TOOTH PREPARATION FOR CAST RESTORATIONS

Seminar ByDr. S. HIMAGIRI

Postgraduate Student

DEPARTMENT OF CONSERVATIVE DENTISTRY & ENDODONTICSSRI RAMACHANDRA DENTAL COLLEGE AND HOSPITAL

CHENNAI

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CONTENTS

Page No.

Introduction 1

Definition 1

Indications 2

Contra indications 2

Advantages 3

Disadvantage 4

Principles of cavity preparation for cast restoration 4

Preparation features of the circumferential tie 7

Types and design features of occlusal gingival bevel 8

Circumferential tie constituents for extra coronal preparation 11

Types and design features of facial and lingual flares 13

Mechanical problems and preparation design solutions 15

Design of cavity and tooth preparations for cast restorations 20

Modifications for class V cast metals 27

Tooth preparation for only cast restorations 28

Modifications for class – IV and class III 33

Modification for class V cast materials 33

Tooth preparation for cast restoration with surface extension 34

References 39

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INTRODUCTION

Dr. D. Philbrook in 1897 was credited with casting the first

restoration, which was without any evidence.

Taggart in 1907 was credited for improving cast restoration with lost

wax procedure, giving details of the casting procedure.

DEFINITION (Vimal K. Sikri)

An inlay is primarily an intra coronal cast restoration that is designed

mainly to restore occlusal and proximal surfaces of posterior teeth

without involving the cusps and rarely the proximal surface anterior

teeth.

Onlay is a combination of intra coronal and extra coronal cast

restoration when one or more cusps are covered.

Full veneer crown is an extra coronal cast restoration where all cusps

are covered.

ACCORDING TO STURDEVANT

Inlay

Class II inlay involves the occlusal and proximal surface(s) of

posterior teeth and May cap one or more but not all of the cusps.

Onlay

The class II onlay involves the proximal surface(s) of a posterior

tooth and caps all of the cusps.

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INDICATIONS:

Extensive tooth involvement; restorations are efficient in replacing

lost tooth structure and also for supporting remaining tooth surface.

Fractured amalgam restorations.

Adjunct to periodontal therapy to correct tooth anomalies, which

predisposes to periodontal problems.

Restoration of endodontically treated teeth.

Retainers for fixed prosthesis.

Sub gingival lesions: properly finishing and polished gold alloys are

more compatible with the periodontium.

Patients with low incidence of plaque accumulation.

Fracture lines: fracture lines in the enamel, especially in teeth having

extensive restorations, should be recognized as cleavages planes for

possible future fracture of the tooth.

Esthetics: all metallic restorations (except gold) properly fitted cast

restorations are most pleasant esthetically.

CONTRA INDICATIONS:

Physiologically, young dentition with large pulp chambers and

incompletely mineralized dentin are contra indications:

Developing and deciduous teeth

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High plaque / caries indices patient with rampant caries and poor

oral hygiene should not be given cast restoration.

Should not be used in-patient with severing occlusal interference or

other defects in the stomatognathic system.

Dissimilar metals: gold-based castings are avoided in patients

already having silver restorations.

ADVANTAGES:

Yield strength, compressive strength, tensile strength and shear

strength of alloys used for cast restorations are for greater than those

of any materials used intra orally.

Cast alloys are five times the ultimate strength of amalgam.

In impart resistance to the tooth rather than depending on tooth

structure to produce resistance form to the restoration.

Are capable of reproducing precise form and minute detail .

As the cast inlay contains one or more noble metals, they are not

significantly affecting by tarnish and corrosion process.

Restorations can be finished and polished outside the oral cavity there

by producing surface with maximum biological acceptance.

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DISADVANTAGE:

Being a cemented restoration, several inter phases will be created at

the tooth cement casting junction, this leakage is pronounced

gingivally than the other parts of the restoration.

Restorations necessitate extensive tooth involvement in the

preparation, which creates possible hazards for the vital dental tissues.

The cathode nature of cast dental alloys towards amalgam may lead to

galvanic deterioration of amalgam. It these two restorations are

placed adjacent to or opposite to each other.

