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INTRODUCTION: Many of the restorative problems that need solution cannot be resolved using amalgam or composite resin. A cast restoration is usually the restoration of choice when there is no support for materials like amalgam, Direct filling gold or resin. Dr. John Murphy (1837) first tried to form cast restorations by platinum foil matrix technique. Dr Philbrook (1897) is first introduced the technique of investing and casting an inlay. But it was Dr. Taggart (1907) who permanently changed the practice of restorative dentistry by introducing pneumatic pressure casting technique and other refinements. The cast metal restoration is extremely versatile and is especially applicable to class II preparations. The restoration procedure requires meticulous care both in cavity preparation and in proper manipulation of dental materials. DEFINITION An inlay may be defined as a restoration which has been constructed out of mouth from gold, porcelain or other material and then cemented into the prepared cavity of a tooth. 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. 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.

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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.

DISADVANTAGE: •

Being a cemented restoration, several interphases will be created at the tooth cement casting junction, this microleakage 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 and the procedure is technique sensitive

Much more expensive than other restorative materials.

Some cast alloys have a very high abrasive resistance caused wear of opposing natural tooth.

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.

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Subgingival lesions: properly finishing and polished gold alloys are more compatible with the periodontium.

Patients with low incidence of plaque accumulation.

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 severe occlusal interference or other defects in the stomatognathic system.

Dissimilar metals: gold-based castings are avoided in patients already having silver restorations.

MATERIALS FOR CAST RESTORATIONS: Until recently gold based alloys have been the only ones used for cast dental restorations. The ADA Sp. #5 still requires 75% of gold and platinum group metals to be present in alloys for a cast restoration. There are a number of other materials available to the profession. According to Sturtevant there are 5 distinct groups of alloys. 1. The traditional high gold alloys. 2. Low gold alloys. 3. Palladium-silver alloys. 4. Base metal alloys. 5. Recent-modified brasses and bronzes. According to Marzouk: Class I – Gold and platinum group based alloys. They are Type I, II, III, IV gold alloys. a) Type I- Soft, VHN = 50-90 b) Type II – Medium, VHN= 90-120 c) Type III- Hard, VHN= 120-150 d) Type IV- Extrahard, VHN>150

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Class II – Low gold with gold content less than 50%. Some may contain as less as 5% gold. Class III – Non-gold palladium based alloys. Class IV – Nickel-chromium based alloys. Class V – Castable, moldable ceramics. MOUTH PREPARATION PRIOR TO CAST RESTORATIONS For a single tooth a cast restoration should be the ultimate and final restoration. Every precaution should be taken to insure the longevity of such a restoration. Following are some of the measures to be taken before preparing a tooth to receive a cast restoration. 1. Control of plaque Due to the vulnerability of the cast/cement/tooth structure junction, patients to receive a cast restoration should exhibit the ability, willingness and practice of control measures for their plaque. As a rule, patients should show a 10% or less plaque index prior to fabricating cast restorations for their teeth, or the result will be a futile restorative attempt. 2 . Control of caries Understandably, before planning a cast restoration, which may be more durable from the physical and mechanical aspects, but not so from the cariogenic aspect, rampant or uncontrolled carious processes should be halted. Indirect pulp capping and/ or amalgam-composite resin restorative procedures should be employed until the patient demonstrates the ability to control plaque, and, subsequently, demonstrates little or no incidence of decay recurrence. 3. Control of periodontal problems •

It is ideal to start cast restoration fabrication with a sound periodontium, unless, of course, these restorations are part of the periodontal therapy and maintenance. In the latter case, the periodontal disease should be under control.

The pockets should be eradicated, bone resorption arrested, defects corrected, exposed roots and crown surfaces free from deposits, gingival tissues healed, and apparent clinical crown dimensions stable.

4. Proper foundation

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Badly broken down teeth will need a substructure or a foundation before preparing them for a cast restoration.

The need for such a foundation should be diagnosed and proper material and techniques should be employed before preparing the tooth for a cast restoration.

Nothing is more frustrating than attempting to build up a tooth in a foundation form after unsuccessfully attempting to prepare the tooth for a cast restoration.

5. Control of the pulpal condition of the tooth •

Cast restorations are used for teeth with extensive defects, teeth that usually have been previously restored one or more times with amalgam or other materials. The pulpdentin/root-canal complex of these teeth has been subjected to numerous traumas which invariably affects their physiology.

In many instances, these teeth, after being subjected to the additional trauma of cast restoration procedures, will undergo irreversible pathologic changes necessitating endodontic therapy.

Therefore a proper preoperative evaluation of the condition of the pulp-dentin/rootcanal system is essential. If irreversible pathologic pulpal changes are present, endodontic therapy should be part of the mouth preparation prior to the cast fabrication.

6. Occlusal equilibration Prior to preparing teeth for cast restoration, is vitally important to equilibrate the natural dentition. There must be no interfering or premature contacts, and there should be a pattern of a reliable protective mechanism for mandibular disclusion 7. Diagnostic wax-ups and temporary restorations •

Occasionally, cast restorations are part of the overall therapy to create a physiologically functioning stomato-gnathic system. This may involve certain changes in the anatomy of occluding surfaces.

Full arch study models should be made, and properly mounted on a semi- or fullyadjustable articulator. The involved teeth should be reduced and diagnostic wax-ups made in the desired occlusal shape and relationship.

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Duplicate stone models are then fabricated to serve as an aid in the construction of both temporary and final restorations.

The teeth are roughly prepared and restored with temporary restorations that create the desired features that will be incorporated in the final cast restorations. These temporary restorations are usually made of reinforced resinous materials.

Patients should wear these temporaries and be examined periodically. Changes can then be made in the temporaries to achieve the utmost compatibility between the stomato-gnathic system and these restorations.

When this is achieved and verified, the teeth are finally prepared, and cast restorations are fabricated as replicas of the temporaries, which proved to be physiologic and therapeutic to the stomato-gnathic system.

Because of the permanency of cast restorations, it is mandatory to plan the restorative treatment for the entire mouth prior to fabrication of cast restorations.

Treatment may include full dentures, partial dentures, fixed bridges, etc.

So it is necessary to know in advance the location of the contemplated cast restoration, so that modifications may be made in the tooth preparation, restoration dimension and shape, etc., to satisfy the required role of the cast restoration in the restorative treatment for the entire stomato-gnathic system.

PRINCIPLES

OF

CAVITY/TOOTH

PREPARATION

FOR

CAST

RESTORATION Unlike amalgam cast alloys and ceramics can restore teeth via intra and extracoronal preparations. •

Intracoronal preparations are 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 cases ending gingivally with no definite flat floor.

