Tooth preparation for cast restorations seminar/ dental implant courses by Indian dental academy

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TOOTH PREPARATION FOR CAST RESTORATIONS

Seminar By Dr. R.SUNITHA Postgraduate Student

DEPARTMENT OF CONSERVATIVE DENTISTRY & ENDODONTICS CKS TEJA INSTITUTE OF DENTAL SCIENCES AND RESEARCH TIRUPATHI

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CONTENTS

Introduction Definition Indications Contra indications Advantages Disadvantage Principles of cavity preparation for cast restoration Preparation features of the circumferential tie Types and design features of occlusal gingival bevel Circumferential tie constituents for extra coronal preparation Types and design features of facial and lingual flares Mechanical problems and preparation design solutions Design of cavity and tooth preparations for cast restorations Modifications for class V cast metals Tooth preparation for only cast restorations Modifications for class – IV and class III Modification for class V cast materials Tooth preparation for cast restoration with surface extension References

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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.

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.

4  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  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.

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.

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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.  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: Partial bevel:  Involves part of the enamel not exceeding 2/3 rd 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.

8  Used for class I, II and III alloys.  It preserves the internal boxed-up resistance and retention fracture of the preparation.

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)  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.

9 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.

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-burnishible type of alloy (Gold alloy).

It should be located on

assessable areas of the tooth surfaces for proper finishing. indicated when minimal axial depth is required.

Disadvantage: possibility of indefinite termination for casting.

It is most

10 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 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 are 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.

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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: 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.  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.

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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.  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.

Skirt It involves a part of the axial angles of tooth.

14 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.

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

15 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.

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.

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,

16 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 walls should taper from each other on the average of 2-5째, 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 30-45째 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.

17 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 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.

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.

18 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.

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.

19 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”.

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.

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.

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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, oneplaned 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 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.

21 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.

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 nonfunctional cusps.

Location of margin: Occluso-facio-lingual portion.

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

2. They must be located for enough gingivally away from contact with the opposing tooth surface.

3. Normally this will involve Âź 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:

22 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.

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

The type of cast alloy, the less the castability of the alloy the greater will be the angle.

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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.

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.

24 4. The gingival, buccal, and lingual walls proximally should be similar to inlays.

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.

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.

25  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.

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.  The groove should be completely rounded in mesio-distal direction for cast ceramics and surrounding walls for cast alloys.

26  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 multicuspal 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.

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.

27  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.

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.

3 rd 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. 243256.

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

Operative dentistry

modern theory and practice. All India Publishers and Distributors.