The procedure is lengthy and requiring more than one visit.

Much more expensive than other restorative materials.

Some cast alloys have a very high abrasive resistance caused wear of

opposing natural tooth.

PRINCIPLES OF CAVITY PREPARATION FOR CAST

RESTORATION

Cast alloys and ceramics can restore teeth via intra and extra coronal

preparations.

Intra coronal is mortise shaped having definite walls and floors

joined at line angle and point angle.

Extra coronal preparations are created by occlusal and axial surface

reduction, in many casts ending gingivally with no definite flat floor.

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CAST PREPARATIONS SHOULD HAVE THE FOLLOWING

FEATURES:

Preparation path:

The preparation will have a single path, an opposite to the direction of

the occlusal loading.

Path is usually parallel to the long axis of the tooth, and it will help

restoration between and decreases its micro movements during

function.

APICO-OCCLUSAL TAPER OF A PREPARATION

Preparation should be without any under cuts.

Maximum retention is achieved by placing opposing walls parallel to

each other.

Since exact parallelism can create technical problem in processing and

in getting final materials into and out of the preparation.

Slight divergences of opposing walls are essential to facilitate cast,

fabrication with minimum errors.

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A taper should be an average of 2 to 5 from the path of the

preparation; it can be decreased or increased according to the

following factors:

1. LENGTH OF THE PREPARATION

The greater the depth of preparation, the more taper will be but it

should not exceed 10.

2. DIMENSIONS AND DETAILS OF SURFACE INVOLVEMENT

The greater the surface involvement is and the more detailed that the

internal anatomy is, the greater will be the frictional component

between the preparation and the materials contacting it.

To diminish friction, the taper is increased but not exceed 10.

3. THE NEED FOR RETENTION

The greater the need for retention is, the more will be the need to

approach exact parallelism.

If carcinogenic and anatomical conditions dictate two different types

for opposing walls, it is preferable to create two planes for each

involved wall, i.e., inner planes parallel to each other and outer planes,

satisfying the needs compelling the different tapers. The inner plane

assures the single insertion of path of preparation.

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PREPARATION FEATURES OF THE CIRCUMFERENTIAL TIE

Peripheral marginal anatomy of the preparation is called

“circumferential tie” and should have the following features,

advocated by NOY:

It the margin ends on enamel

Enamel must supports by dentin

Rods should be continuous with dentin

Rods should be covered with restorative material

Circumferential tie in inlays will be in the form of bevel. It’s directed

away form the cavity preparation.

TYPES AND DESIGN FEATURES OF OCCLUSAL GINGIVAL

BEVEL:

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Partial bevel:

Involves part of the enamel not exceeding 2/3rd of its dimension.

Used to trim weak enamel rods from margin peripheries.

Short bevel

Includes the entire enamel wall employs for class I alloy.

Long bevel

Includes all of the enamel and upto ½ of the dentinal wall.

Used for class I, II and III alloys.

It preserves the internal boxed-up resistance and retention fracture

of the preparation.

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Full bevel

Includes all of the enamel and dentinal wall

Deprives the preparation of its internal resistance and retention.

Its use should be avoided unless impossible to use any other form

of bevel.

Counter bevel:

When capping cusps to protect and support them, this type of bevel

is used, opposite to an axial wall on the facial or lingual surface of

the tooth and it will have a gingival inclination facially or

lingually.

HOLLOW GROUND BEVEL (CONCAVE BEVEL)

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Bevel portion of circumferential tie must have a specific angulation

relative to the remaining portion of the wall.

This allows more space for cast material bulk, to improve materials

castability, retention and better resistance to stresses.

FUNCTION OF OCCLUSAL AND GINGIVAL BEVEL:

Bevels are the flexible extension of a cavity preparation, allowing the

inclusion of surface defects, supplementary grooves, or other areas on

the tooth surface.

Bevels create obtuse angled marginal tooth structure, which is the

bulkiest and the strongest configuration of any marginal tooth

anatomy and produce acute angled marginal cast alloy this

configuration will be the most amenable to burnishing for that alloy.