Most single-tooth restorations are combinations of these intra- and extracoronal types. The general principles of cavity-tooth preparation may be applied without deviation to cast a restoration: 1) Outline form 2) Retention form and resistance form 3) Removing carious dentin

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4) Convenience form 5) Finishing enamel wall and margin 6) Cleaning and critical appraisal of the cavity. In the preparation for a cast restoration there is greater surface extension in the outline form than in the case of amalgam. Besides all the general principles of a cavity and tooth preparation, cast restoration should have the following features: 1) Preparation path 2) Apico-occlusal taper of a preparation 3) Preparation features of the circumferential tie 4) Mechanical problems and preparation design solutions 1. Preparation path The preparation will have a single insertion (draw) path, opposite to the direction of the occlusal loading. This path is usually parallel to the long axis of the tooth. So that the completed cavity will have draft (no undercut).

2. Apico-occlusal taper of a preparation: •

For maximum retention in a cast restoration opposing walls and opposing axial surfaces of a tooth preparation should be perfectly parallel to each other.

Since exact parallelism can create technical problems a slight divergence of opposing axial wall intracoronally and a slight convergence of opposing axial surfaces extracoronally are essential to facilitate cast fabrication with minimum errors.

This taper should be an average of 2-5° from path of preparation.

It can be decreased or increased according to the following factors : 1. Length of the preparation wall /axial surface. Greater the wall – more taper will be necessary although it should not exceed 10°. Less the length, less the taper will be.

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2. Dimensions and details of surface involvement and internal anatomy in the preparations. The greater the surface involvement is and more detailed 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. 3. The need for retention. The greater the need for retention is the more will be the need to approach parallelism (i.e.) less taper. If 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 3. Preparation features of the circumferential tie: The peripheral margin anatomy of the preparation is called circumferential tie. This should have the following features advocated by Noy for an ideal cavity wall namely 1. Enamel must be supported by sound dentin. 2. Enamel rods forming the cavosurface margin should be continuous with sound dentin. 3. Enamel rods forming the cavosurface margin should be covered with the restorative material. 4. Angular cavosurface angles should be trimmed. For the occlusal and gingival walls in intracoronal cavity preparation the tooth circumferential tie will be in the form of a bevel, which is a plane of a cavity wall or floor directed away from the cavity preparation. Types and design features of occlusal and gingival bevels. a. Partial bevel – involves part of enamel wall, not exceeding 2/3rds its dimension. b. Short bevel – includes entire enamel wall, but not dentin. This level is used with class I alloys especially for type 1 and 2.

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c. Long bevel – includes all enamel and upto ½ of the dentinal wall. Most frequently used level for the first 3 classes of cast material. Its major advantage is that it preserves the internal ‘boxed-up’ resistance. d. Full bevel – includes all of the dentinal and enamel walls of the cavity wall or floor. Its use should be avoided except in cases where it is impossible to use any other form of bevel. e. Counter bevel – when capping cusps to protect and support these, this type of bevel is used, opposite to an axial cavity wall, on the facial or lingual surface of the tooth. f. Hollow ground (concave) bevel – All the types of bevels are in the form of a flat plane, but any of them especially the last three can be prepared in a concave form. 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.

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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

EXTRACORONAL

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. 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 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. Types and design feature of facial and lingual flare For the facial and lingual proximal walls in intracoronal cavity preparation for castings, flares are used, which are the flat or concave peripheral portions of the facial and lingual walls. There are 2 types of flares; -

The primary flare

-

The secondary flare

primary flare -

is the conventional and basic part of the circumferential tie facially and lingually for an intracoronal preparation.

-

It is very similar to a long bevel formed of enamel and part of dentin on the facial or lingual wall. 11


-

Primary flares also have a special angulation i.e. 45째 to the inner dentinal wall proper.

They bring the facial and lingual margins of the cavity preparation to cleansable finishable areas. They are indicated for any facial or lingual proximal wall of an intracoronal cavity preparation.

Secondary flare: -

It is almost always a flat plane super imposed peripherally to a primary flare.

-

It is usually prepared solely in enamel.

-

Unlike primary flares, secondary flares may have different angulations, involvement and extent depending on their function.

Indicated in very widely extended lesions bucco-lingually and in very broad contact areas or malposed area.

4. Mechanical problems for cast restorations and preparation design solutions in general. The numerous auxiliary means of retention for cast restorations are: a. Luting cements -

Action is primarily mechanical, locking the casting

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.

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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.

-

Grooves can also be located externally for extracoronal preparation

c. 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.

d. 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.

e. External box -

can be proximal, facial or lingual

-

can be stepped gingivally or occlusally

-

Internal cavity within a floor of the preparation having a continuous

f. Slot surrounding wall and floor, junction between the floor and the surrounding walls is very rounded.

-

Less locking than internal box

-

Indicated for replacing an occlusal dovetail

-

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

g. Pins and horizontal. h. Collar -

This is a surface extension completely surrounding a cusp or a surface of a tooth.

i. Skirt – It is specific extension involving a part of the axial angles of tooth.

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j. Post- They are used as retention mode for as core foundation to be covered with cast restoration. k. 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.

-

They can be in form of opposite groove or internal box to oppose an internal box, a dove tail to oppose a proximal external box.

l.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.

m.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.

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n.Electrolytic etching of tooth surface of the casting The internal surface of class V cast material restoration can be subjected to electrolytic etching.

The margins and external surface of the restoration are

covered with sticky wax, 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. o. Capping of cusps- encompassing a cusp with casting for resistance and or retention p. Capping of marginal ridge if there is no adjacent tooth , the marginal ridge may be capped for added retention and resistance.

q. Grossly roughening , irregularizing or multiple leveling the surface of the preparation.

The other preparation features 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.

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Designs of cavity and tooth preparations for cast restorations: There are 5 general designs of tooth preparation to accommodate a cast restoration namely: 1. Inlays. 2. Onlays. 3. Cast restoration with surface extensions. 4. Pin-lays. 5. Full veneer cast or cast based restoration. Tooth preparations for Inlay Cast restoration The class II inlay involves the occlusal and proximal surfaces of a posterior tooth and may cap one or more cusps but not all of the cusps. General shape: The outline of the occlusal portion of this preparation is dove-tailed. The proximal portion is usually boxed in shape.

Location of margins: 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 – the most peripheral margins of the preparation are located away from contact with the opposing tooth surfaces during centric closure. All adjacent wear facets, supplementary grooves, and areas of decalcifications, or any defect in the adjacent parts of the occlusal surface should be included in the beveled part of the cavity preparation only.

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Proximal portion: The facial and lingual margins are each in the corresponding embrasures. The more inaccessible this portion of the tooth preparation is the more should be this proximal extension, but it should always stop short of the axial angle of the tooth. Extensions should be made in and with the flared portion, not with the wall proper. Extension gingivally should be accomplished with the bevel and not with the wall proper. Internal anatomy: Occlusal portion: The facial and lingual walls and sometimes the proximal walls will be formed of 2 parts. 1. The wall proper, constituting about the pulpal 2/3 of the facial or lingual wall is formed completely of dentin. These walls should taper from each other on the average of 2-5° or be parallel to each other. Each wall should make a right angle or a slightly obtuse angle with the pulpal floor. 2. The occlusal bevel, which is a long bevel constitute almost one-third of the facial and lingual walls. •

This beveled outer plane of the walls will have an average angulation of 3045° to the long axis of the crown. This angulation should increase with the width of the cavity preparation in order to: -

Accommodate more bulk of cast alloy.

