and may be resolved into two

components.

F Cosθ - perpendicular to cavity wall.

And F Sinθ - parallel to the wall.

Assuming the coefficient of friction is ‘µ’ between

gold of the inlay and tooth structure than µF Cosθ becomes

the frictional force between gold and tooth structure which

prevents the movement of the two with respect to each other.

The component FSinθ parallel to the cavity wall tends to

push the inlay back to the cavity wall.

Thus the total force of frictional retention tending to

hold the inlay in its cavity is:

P = µ FCosθ - F Sinθ

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This shows that as the value of θ increases greatly, the

inlay will bounce out of the cavity and this angle is known

as the “Critical Angle” θc.

In a proximo-occlusal restoration one of the proximal

wall is absent and opposite retentive stresses are developed

only on the buccal and the lingual surfaces whereas the

gripping power has been lost in proximal direction, due to

which there is a force tending to push the restoration out

through the absent wall. This displacing force can be

counteracted by the retaining stresses present in the buccal

and lingual walls and can be supplemented clinically by

placing a gingival groove in the gingival wall. The occlusal

dovetail lock also resists lateral displacement of the key by

the additional tensile stresses developed within the lock.

With the help of the diagram it is seen that by

increasing the angulation to 35°-45° of the gingival bevel the

resistance to displacement is offered by that portion of

dentin which comes in the path of the arc formed by radius

FE and FF with P as the rotation center. Keeping the gingival

bevel at 15° won’t serve any purpose as the filling may be

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rotated out of the cavity because no resistance to

displacement is offered by either the axial wall CG or DG.

Where the buccal and the lingual walls instead of

flaring from the axial line angle to the cavo-surface margin

in a continuous plane are changed into two narrow-and two

smaller diverging planes even with such a modification of

the buccal and lingual proximal walls, it is possible to retain

the retentive stresses of a preparation since the

supplementary diverging planes are mostly line angles

leaving the balance of the wall in the elastic dentin.

If the proximal walls diverge excessively occlusally

from the gingival wall. The reacting stresses in the dentin

since all forces react as displacing stresses. Hence, such a

divergence is not acceptable and every effort should be made

to approach parallelism not exceeding 2°-5° gingivo-

occlusally.

BEVELLING

Bevelling plays a very important role in reducing the

stresses on the remaining tooth structure, thus maintaining

the integrity of both the tooth and the restoration.

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The lower surface bevel helps to seal and protect the

margin resulting in a strong enamel margin with an 140°-

150° angulation. This leaves 30°-40° marginal metal on the

inlay. The marginal gold alloy is too thin and weak if its

angle is less than 30°, conversely the metal at the margins is

too bulky and difficult to burnish, if its angle is greater than

40°.

In small teeth such as premolar, the joining of mesial

and distal cavity across the occlusal surface results in a

considerable weakening of the tooth and an occlusal stress is

liable to produce a vertical fracture.

When it is thought that there is risk of this occurring

the occlusal bevel should be increased so as to extend

beyond the summit of the cusps. Occlusal stresses will then

be taken entirely by the inlay and transmitted to the flat

floor, splitting strains thus being much reduced.

CONCLUSION

Tooth is a engineering marvel, which can withstand

forces because of resiliency of dentin.

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Increased amount of dentin increases the retention of a

restoration and better resistance to forces.

An intact tooth can best withstand forces but when lost

due to caries, has to be replaced by a restorative material.

There are various forces that can act on these restorations

hence based on sound principles these restorations should be

placed so as to prevent their dislodgement and increase the

resistance of tooth as well as restoration to forces.

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FORCES ACTING ON RESTORATIONS

CONTENTS

Introduction

Retention and Resistance forms in general

Forces acting on Class I, Class II, Class V

Direct tooth colored restorations

Cast restorations i.e. Inlay

Conclusion

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