Bevels are major retention forms for cast restorations.

Reduce the error factor to three or more folds at the margins.

Some bevels like hallow ground and counter bevel, are used for the

resistance form of the tooth-restoration complex by encompassing

cusps.

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CIRCUMFERENTIAL TIE CONSTITUENTS FOR EXTRCORONAL

PREPARATION:

a). The chamfer finishing line; it is the most universal design for class I,

II and III cast metals. It involves bulk and definite termination for

preparation marginally, with little tooth involvement (0.5mm maximal

depth).

Disadvantage: Its only disadvantage is the limited burnishability of the

marginal cast alloy and liability of transitional continuation of a

circumferential tie and adjacent bevel tie. It is most practical type of

finishing line for sub gingival extra coronal preparation. It is

contraindicated for class IV, V cast material due to poor castability.

b). The knife-edge finishing line: Is circumferential tie with least tooth

structure involvement. It should only be used to accommodate a very

castable-burnishAble type of alloy (Gold alloy). It should be located on

assessable areas of the tooth surfaces for proper finishing. It is most

indicated when minimal axial depth is required.

Disadvantage: possibility of indefinite termination for casting.

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There is a chance of the margin not being covered with a casting

made of certain alloys due to lack of bulk space to accommodate less

wetting alloy. There is possibility of fracturing the alloy part of the

circumferential tie during burnishing finishing polishing. It is

contraindicated for class III, IV, V cast material

c). The beveled shoulder finishing line: It is involve with most of tooth.

It is exactly a gingival floor of an intra coronal preparation but on a

smaller scale

1. It is indicated when a definite gingival floor, with all its

components is needed for resistance – retention purposes.

2. Also when maximum bulk of the cast is needed marginally for

material that are limited in their castability or are difficult to

burnish. It is ideal design for sub gingivally located margin

because maximum predictability of the casting termination

gingivally.

It can be used for any cast material its bevel portion could be

hollow ground, as this is most suitable for class IV and V cast

material.

d). The hollow ground (concave) bevel: Is actually an exaggerated

chamfer of a concave beveled shoulder. Its tooth involvement is greater

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than a chamfer and less than a beveled shoulder. It is mechanically

comparable to beveled shoulder and superior to a chamfer care must be

taken to ensure there is no residual friable enamel or thinned tooth

structure at the periphery of this finishing design. It is ideal finishing line

for class IV and V cast material.

Mechanical problems for cast restoration and preparation design

solution in general cast restoration is used for compound and complex

tooth involvement. In addition to the principal retention form previous

described (parallelism, dovetail, surface area frictional retention,

circumferential tie, masticatory load, directed to seat the restoration),

there are numerous axillary’s mean for cast restoration.

TYPES AND DESIGN FEATURES OF FACIAL AND LINGUAL

FLARES:

Flares are the flat or concave peripheral portions of the facial and

lingual walls.

There are two types of flares:

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1. Primary flare

2. Secondary flare

PRIMARY FLARE:

It is the conventional and basic part of circumferential tie facially and

lingually for an intra coronal preparation. It is very similar to long

bevel formed of enamel and dentin on the facial or lingual wall,

primary flares always have a specific angulation, i.e., 45 to the inner

dentinal wall proper.

Main function of flare is to bring the facial and lingual margins of the

cavity preparations to cleansable – finishable areas.

SECONDARY FLARE:

Flat – plane super imposed peripherally to a primary flare

Sometimes it is prepared in a hollow ground form to accommodate

materials with low castability.

Secondary flares may have different angulations; involvement and

extent depend on their function.

Indicated in very widely extended lesions bucco-lingually, results in

primary flare ends with an acute angle marginal tooth structure.

A secondary flare with correct angulation can create the needs obtuse

angulation of marginal tooth structure.

In very broad contact areas or malposed contact area.