-

To be able to resist increased stresses near the cusps on the inclined planes.

This increased angulation is necessary to bevel enamel rods, which are inclined towards the cusps, in the occlusal, one third of inclined planes. •

The angulation of the bevel should decreases with the increased steepness of the cusp. Bevels are not needed at all in very steep cusps, especially in a very narrow preparation. An increased angulation is necessary also for a direct wax pattern as compared to a cavity preparation for an indirect pattern. Therefore more marginal bulk is required for the direct wax pattern technique.

The bevel part of the facial and lingual and sometimes the proximal walls of the inlay cavity preparation will usually be half of the cavity wall proper.

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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 grooves and to move the margin away from the occlusal contact.

In wider cavities and in deeper ones they are extended to improve the taper and reduce frictional components for easier material manipulation.

In the inlay cavity preparation, the pulpal floor should be flat over most of its extent. If this is not possible at least peripheral portion should be flat.

According to Marzouk the conventional pulpal depth should be little more than that for amalgam in order to create more length for surrounding walls. According to Gillmore the depth is limited when compared with the amalgam restoration because bulk is not necessary to resist fracture. The depth of the cavity should be 1 - 1.5 mm from the DEJ. The pulpal floor should meet all surrounding walls in a definite line angle, except at its junctional with the axial wall, where the joint should be rounded.

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Proximal portion: •

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.

The depth axially should be 1 - 1.5 mm from the DEJ. However different depths may be necessary according to the cariogenic pattern of the dentinal lesion proximally.

Proximally the facial and lingual walls are comprised of two planes. 

In the axial half it is formed completely of dentin and 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 3rd plane in the form of secondary flare, placed on enamel peripherally. This servers to simplify impression and wax pattern manipulation. Secondary flare should not be used if a direct wax pattern technique is to be used.

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 axioproximal direction, it is formed of 2 planes. 

The axial half consists of gingival floor (wall) proper, being perfectly flat, formed of dentin, and making either a right angle or 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. 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. 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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The junction between the occlusal bevel and the secondary or primary flare proximally and also the junction between the primary or secondary flares proximally and the gingival bevel should be very rounded and smooth.

Some of the retention means eg. Facial, lingual and gingival grooves proximally, internal boxes or slots occlusally, capping corners of cusps etc, can be used.

In modifications of Class II cavity preparation for esthetics the secondary flare is omitted for esthetic reasons on the mesio-facial proximal wall. The wall is completed with minimal extension by using hand instrument only 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.

1. 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. 2. Surrounding walls should be more parallel to one another. i.e., less taper 3.

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.

4. 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 Materials The internal anatomy of cavity preparations for these alloys will include the following additional changes. 1. Definitely flat pulpal and axial walls meet surrounding walls in a very rounded line angles. 2. There is no decisive differentiation between surrounding walls and the circumferential tie. Both can be in the form of concave surrounding walls. 3. To improve retention, the preparation should be relatively deeper and with minimal or no taper. Auxillary retention means should be used heavily. 4. 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 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 Class II cast metal inlay Initial Preparation. •

Carbide burs used to develop the vertical internal walls of the preparation for cast metal inlays and onlays are plane cut, tapered fissure burs. Recommended dimensions and configurations of the burs to be used are No. 271 and the No. 169L (Brassier USA, Inc., Savannah, Georgia). Note that the sides and end surface of the No. 271 bur meet in a slightly rounded manner so that sharp, stress-inducing internal angles will not be formed in the preparation. The marginal bevels are placed with a slender, finegrit, flame shaped diamond instrument, such as the No. 8862 (Brassier USA, Inc., Savannah, Georgia). 22


Throughout preparation for a cast inlay, the cutting instruments used to develop the vertical walls are oriented to a single "draw" path, usually the long axis of the tooth crown, so that the completed preparation will have draft (no undercuts).

The gingival-to-occlusal divergence of these preparation walls may range from 2 to 5 degrees per wall from the line of draw.

If the vertical walls are unusually short, a maximum of 2 degrees occlusal divergence is desirable to increase retention potential. As the occlusogingival height increases, the occlusal divergence should increase because lengthy preparations with minimal divergence (more parallel) may present difficulties during pattern withdrawal, trial seating and withdrawal of the casting, and cementing.

Occlusal Step. •

With the No. 271 carbide bur held parallel to the long axis of the tooth crown, enter the fossa/pit closest to the involved marginal ridge, using a punch cut to a depth of 1.5 mm to establish the depth of the pulpal wall.

In initial preparation do not exceed this specified depth, regardless of whether the bur end is in dentin, caries, old restorative material, or air.

The bur should be rotating at high speed (with air-water spray) before application to the tooth and should not stop rotating until it is removed.

A general rule is to maintain the long axis of the bur parallel to the long axis of the tooth crown at all times .For mandibular molars and second premolars whose crowns tilt slightly lingually, this rule dictates that the bur should tilt slightly (5 to 10 degrees) lingually to conserve the strength of the lingual cusps.

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Maintaining the 1.5-mm initial depth and the same bur orientation, extend the preparation outline mesially along the central groove/fissure to include the mesial fossa/pit .

Occasionally a fissure extends onto the mesial marginal ridge. This defect, if shallow, may be treated with Enameloplasty, or it may be included in the outline form with the cavosurface bevel, which is applied in a later step in the tooth preparation.

The facial and lingual extension in the mesial pit region should provide the desired dovetail retention form, which resists distal displacement of the inlay .

If major facial or lingual extension is required to remove undermined occlusal enamel, capping the weak remaining cuspal structure may be indicated, as well as additional features in the preparation to provide adequate retention and resistance forms.

Continuing at the initial depth, extend the occlusal step distally into the distal marginal ridge sufficiently to expose the junction of the proximal enamel and the dentin.

While extending distally, progressively widen

the preparation

to the desired

faciolingual width in anticipation for the proximal box preparation. •

The increased faciolingual width enables the facial and lingual walls of the box to project (visually) perpendicularly to the proximal surface at positions that will clear the adjacent tooth by 0.2 to 0.5 mm.

If the occlusal step has been prepared correctly, any caries on the pulpal floor should be uncovered by facial and lingual extension to sound enamel (supported by dentin).

Proximal Box. •

Continuing with the No. 271 carbide bur, isolate the distal enamel by cutting a proximal ditch.

24


The mesiodistal width of the ditch should be 0.8 mm (the tip diameter of the bur) and prepared approximately two thirds (0.5 mm) at the expense of dentin and one third (0.3 mm) at the expense of enamel.

The gingival extension of this cut may be checked with the length of the bur by first measuring the depth from the height of the marginal ridge and then removing the bur and holding it beside the tooth. A periodontal probe also may be used for this measurement.

While penetrating gingivally, extend the proximal ditch facially and lingually beyond the caries to the desired position of the facioaxial and linguoaxial line angles. If the carious lesion is minimal, the ideal extension facially and lingually will be as previously described. Ideal extension gingivally of a minimal, cavitated lesion will eliminate caries on the gingival floor and provide 0.5-mm clearance of the unbeveled gingival margin with the adjacent tooth.