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MECHANICAL PROBLEMS AND PREPARATION DESIGN

SOLUTIONS:

There are numerous auxillary means of retension for cast

restorations are present.

a. Luting cements

Action is primarily mechanical, locking the cast to tooth structure

by filling the space between them, wetting the details of both the casting

and tooth preparation and filling in the vacancies or irregularities.

b. Grooves

Should be located completely in dentin

Prepare at the expense of the dentinal portion of the facial or

lingual walls or gingival floors proximally.

It helps to prevent lateral displacement of mesial / distal, facial and

lingual parts of restorations.

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It also improves seating of the restoration and minimize marginal

discrepancies.

Grooves are Prepared with tapered fissure bur and they should not

exceed 2mm in depth.

Reverse bevel

Placed at the expense of the gingival floor, creating an internal

dentinal plane inclining gingivally – axially.

It provides locking the restoration and preventing proximal displacement

of restoration.

Internal box

Prepared in dentin with four vertical surrounding walls joining a floor

at definite line and point angles, it can impart four to twelve times the

retention of an external box of same dimension.

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It is advisable to place internal boxed at the very peripheries of a

cavity preparation.

Should have minimum 2mm in three dimensions.

Slot

Internal cavity within a floor of the preparation having a

continuous surrounding wall and floor, junction between the floor and the

surrounding walls is very rounded.

Pins

Pins can be cemented and threaded, parallel and non-parallel,

vertical and horizontal.

Collor

This is a surface extension completely surrounding a cusp or a

surface of a tooth.

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Skirt

It involves a part of the axial angles of tooth.

Post

They are used as retention mode for as core foundation to be

covered with cast restoration.

Reciprocal retention

In a cavity or tooth preparation to accommodate a cemented type of

restoration, every retention mode must have an opposite retention mode

to completely immobilize the restoration placing retention modes at every

end of the preparation or parts of the preparation is called reciprocal

retention, a basic and design preparation for cast restoration.

Capping the occluso-proximal facial or lingual corner of the

preparation:

This is done for dual purpose of protecting thinned corners, due to

over preparation or wide preparation in an ovoid tooth, and adding to the

restoration retention by locking it, over a facial or lingual corner, a bevel

extension facially or lingually a knife edge or chamfer finishing line.

This procedure is not indicated for class IV and V material.

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Pre-cementation grooving of the casting and the adjacent tooth surface

or walls

To add to the retaining capability of the luting agent, after the

casting is ready for cementation an inverted truncated cone groove is cut

on one or more of the sides of casting and opposite it on the preparation a

similar groove is cut. If they are not opposite one another, the retaining

effect will be minimized. A sufficient amount of cement should be mixed

and flown into the grooves when cementing the casting. It has been

proven that this technique will enhance the retaining effect of the cement

several fold, and it is most effective for class V.

Electrolytic etching of tooth surface of the casting

The internal surface of class IV cast material restoration can be

subjected to electrolytic etching. The margins and external surface of the

restoration are covered with sticky way, and the restoration is used as an

anodic electrode in an electrolytic cell composed of 0.5N nitric acid and a

cathode of another non noble alloy higher in the electromotive force table

a voltage current is passed into cell, leading selective etching of non

noble alloy. Etching process takes 10-15 minutes, to create irregularities.

The other preparation feature that will help solve mechanical

problems of cast restoration.

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All line and point angle should be definite, but not angular.

Roundness for class V material.

Axial wall should slant toward pulpal floor, together with rounding

axio pulpal line angle can reduce stress.

Maximum reduction should be at the occluding surface, especially

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

relation of the mandible average of 1mm should be cleared for metallic

casting in inclined place of cusp. This reduction is 1.5 mm for ceramic

restorations.

DESIGN OF CAVITY AND TOOTH PREPARATIONS FOR CAST

RESTORATIONS

General Shape:

The outline of the occlusal portion of the preparation is the dove-

tailed. The proximal portion is usually boxed in shape.

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Location of margins

In the occlusal portion the facial, lingual, and, sometimes, proximal

margins are located on the inclined planes of the corresponding cusps,

triangular ridges or the marginal ridges (crossing ridges). This is

designed so that the bucco-lingual width of the cavity preparation

(distance measured between the buccal and lingual wall proper),

especially at the isthmus portion, does not exceed one-third the

intercuspal distance. The most peripheral margins of the preparation are

located away form contact with the opposing tooth surfaces during centric

closure and excursive movements of the mandible. All adjacent wear

facet, supplementary grooves and areas of decalcifications, or any defect

in the adjacent parts of the occlusal surface, should be included in the

beveled portion of the cavity preparation only.

In the proximal portion the facial and lingual margins are each in

the corresponding embrasure. This is designed so that the full length of

an explorer can be passed freely in the occluso-gingival direction, and so

that all undermined enamel, surface defects, and peripheral marginal

undercuts are eliminated.

The wall proper, constituting about the pulpal two-thirds of the

facial or lingual (proximal) walls, is formed completely of dentin. These

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walls should taper from each other on the average of 2-5, or be parallel

to each other, if necessary. Each wall should make a right angle or

slightly obtuse angle with the pulpal floor.

The occlusal bevel, which is a long bevel, constituting almost one-

third of the facial and lingual (proximal) walls, this beveled outer pane of

the walls will have an average angulation of 30-45 to the long axis of the

crown.

This angulation should increase as the width of the cavity

preparation increases, in order to accommodate more bulk of cast alloy,

and to be able to resist increased stresses near the cusps on the inclined

planes. Also, this increased angulation is necessary to bevel enamel rods,

which are inclined toward the cusps.

The angulation of the bevel should decrease with increased

steepness of the cusps. Sometimes, bevels are not needed at all in very

steep cusps.

The “bevel” part of the facial and lingual, and, sometimes, the

proximal walls of the inlay cavity preparation will usually be half that of

the cavity “wall proper”. This bevel is extended to include wear facets

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and occlusal defects or decalcifications, if they are confined to the

occlusal surface. They are also extended to include supplementary

groves and to move the margin away from occlusal contacts.

In the inlay cavity preparation, the pulpal floor should be flat over

most of its extent. If this is not possible, at least the peripheral portions

should be flat.

The conventional pulpal depth of the inlay cavity preparation is a

little more than that for amalgam in order to create more length for

surrounding walls. Generally speaking, this depth should be 1-1. 5 mm

from the dentinoenamel junction. The pulpal floor should meet all

surrounding walls in a definite line angle; expect its junction with the

axial wall, where the joint should be very rounded.

In the proximal portion of the inlay cavity preparation, the axial

wall should be either flat or slightly rounded in the bucco-lingual

direction, and either vertical or slightly divergent (5-10) towards the

pulpal floor in the gingivo-occlusal direction. Divergence here is

important only insofar as it imparts some taper on the preparation,

facilitating the procedural steps for the restoration.

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The axial wall should, meet the pulpal floor in an extremely

rounded junction as in amalgam. This prevents stress concentration in the

tooth and the casting. Furthermore, the depth, axially, should ideally be

1-1.5mm from the dentinoenamel junction. However, different depths

may be necessary according to the cariogenic pattern of the dentinal

lesion proximally.

Proximally, facial and lingual walls are comprised of two planes.

In the axial half (i.e., the facial or lingual “wall proper”) it is formed

completely of dentin ad meets the axial wall at a right angle relationship.

This is the main resistance and retention feature of that part of the

cavity preparation. The proximal half of the facial and lingual walls is

formed of a primary flare, comprised of enamel and dentin with an

unchanged 45 angle to the “wall proper.

Sometimes it is necessary to impose a third plane in the form of a

secondary flare, placed on enamel peripherally. This serves to simplify

impressions and wax patter manipulations ad for the other reasons

previously mentioned. Secondary flares should not be used if a direct

wax pattern technique is to be used. The secondary flare can have

variable angulation and extent to achieve its objectives.

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The gingival floor, proximally should be flat in the bucco-lingual

direction, making a slightly obtuse angle with the buccal and lingual

walls.