Moderate-to-extensive caries on the proximal surface dictates continued extension of the proximal ditch to the extent of the caries at the dentinoenamel junction (DEJ), but not pulpally.

When preparing the proximal portion of the preparation, maintain the side of the bur at the specified axial wall depth regardless of whether it is in dentin, caries, old restorative material, or air.

Guard against overcutting the facial, lingual, and gingival walls, which would not conserve tooth structure and could result in: (1) overextension of the margins in the completed preparation, (2) a weakened tooth, and (3) possible injury of the soft tissue.

The axial wall should follow the contour of the tooth faciolingually. Any carious dentin on the axial wall should not be removed at this stage of preparation.

Then with the No. 271 carbide bur, make two cuts, one at the facial limit of the proximal ditch and the other at the lingual limit, extending from the ditch perpendicularly toward the enamel surface (in the direction of the enamel rods.

25


Extend these cuts until the bur is nearly through the marginal ridge enamel (the side of the bur may emerge slightly through the surface at the level of the gingival floor). This weakens the enamel by which the remaining isolated portion is held.

At this time, however, the remaining wall of enamel often breaks away during cutting, especially when high speeds are employed. If the isolated wall of enamel is still present, it can be fractured out with a spoon excavator.

Planing the distofacial, distolingual, and gingival walls by hand instruments to remove all undermined enamel may be indicated if minimal extension is needed to fulfill an esthetic objective.

Depending on access, use a No. 15 (width) straight chisel, binangle chisel, or enamel hatchet. Plane the wall by holding the instrument in the modified palm-and-thumb grasp, and use a chisel-like motion in an occlusal-to-gingival direction.

Plane the gingival wall by using the same instrument as a hoe, scraping in a lingualto-facial direction.

26


When proximal caries is minimal, ideal facial and lingual extension at this step in the preparation results in margins that clear the adjacent tooth by 0.2 to 0.5 mm.

The experienced operator usually does not use chisel hand instruments during preparation for inlays, inasmuch as the narrow, flame-shaped, fine-grit diamond instrument, when artfully used, will remove ragged, weak enamel during application of the cavosurface bevel and flares and will cause the patient to be less apprehensive.

Shallow (0.3 mm deep) retention grooves may be cut in the facioaxial and linguoaxial line-angles with the No. 169L carbide bur. These grooves are indicated especially when the prepared tooth is short.

Final Preparation Removal of Infected Carious Dentin and Pulp protection. •

After the initial preparation has been completed, evaluate the internal walls of the preparation visually and tactilely (with an explorer) for indications of remaining carious dentin.

If carious dentin remains, and if it is judged to be infected, but shallow or moderate (1 mm or more of remaining dentin between the caries and the pulp), satisfactory isolation for removal of such caries and the application of any necessary base may be attained by the reduction in salivation resulting from anesthesia and the use of cotton rolls, a saliva ejector, and gingival retraction cord.

Use a slowly revolving round bur (No. 2 or No. 4) or spoon excavator to remove the carious infected dentin. If the bur is used, improve visibility by using air alone. This excavation is done just above stall-out speed with light, intermittent cutting.

Light-cured glass-ionomer cement may be mixed and applied with a suitable applicator to these shallow (or moderately deep) excavated regions to the depth and form of the ideally prepared surface.

27


The material is applied by conveying small portions on the end of a periodontal probe and is light-cured when the correct form has been achieved . Any excess cement can be trimmed back to ideal form with the No. 271 carbide bur after the cement has hardened.

If the carious lesion is judged to closely approach the pulp, a rubber dam should be applied before the removal of infected dentin.

When excavating extensive caries, attempt to remove only the infected dentin and not the affected dentin, since removal of the latter might expose a healthy pulp

If removal of soft, infected dentin leads directly to a pulpal exposure (carious pulpal exposure), then root canal treatment should be accomplished before completing the cast metal restoration.

If the pulp is inadvertently exposed as a result of operator error or misjudgment (mechanical pulpal exposure), then it must be decided whether to proceed with root canal treatment or attempt a direct pulp capping.

A fourable prognosis for the pulp after direct pulp capping may be expected if the following criteria are met : •

The exposure is small (less than 0.5 mm in diameter).

The tooth has been asymptomatic, showing no signs of pulpitis.

Any hemorrhage from the exposure site is easily controlled.

The invasion of the pulp chamber was relatively «i traumatic with little

physical irritation to the pulp tissue.

28


A clean, uncontaminated operating field is maintained (i.e., a rubber

dam). •

It the excavation closely approaches the pulp or if a direct pulp cap is indicated, first apply a lining of calcium hydroxide using a flow technique (without pressure). This calcium hydroxide liner should cover and protect any possible near or actual exposure and also extend over a major portion of the excavated dentin surface.

Leave the peripheral 0.5 to 1 mm of the dentin excavation available for bonding the light-cured glass-ionomer cement base subsequently applied

Also small mechanical undercuts can increase the retention of the glass-ionomer base. If suitable undercuts are not present after removal of infected dentin, retention coves are placed with the No. 1/4 carbide bur. These coves are placed in the peripheral dentin of the excavation and are as remote from the pulp as possible.

The light-cured glass-ionomer cement should be applied without pressure. It should completely cover the calcium hydroxide lining and some peripheral dentin for good adhesion.

Remaining old restorative material on the internal walls should be removed if any of the following conditions are: (1) the old material is judged to be thin and or nonretentive, (2) there is radiographic evidence of caries under the old material. (3) the pulp was symptomatic preoperatively. or (4) the periphery of the remaining restorative material is not intact, (i.e., there is some breach in the junction of the material \with the adjacent tooth structure that may indicate caries under the material). If none of these conditions is present, the operator may elect to leave the remaining restorative material to serve as a base, rather than risk unnecessary removal of sound dentin or irritation or exposure of the pulp.

The future need of root canal therapy is a possibility for any tooth treated for deep caries that approximates or exposes the pulp. When treating a tooth that has had such extensive caries, consider (1) reducing all cusps to cover the occlusal surface with metal, for better distribution of occlusal loads and (2) adding skirts to the preparation

29


to augment resistance form because teeth are more prone to fracture after root canal therapy. Preparation of Bevels and Flares. •

After the cement base (where indicated) is completed, the slender, flame-shaped, finegrit diamond instrument is used to bevel the occlusal and gingival margins and to apply the secondary flare on the distolingual and distofacial walls. This should result in 30- to 40-degree marginal metal on the inlay.

This cavosurface design helps seal and protect the margins and results in a strong enamel margin with an angle of 140 to 150 degrees.

A cavosurface enamel angle of more than 150 degrees is incorrect because it results in a less defined enamel margin (finish line), and the marginal cast metal alloy is too thin and weak if its angle is less than 30 degrees.

Conversely, if the enamel margin is 140 degrees or less, the metal is too bulky and difficult to burnish when its angle is greater than 40 degrees.