In the axio-proximal direction, it is formed of two planes. The

axial half consists of gingival wall (floor) proper, being perfectly flat,

formed of dentin, and making either a right angle or a slightly obtuse

angle with the axial wall. The proximal half should be beveled in the

form of a long bevel inclining gingivally. This bevel is usually angulated

on the average of 30-45 to the wall proper. However, this angulation can

be increased by an increase in the gingival extent and length of the

surrounding walls. This will serve to minimize marginal discrepancy.

Although this bevel is usually equal in extent to the “wall proper”,

it can be increased to include notches or surface defects, or decreased if

the margin ends on cementum rather than enamel.

The extent of the bevel may also be decreased if the cavity is

increased in length occluso-apically. This creates more room for the flat

“wall proper”.

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Te junction between the occlusal bevel and the secondary or

primary flares proximally, and also, the junction between the primary or

secondary flares proximally and the gingival bevel should be very

rounded and smooth.

Modifications for Class IV and some Class III materials

Although the general shape, location of margins, and most of the

internal anatomy of preparations for cast alloys in the Class IV (and

sometimes Class III) category are similar to those to be described for

Class I and II alloys, certain specific modifications must be enumerated.

Although the preparation will still contain internal boxed portions

(buccal, lingual, and proximal “wall proper”) occlusally and proximally,

the internal line and point angles should be more rounded.

Surrounding walls should be more parallel to one another. All

circumferential tie constituents (primary or secondary flares, occlusal or

gingival bevels) should be hollow-ground to improve the capability of

these alloys to replicate marginal details during casting.

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Tooth preparation should be deeper axially and pulpally to

compensate for the loss of retention that results from the relatively poor

castability of these alloys.

Modifications For Class V Cast Metals

The internal anatomy of cavity preparations for these alloys will

include the following additional changes.

Definitely flat pulpal and axial walls meet surrounding walls in a very

rounded line angles.

To improve retention, the preparation should be relatively deeper and

with minimal or no taper.

The gingival floor, if its margins end on cementum (dentin) or at the

occlusal or middle third of the anatomical crown may be made a flat,

one-planed floor completely formed of dentin (after cementum

removal), or enamel and dentin terminating in a 90 cavosurface

margin. Although this might reduce the negating effect of bevels on

internal discrepancies shown marginally, exactness of the fit of cast

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ceramics would counteract this problem. This could simply be done

by preparing these surrounding walls in a purely wall proper

configuration with no bevel or flare components. This simplified

cavity preparation could be done provided that.

No undermined enamel is left marginally.

The margins are placed in finish able, cleansable areas

Joint angles are prepared extremely rounded and the

cavosurface ones are made right angles.

Tooth preparation for only cast restorations:

Onlays are the most and universally used cast restoration for

individual teeth. It is partly intra coronal and partly extra-coronal type of

a restoration, which has cuspal protection as the main feature.

Indications:

1. Cuspal protection is to be considered if the width of the lesion is

1/3 to ½ of inter cuspal distance.

2. In the cast restoration cuspal protection is mandatory, if the width

of the lesion is exceeds ½ of inter cuspal distance.

3. In tooth preparation the length width ratio of cuspal is more than

1:1, but not exceeding 2:1, cuspal protection is to be considered.

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4. If length; width ratio is more than 2:1 cuspal protection is

mandated.

5. When need to change the dimension, shape and interrelationship of

the occluding tooth surface the onlay cast restoration are ideal.

6. Onlay is ideal restoration for abutment teeth for a R.P.D (or) fixed

prosthesis.

7. They are ideal supporting restorations for remaining tooth

structure.

8. Onlays are necessary to include wear facets that exceeded the cusp

tips and triangular ridge crests.

General shape:

1. Onlays are dovetailed internally and follow cuspal anatomy

externally.

2. Proximally they appear as box (or) cone shaped.

3. The main feature is to capping of the functional and the shoeing f

non-functional cusps.

Location of margin:

Occluso-facio-lingual portion.

1. On the functional side cusps are capped for the additional retention

and protection.

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2. They must be located for enough gingivally away from contact

with the opposing tooth surface.

3. Normally this will involve ¼ to 1/3 the facial (or) lingual surfaces.

4. Gingivally, margins include all facial (or) lingual grooves and

should be parallel to the contour of cusp tips and crests of adjacent

ridges.

5. On non-functional side, the facial (or) lingual margins located just

gingival to the tip and ridge crests of the cusps and away from

occlusal contact.

Proximal portion:

1. Secondary flares are used in all situations

Internal anatomy:

In occluso-facio-lingulal – portion:

1. The pulpal floor is deeper, and the facial (or) lingual wall will be

formed by four planes.

a. Wall proper : is an a intra coronal portion of the wall which

constitutes atleast half of the vertical height of the total wall.

It is completely in dentin and slightly tapered from the

opposing wall proper by 2 to 5, making a definite angle with

the pulpal floor.

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2. The table is the transitional area between the intra-coronal and

extra-coronal parts of the preparation. It is partly in dentin and in

enamel. It is relived form opposing cusps by at least 1.5mm in

both static and functional contacts. At any location the table

should be flat, following cuspal direction in mesio-distal direction.

The table is one of the major resistance forms of the entire

structure.

Counter Bevel

The counter bevel is formed in enamel and Dentin, and it should be

relieved from opposing cuspal element at least 1mm in both static and

functional occlusal contact.

The hollow ground bevels are on the capped side will have four

different hollow ground bevels corresponding inclined planes of the cusp.

The angulation is from 30-70 between the bevel and long axis of

the teeth.

The angle varies according to:

The amount indicated for facial or lingual surface

The amount of needed retention.

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The type of cast alloy, the less the castability of the alloy the greater

will be the angle.

The shoe

It performs same functions as table. In some situations it end in

facially or lingually with acute angle leaving frail enamel. This should

incline away facially or lingually –gingivally and follows cuspal

anatomy. The peripheral partial bevel is indicated with difficult

continuation between a shoe and a primary (or) secondary flare

proximally.

Proximal portion

The internal anatomy of tooth preparation for on lay proximally is

very similar to proximal part in cavity preparation for inlays. For onlays,

however the secondary flare with its flexible angulation is feature of

cavity preparation.

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Modifications for class – IV and class III

1. The occlusal reduction must be greater, in order to accommodate

bulkier cast material.

2. All circumferential tie constituents must be hollow ground

3. All cusps must be capped rather than shoed, as a means of cuspal

protection.

4. The preparations should not feature any small-complicated internal

(or) external details.

5. The concavity of hollow-ground bevels should include enamel and

dentin.

Modification for class v cast materials:

1. Cuspal protection must take the form of capping rather than

shoeing.

2. In most cases, capped cusps consist of 3 distinct planes an hollow

ground bevel extending to pulpal floor to the table.

The counter bevel should be more acute than that described for

class I and class II alloys.

3. There is more occlusal reduction for the table and counter bevel to

accommodate sufficient bulk of cast ceramic.

4. The gingival, buccal, and lingual walls proximally should be

similar to inlays.

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5. The preparation should be deeper than that for class – I and II cast

alloys due to absence of “boxed up” internal portions of

restorations.

6. No tapering should be exhibited by any wall

7. In cases of extreme occlusal involvement bucco-lingually this

modification may be accomplished.

8. The junctions between the walls and pulpal floor and table are

very rounded.

9. The facial (or) lingual margins ends in the middle or occlusal 1/3

of that surface.

Tooth preparation for cast Restoration with surface extension.

These are modification for basic onlay and inlay tooth preparations

and restoration involving part (or) all the axial surfaces.

There are three types of surface extensions that may be imposed on

any type of tooth.

Reverse secondary flare:

This is a surface extension of the basic intra coronal inlay (or)

onlay.

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Indications:

Surface extensions are required to include facial (or) lingual defects

beyond the axial angle of tooth.

They are used to eradicate peripheral undercuts.

To add retentive capability of the restoration proximally.

This type is contraindicated in class – IV and V restoration.

Reverse secondary flares can be added to cavity preparation in lieu of

a secondary flare, directly over primary flare.

It can also be placed in a cavity preparation upon a secondary flare.