30


Usually it is helpful to insert a gingival retraction cord of suitable diameter into the gingival sulcus adjacent to the gingival margin, and leave it in place for several minutes just before the use of the flame-shaped diamond instrument on the proximal margins.

Immediately before the flame-shaped diamond instrument is used, the cord may be removed, resulting in an open sulcus that improves visibility for beveling the gingival margin and helps prevent injury and subsequent hemorrhage of the gingival tissue.

Using the flame-shaped diamond instrument, rotating at high speed, prepare the lingual secondary flare.

Approach from the lingual embrasure, moving the instrument mesiofacially. Compare the direction of the distolingual wall and the position of the distolingual margin before and after this extension.

Notice that the distolingual wall extends from the linguoaxial line angle into the lingual embrasure in two planes. The first is termed the lingual primary flare; the second is named the lingual secondary flare.

During this (secondary) flaring operation, the long axis of the instrument is held nearly parallel to the line of draw, with only a slight tilting mesially and lingually for assurance of draft, and the direction of translation of the instrument is that which results in a marginal metal angle of 40 degrees.

Bevel the gingival margin by moving the instrument facially along the gingival margin. While cutting the gingival bevel reduce the rotational speed to increase the sense of touch; otherwise overbeveling may result. The instrument should be tilted

31


slightly mesially to produce a gingival bevel with the correct steepness to result in 30degree marginal metal. •

Although the instrument is tilted mesially, its long axis must not tilt facially or lingually. The gingival bevel should be 0.5 to 1 mm wide and should blend with the lingual secondary flare.

Complete the gingival bevel, and then prepare the facial secondary flare. The long axis of the instrument during this secondary flare is again returned nearly to the line of draw with only a small tilting mesially and facially, and the direction of translation of the instrument is that which results in 40-degree marginal metal.

When the adjacent proximal surface (mesial of the second premolar) is not being prepared, care must be exercised neither to abrade the adjacent tooth nor to overextend the distofacial margin. To help in preventing such abrasion or overextension, 1. - the instrument may be raised occlusally (thus using the narrower portion at its tip end) to complete the most facial portion of the wall and margin . 2.

Also, the more slender No. 169L carbide bur may be used rather than the flame-shaped diamond instrument.

3.

When access permits, a fine-grit sandpaper disc may be used on the facial and lingual walls and margins of the proximal preparation, especially when minimal extension of the facial margin is desired. This produces smooth walls and helps create respective margins that are straight (not ragged) and sound.

In the flaring and beveling of the proximal margins, as described in the previous paragraphs, the procedure began at the lingual surface and proceeded to the facial

32


surface; however, the direction may be reversed, starting at the facial surface and moving toward the lingual surface. •

The gingival bevel serves the following purposes: •

Weak enamel is removed. If the gingival margin is in the enamel, it would be

weak if not beveled because of the gingival declination of the enamel rods •

The bevel results in 30-degree metal that is burnishable (on the die) because of

its angular design. Bulky 110-degree metal along an un-beveled margin is not burnishable. •

A lap, sliding fit is produced at the gingival margin. This helps improve the fit

of the casting in this region. With the prescribed gingival bevel, if the inlay fails to seat by 50 μm, the void between the bevel metal and the gingival bevel on the tooth may be as small as 20 μm; however, failure to apply such a bevel would result in a void (and a cement line) as great as the failure to seat.

LAP SLIDING FIT •

Uninterrupted blending of the gingival bevel into the secondary flares of the distolingual and distofacial walls results in the distolingual and distofacial margins joining the gingival margin in a desirable arc of a small circle; also, the gingivofacial and gingivolingual line angles no longer extend to the marginal outline. If such line angles are allowed to extend to the preparation outline, early failure may follow because of an "open" margin, dissolution of exposed cement, and eventual leakage, all potentially resulting in caries.

The secondary flare is necessary for several reasons: (1) The secondary flaring of the proximal walls extends the margins into the embrasures, making these margins more self-cleansing and more accessible to finishing procedures during the inlay insertion appointment, and does so with conservation of the dentin. (2) The direction of the flare results in 40-degree marginal metal . Metal with this angular design is burnishable; however, metal shaped at a larger angle is 33


unsatisfactory for burnishing; metal with an angle less than 30 degrees is too thin and weak, with a corresponding enamel margin that is too indefinite and ragged.

(3) A more blunted and stronger enamel margin is produced because of the secondary flare. •

The secondary flare is omitted for esthetic reasons on the mesiofacial proximal wall of preparations on premolars and first molars of the maxillary dentition. In this location the wall is completed with minimal extension by using either hand instruments (straight or binangle chisel) followed by a fine-grit sandpaper disc or very thin rotary instruments.

The flame-shaped, fine-grit diamond instrument also is used for occlusal bevels. The width of the cavosurface bevel on the occlusal margin should be approximately one fourth the depth of the respective wall. •

The resulting occlusal marginal metal of the inlay should be 40-degree metal; thus the occlusal marginal enamel is 140-degree enamel.

•

Beveling the occlusal margins in this manner increases the strength of the marginal enamel and helps seal and protect the margins.

34


While beveling the occlusal margins, a guide to diamond positioning is to maintain an approximate 40-degree angle between the side of the instrument and the external enamel surface. This also indicates when an occlusal bevel is necessary. For example, if the cusp inclines are so steep that the diamond instrument, when positioned at a 40degree angle to the external enamel surface is parallel with the enamel preparation wall, then no bevel is indicated.

Using this technique demonstrates that margins on the proximal marginal ridges always require a cavosurface bevel. Failure to apply a bevel in these regions leaves the enamel margin weak and subject to injury by fracture, both before the inlay insertion appointment and during the try-in of the inlay when burnishing the marginal metal.

Similarly, the importance of extending the occlusal bevel to include those portions of the occlusal margin that cross over the marginal ridge cannot be overemphasized.

These margins are beveled to result in 40-degree marginal metal. Otherwise fracture of the enamel margin in such stress-vulnerable regions may occur in the interim between the preparation and the cementation appointment.

The diamond instrument also is used to lightly bevel the axiopulpal line angle. Such a bevel provides a thicker and therefore stronger wax pattern at this critical region.

Thus the desirable metal angle at the margins of inlays is 40 degrees, except at the gingival margins, where the metal angle should be 30 degrees. This completes the preparation.

35


Variations in proximal margin design: The design of the proximal margins will vary with 1. the extent of tooth tissue loss, 2. the location of that loss, 3. tooth form, 4. the positional relationship with adjacent teeth, 5. the need for retention form, and 6. convenience Several basic designs are used to finish and extend walls and margins of the proximal box that has resulted from the removal of dental caries or old restorations. Each design may have a specific advantage because of the factors present. These designs include the box, slice, auxiliary slice and modified flare. 1. Box preparation: •

There are two common methods by which cast gold restorations may be fabricated. With the direct method the wax pattern is formed directly in the mouth. The indirect method requires that an impression of the prepared cavity be taken, from which a die is constructed.

A wax pattern is formed on the die, and the casting is subsequently finished and polished on the die with all the convenience afforded by an extra oral procedure.

The direct wax technique requires a proximal margin be designed for ease of wax manipulation in the mouth. Margins are prepared to permit as great a bulk of wax as is consistent with their subsequent finishing and adaptation.

A proximo-occlusal cavity, for example, will have its buccal and lingual proximal walls finished so that the cavosurface angle formed by the proximal flare and tooth surface will be, for histologic reasons, at right or slightly obtuse angles.

A cervical bevel is required.

The bevel is placed most often with hand

instruments, thus forming a lap joint with a bulk of wax suitable for direct carving.

36


The box design is principally used with the proximo-occlusal preparation for the direct method of wax pattern formation.

Type-I gold alloys permit an ease of

manipulation of the cervical, buccal, and lingual margins of a casting formed from a well-condensed and adapted wax pattern. 2. Slice preparation: •

Historically, a slice referred to the placement of extracoronal taper using a disk of adequate diameter to contact nearly the entire proximal surface. Not only did the slice establish a cervical finish line for the preparation, but is also eliminated much of the proximal anatomic undercut which facilitated taking an impression with a non-elastic material, dental compound.

With the introduction of the elastic impression materials such as gross reduction of the proximal contour was unnecessary.

Presently, the slice preparation involves conservative disking of the proximal surface to establish the buccal and lingual extent of the finish lines and provide a lap joint for finishing.

These slices are generally placed on the buccal and lingual proximal surfaces independently. The slice may extent to the cervical floor, or more frequently will terminate at some point occlusal to the cervical floor.

The preservation of tooth tissue is a major factor in deciding the cervical extent of the slice.

Tooth form, as observed intraorally and radiographically, will suggest how much cervical extension is consistent with the goals of finishing convenience and conservation of tooth tissue.

In general, teeth with proximal contours of the square tooth form will permit the use of a slice that extends to the cervical floor. Convenient external outline is reached with minimal loss of tooth tissue when such flattened areas

37


are disked. Teeth with tapering or ovoid forms generally will indicate a slice preparation extending short of the cervical floor. Excessive tooth tissue would be removed if disking were carried further cervically.

Square teeth

Tapering/ovoid teeth

Placement of a proximal slice for the indirect inlay produces excellent definition for the finishing line. Beveling of the proximal cavosurface with the slice not only assures a sound enamel margin but also yields a casting margin that is adapted and finished with relative ease. 3. Auxiliary slice: •

The auxiliary slice, wraps partially around the proximal line angles, thus providing additional tooth support.

Minimal bulk of tissue is lost, yet

resistance form is greatly enhanced, reducing the possibility of tooth fracture. •

The auxiliary slice can be employed similarly to provide external retention form. An auxiliary slice around the lingual proximal line angle of a tooth, for example, will aid in preventing a buccal displacement of the casting. Such a retentive factor may be used when a buccal proximal wall in impossible to establish because of the loss of tooth tissue from dental caries or fracture.

38


In a like manner an auxiliary slice around a buccal line angle will provide an external cavity wall to aid in preventing the lingual displacement of the casting.

An auxiliary slice also can conservatively include within the inlay preparation the margins of a previous amalgam cavity and cement base as well as areas of demineralized enamel.

4. Modified Flare Preparation: •

The modified flare preparation is somewhat of a hybrid between the box and slice preparations.

Buccal and lingual proximal walls are initially formed with minimal extension, then disked in a plane that only slightly reduces the proximal wall dimension.

Excessive disking will reduce the retention of these walls. The obtuseness of the cavosurface angle is enhanced.

A box, the slice preparation, or the modified flare preparation may be selected as being the most suitable because of any one of a number of mechanical or biologic considerations, or for esthetic reasons.

The mesial buccal proximal margins of the maxillary bicuspids and first molar, for example, frequently display better esthetic effects when no slice is used.

The particular type of preparation is seldom arbitrary, and the decision should be made at the time of the specific treatment planning of the case.

Modifications in Inlay Tooth Preparations.

39


Again, realizing that the indications for small inlays are rare, the following sections provide procedural information that may promote better understanding of their applications in more complex and larger inlay or onlay restorations. 1.Mesioocclusodistal Preparation. If a marginal ridge is severely weakened because of excessive extension, the preparation outline often should be altered to include the proximal surface. The decision to extend the preparation in this manner calls for clinical judgment as to whether the remaining marginal ridge withstands occlusal forces without fracture.

2. Modifications of class II cavity preparation for esthetics: For esthetic reasons, minimal flare is indicated for the mesiofacial proximal wall of maxillary first premolar and first molar. To accomplish this, secondary flare is omitted, and the wall and margin are developed with a chisel or enamel hatchet and final smoothing done with fine grit or a narrow diamond or bur.

Minimal flare facially, No secondary flare 3. Facial and lingual groove extension: Frequently a faulty facial groove on the occlusal surface is continuous with a faulty facial surface groove. This indicates extension of the cavity outline to include the fissure to its termination.

40


4. Class-II cavity preparation for abutment teeth and extension gingivally to include root surface lesions. Extending the gingival margin into the gingival sulcus is usually indicated on the proximal surfaces adjacent to connectors or RPD because of the difference in keeping those regions clean.

In addition the occlusal outline form must be wide enough faciolingually to accommodate any contemplated restoration preparation without involving the margins of the restoration. The same is indicated when further gingival extension is indicated to include a root lesion on the proximal surface. 5. Maxillary first molar with unaffected strong oblique ridge. When a maxillary first molar is to be restored consideration should be given to persevering the oblique ridge if it is strong and unaffected. If a distal surface cavity appears subsequently to a insertion of a mesioocclusal restoration the tooth should be prepared for a disto-occluso-lingual inlay. This is preferable to the distoocclusal restoration because it prevents the miniature disto-lingual cusp from subsequent fracture.

41


6. Fissures in the facial and lingual cusp ridges or marginal ridges Proper Outline form dictated that the preparation margin should not cross such fissure but include them. It can be done with a No.271 carbide bur until only 2mm of tooth structure remains between the bur and the tooth surface(buccal, proximal or lingual).This treatment particularly applies to a mesial fissure of maxillary first premolar.

7. Capping cusp •

The facial and lingual margins on the occlusal surface frequently must be extended toward the cusp tips to the extent of existing restorative materials and to uncover caries.

When the occlusal outline is extended up the cusp slopes more than half the distance from any primary occlusal groove (central, facial, or lingual) to the cusp tip, capping the cusp should be considered.

If the preparation outline is extended two thirds of this distance or more, capping the cusp is usually necessary to: (1) protect the weak, underlying cuspal structure from fracture caused by masticatory force and (2) remove the occlusal margin from a region subjected to heavy stress and wear.

42


At this point in preparation the pulpal floor, depth can be increased from 1.5 mm to 2 mm. This additional pulpal depth ensures sufficient reduction in an area that is often underreduced, and will result in greater strength and rigidity to the wax pattern and cast restoration. The following section describes the technique for capping less than all of the cusps of a posterior tooth.