The reverse secondary flare can fulfill the form of partial bevel. It

involves enamel only, with the main cavity preparation.

It ends on the facial (or) lingual surface with a knife-edge finishing

line and its extent should not exceed the height of contour of facial

(or) lingual surface in mesio-distal direction nor should it included the

tip of cups.

Skirt

This is more extensive surface extension than reverse secondary

flare, also super imposed on the basic intra-coronal inlay or onlay cavity

preparation facially (or) lingually.

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Indications

Skirting is to involve defects with more dimensions than those that can

be involved in a reverse secondary flare.

It is required to impart resistance and retention on a cast restoration.

It is necessary when contact areas and contour of proximal surfaces

are to be changed in the contemplated restorations.

Skirts are essential facially and lingually for tilted Teeth in order to

restore the occlusal plane and are prepared at the side forwards, which

the tooth is tilted.

Features:

Skirts include facial and lingual surfaces at axial angles to a depth 0.5-

1 mm in class –I and II alloys, and a depth of 1.5 – 2 mm for class III,

IV and V cast materials.

The maximum depth of skirts should be at the junction of the surface

extension with the cavity preparation.

For class – I, II and III skirt ends at mesially (or) distally in a chamfer

line. For class IV and V the skirt ends in a hollow ground bevel.

It is sometimes preferable to terminate the skirt mesially or distally in

a vertical groove, and is used to accommodate bulky material.

The depths should be 1-2mm for classes I and II and 2mm for cast

ceramics.

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The groove should be completely rounded in mesio-distal direction for

cast ceramics and surrounding walls for cast alloys.

Every effort should be made to make axial reduction of the skirt

parallel to the rest of cavity preparation and some times it is even

tapered to remainder of preparation.

For class I, II, III and IV cast alloys, intervening facial or lingual wall

proximally between cavity preparation proper and the skirt extension.

Should have boxed portion and a primary flare.

If the skirt to be used to change the contact and contour of the tooth, it

should be extended far enough on the facial and lingual surface of a

tooth create sufficient retention for the cast material.

Collar

This type of surface extension is the most involving surfacewise and

depthwise. And it may be one of two types.

Cuspal collars involve facial (or) lingual surfaces of one cusp only in

multi-cuspal tooth.

Tooth collars, which involve the entire facial or lingual surface of the

tooth.

Either type can surround the cuspal elements or apical to an already

lost cuspal element.

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Indications:

They help in retention and resistance when an entire cusp is lost prior

to tooth preparation.

They help retention in shortened teeth.

They help resistance and to enhance support for the tooth that is

endodontically treated.

They are used in situations where pins are contra indicated.

They are used in teeth with large foundations replacing cuspal

elements.

They are used in a cast alloy rest to be veneered by fused porcelain.

Features:

With axial depth of 1.5 mm- 2mm collar extensions and gingivally in

a beveled shoulder finishing line.

For class IV cast material the beveled portion of the shoulder should

be hollow ground.

For cast ceramic materials the bevel is rounded and no bevel required.

In cast alloy there should be definite line angle gingivally.

Collars should have less tapering toward the cavity preparation and

this improves retention in these shortened teeth.

The shoulder portion of collar should be perpendicular to the long axis

of the crown.

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REFERENCES

1. Gerald T. Charbeneau. Principles and practice of operative

dentistry. 3rd Edition. Varghese Publishing. 351-356.

2. William H. Gilmore. Operative Dentistry. 4th Edition. B.I.

Publication Pvt. Ltd. 260-268.

3. Lioyd Baum. Textbook of Operative Dentistry. 3rd Edition. W.B.

Saunders Company. 470-484.

4. Theodore M. Robertson. Herald O. Heyman. Sturdevants art

and science of operative dentistry. IV Edition. Mosby Company.

801-826.

5. Vimal K.Sikri. Textbook of operative dentistry. CBS Publishers.

243-256.

6. M.A. Marzouk., A.L. Simonton., R.D. Gross. Operative dentistry

modern theory and practice. All India Publishers and Distributors.

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