Reduce the Cusps for Capping as Soon as the Indication for Such Capping Is Determined Because This Improves Access and Visibility for Subsequent Steps in Preparation. •

Before reducing the surface, prepare depth gauge grooves (depth cuts) with the side of the No. 271 carbide bur. Such depth cuts should help to prevent thin spots in the restoration.

With the depth cuts serving as guides, complete the cusp reduction with the side of the carbide bur . The reduction should provide for a uniform 1.5 mm of metal thickness over the reduced cusp. On maxillary premolars and first molars, the reduction should be minimal (i.e., 0.75 to 1 mm) on the facial cusp ridge to decrease the display of metal. This reduction should increase progressively to 1.5 mm toward the center of the tooth to help provide rigidity to the capping metal.

If only one of the two lingual cusps of a molar is reduced for capping, the reduction must extend to just include the lingual groove between the reduced and unreduced cusps. This reduction should terminate with a distinct vertical wall that has a height that is the same as the prescribed cusp reduction.

Applying the bur vertically, establish a vertical wall of proper depth and direction. Similar principles apply when only one of the facial cusps is to be reduced.

43


•

A bevel of generous width is prepared on the facial (lingual) margin of a reduced cusp with the flame-shaped, fine-grit diamond instrument (with the exception of esthetically prominent areas). This bevel is referred to as a reverse bevel or counterbevel. It should be at an angle that results in 30-degree marginal metal

•

The exception is the facial margin on maxillary premolars and the first molar where esthetic requirements dictate only a blunting and smoothing of the enamel margin (a stub margin) by the light application of a fine-grit sandpaper disc or the fine-grit diamond instrument (flame-shaped) held at a right angle to the facial surface.

Cusp reduction appreciably decreases retention form because of decreasing the height of the vertical walls; consequently, proximal retention grooves usually are recommended . It may be necessary to increase retention form by extending facial and lingual groove regions of the respective surfaces, or by collar and skirt features. These additional retention features also provide the desired resistance form against forces tending to split the tooth. 8. Including Portions of the Facial and Lingual Smooth Surfaces Affected by Caries or Other Injury. When portions of both a facial (lingual) smooth surface and a proximal surface are affected by caries or some other factor (e.g., fracture), the treatment may be a large inlay, an onlay, a three-quarter crown, a full crown, or multiple amalgam or composite restorations. Generally, if carious portions are extensive, the choice between the previously listed cast metal restorations is determined by the degree of tooth circumference involved. A full crown is indicated if both the lingual and facial smooth surfaces are defective, especially if the tooth is a second or third molar. When only a portion of the facial smooth surface is carious and the lingual surfaces of the teeth are conspicuously free of caries, an MODFL inlay or onlay with a lingual groove extension is chosen over the crown because the former is more favorable to the health of the gingival tissues and more conservative in the removal of tooth structure.

44


Tooth preparation for onlay cast rstorations The cast metal onlay restoration spans between the inlay, which is primarily an intracoronal restoration, and the full crown, which is totally extracoronal restoration. Definition: the cast metal onlay by definition caps all of the cusps of posterior tooth and can be designed to help strengthen a tooth that has been weakened by caries or previous restorative experiences. 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, if the cuspal length: width ratio 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 mandatory. 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 capping of the functional and the shoeing non-functional cusps.

45


Location of margin: 1. Occluso-facio-lingual portion. 1. On the functional side cusps are capped for the additional retention and protection. 2. They must be located far 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 (Shoed side) side, the facial (or) lingual margins located just gingival to the tip and ridge crests of the cusps and away from occlusal contact. 2 Proximal portion: 1. Secondary flares are used in all situations 2. The margins have the same location as for inlays.

46


Internal anatomy: A. In Occluso-facio-lingul –portion 1. Capped part – on capped side of facial /lingual wall formed of four planes: a. Wall proper: is an 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. b. Occlusal bevel: -

is an intracoronal portion of the wall, which constitutes atleast 1/3 of the total wall height.

-

It takes the form of a hollow ground long bevel with standard angulation paralleling the cusp's occlusal inclined planes.

-

On the average it is 30-45° from the long axis of the tooth crown and approximately the same angulation from its "wall proper".

-

It should be relieved at least 1 mm from the opposing cuspal elements in static and functional occluding contacts.

-

For each cusp, there should be two hollow-ground bevels occlusally: one is directed occluso-distally, and the other occluso-mesially, following the direction of the corresponding occlusal inclined planes.

-

The hollow-ground dual directional features of the bevel on each cusp are extremely advantageous in giving bulk for the restoration at its inclined planes, areas of imminent stresses. Also, they increase the surface area of contact between the tooth

surface

and 47

cast,

helping

to

distrubute

forces


physiologically on underlying tooth structures (following removed tooth anatomy). In addition they help to guide the cast to one specific relation with the preparation surface, and substantially increase the immobilization of the restoration. c. 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. d. Counter Bevel -

is the extra-coronal feature of this part of the preparation. Usually, it is formed in areas of enamel and dentin, or it may be completely in either It should be relieved from opposing cuspal elements by at least 1 mm in both static and functional occlusal contact.

-

It is hollow-ground, inclined away from the intracoronal parts of the preparation gingivally-facially or lingually, and follows the cuspal anatomy (i.e., each of the facial or lingual inclined planes for the cusp will have a hollow-ground bevel).

-

So each cusp on the capped side will have four different hollow-ground bevels, each corresponding to an inclined plane of the cusp.

Four hollow ground bevels -

In addition to the advantages of those features mentioned with the occlusal bevel, the counterbevel will embrace the cuspal elements for retention and/or support.

48


The angulation of the counterbevel is not as standard as that for the occlusal bevel. It is measured by the embracing angle, i.e. the angle between the bevel and the long axis of the crown. On the average, this angulation is from 30-70째, but it varies according to: a. The amount of indicated involvement for the facial or lingual surface. The more the desirable involvement there is, the less will be the embracing angle to preserve tooth structure. b. The amount of needed retention. The shorter the preparation walls (axial surfaces) less will be the embracing angle with the long axis of the tooth. In some situations, the counterbevel will almost be parallel to the intracoronal wall proper, which will add substantial the retention of the restoration c.

The type of cast alloy.

The less the castability of the alloy is, the greater will be the embracing angle, to allow for bulk of material marginally The extent of the counterbevel is least standardized in this part of the preparation.

Embracing angle The junction between these four different parts of the capped side should be definite, but slightly rounded, to improve reproducibility and to prevent stress concentration. 2. The Shoed part The shoed part of the preparation the facial or, , lingual , wall will be formed of three parts :

49


Shoed part 1. The "wall proper”: this is similar to and reciprocating with the “wall proper” on the capped side. 2. The occlusal bevel. This, too, is similar to and reciprocating with its counterpart on the capped side. However, it can be relieved less from opposing teeth, and it usually has more extent than its counterpart on the capped side. The occlusal bevel performs the same function as on the capped side 3. The “shoe” : This may be likened to the table on the capped side but it is relieved in less dimension than the table, and most of the time it is in enamel. It performs the same functions as the table. In some situations the shoe will end facially or lingually with acute-angled marginal tooth structure, leaving frail enamel. In this situation, a fourth plane is placed, in the form of a partial bevel. This should incline away from the cavity preparation facially or lingually-gingivally, and follow cuspal anatomy . This peripheral partial bevel is also indicated when there is difficulty in making a proper continuation between a shoe and a primary or secondary flare proximally. On the other hand, in some locations, e.g., mesio-buccal segments of the upper first or second premolars and first molars, the shoe will end with margins facially that might be unacceptable for esthetic reasons. In this situation, the occlusal bevel (the second plane) should be the terminal circumferential tie constituent at the mesio-occlusal inclined planes of the buccal or mesioocclusal inclined planes of the buccal or mesio-buccal cusps of these teeth. Just as on the capped side, the junction between these two, three or four constituents should be definite, but rounded.

50


B.Proximal portion The internal anatomy of tooth preparation for onlay proximally is very similar to proximal part in cavity preparation for inlays i.e.

with standard "wall proper" and

primary flare. 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 exaggerated hollow ground bevel extending to pulpal floor to the table; a table and an exaggerated hollow-ground counter bevel. The embracing angle of this counterbevel 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. 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.

There will be no reverse flares in these preparations.

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TOOTH PREPARATION FOR CAST METAL ONLAYS(Technique) -

Onlay and larger inlay restorations have many clinical applications and may be desirable by many patients. Although not esthetic, these restorations have a welldeserved reputation for providing the finest dental treatment.

-

It can be designed to distribute occlusal loads over the tooth in a manner that greatly decreases the chance of future fracture.

-

It is more conservative of tooth structure than the full crown preparation and its supragingival margins, when possible, are less irritating to the gingiva.

-

Usually an onlay diagnosis is made preoperatively because of the tooth's status. However, sometimes the diagnosis is deferred until extension of the occlusal step of an inlay preparation facially and lingually to the limits of the carious lesion demonstrates that cusp reduction is mandatory.

Initial Preparation Occlusal Reduction. •

As soon as the decision is made to restore the tooth with a cast metal onlay, the cusps should be reduced because this improves both the access and the visibility for subsequent steps in tooth preparation.

Using the No. 271 carbide bur held parallel to the long axis of the tooth crown, prepare a 2-mm deep pulpal floor along the central groove .

To verify the preoperative

diagnosis

for

cusp

reduction,

this

occlusal

preparation may be extended facially and lingually just beyond the caries to sound tooth structure. •

With the side of the No. 271 carbide bur, prepare uniform 1.5 mm deep depth cuts on the remaining occlusal surface. The depth cuts are usually placed on the crest of the triangular ridges and in the facial and lingual groove regions.

These depth cuts will help prevent thin spots in the final restoration. It should be remembered that if a cusp is in infraocclusion of the desired occlusal plane before reduction, then the amount of cusp reduction is less and needs only that which provides the required clearance with the desired occlusal plane.

Caries and old restorative material that is deeper in the tooth than the desired clearance is not removed at this step in preparation.

52


With the depth cuts serving as guides for the amount of reduction, complete the cusp reduction with the side of the No. 271 bur. This reduction, when completed, should reflect the general topography of the original occlusal surface . Do not attempt to completely reduce the mesial and distal marginal ridges at this time to avoid possibly hitting an adjacent tooth. The remainder of the ridges will be reduced in a later step when the proximal boxes are prepared.

Occlusal Step: •

After cusp reduction there should be a 0.3 mm deep occlusal step in the central groove region between the reduced cuspal inclines and the pulpal floor. Maintaining the pulpal depth (0.5 mm) of the step, extend it facially and lingually just beyond any carious areas, to sound tooth structure (or to sound base/ restorative material if certain conditions, have been met). Extend mesially and distally far enough to expose the proximal DEJ.

Extend the step along any remaining facial (and lingual) occlusal fissures as far as they are faulty (fissured)

The facial and lingual walls of the occlusal step should go around the cusps in graceful curves and the isthmus should only be as wide as necessary to be in sound tooth structure or sound base/restorative material.

Old restorative material or caries that is deeper pulpally than this 0.5-mm step is not removed at this stage of tooth preparation. 53


As the occlusal step approaches the mesial and distal surfaces, it should widen faciolingually in anticipation for the proximal box extensions . This 0.5-mm occlusal step will contribute to the retention of the restoration and will provide the wax pattern and cast metal onlay with additional bulk for rigidity.

Proximal Box: •

Continuing with the No. 271 carbide bur held parallel to the long axis of the tooth crown, prepare the proximal boxes as described for inlays.

Final Preparation 1. Removal of Infected Carious Dentin and Defective Restorative Materials and Pulp Protection, •

If the occlusal step and the proximal boxes have been extended properly, any caries or previous restorative materials remaining on the pulpal and axial walls should be visible. Remove them as described previously.

2. Preparation of Bevels and Flares. •

After the cement base (when indicated) is completed , use the slender, flameshaped, fine-grit diamond instrument to place counterbevels on the reduced cusps, to apply the gingival bevels, and to create secondary flares on the facial and lingual walls of the proximal boxes.

First insert a gingival retraction cord as described in the previous inlay section.

During the few minutes required for the cord's effect on the gingival tissues, use the diamond instrument for preparing the counterbevels on the facial and lingual margins of the reduced cusps. The bevel should be of generous width

54


and should result in 30-degree marginal metal. The best way to judge this is to always maintain a 30-degree angle between the side of the instrument and the external enamel surface beyond the counterbevel. •

The counterbevel should usually be wide enough so that the cavosurfnce margin is beyond (gingival to) any contact with the opposing dentition.

A counterbevel is not placed on the facial cusps of maxillary premolars and first molars where esthetic considerations may dictate using a stubbed margin by blunting and smoothing of the enamel margin by either the light application of a fine-grit sandpaper disc or the fine-grit diamond instrument (flameshaped) held at a right angle to the facial surface. The surface created by this blunting should be approximately 0.5 mm in width.

Beveling the gingival margins and flaring (secondary) the proximal enamel walls, is similar as for the inlay.

After beveling and flaring, slightly round any sharp junctions between the counterbevels and the secondary flares. The fine-grit diamond instrument also is used to lightly bevel the axiopulpal line angles.

The desirable metal angle at the margins of onlays is 40 degrees except at the gingivally directed margins, where the metal angle should be 30 degrees.

When deemed necessary, shallow (0.3 mm deep) retention grooves may be cut in the facioaxial and the linguoaxial line angles with the No. 169L carbide bur. These grooves are especially important for retention when the prepared tooth is short, which is often the case after reducing all the cusps.

Modifications in Onlay Tooth Preparations 1. Facial or Lingual Surface Groove Extension. A facial surface fissure (mandibular molar) or a lingual surface fissure (maxillary molar) is included in the outline in the same manner as for inlay preparation. This extension is sometimes indicated to provide additional retention form even though the groove is not faulty.

55


2. Inclusion of Portions of the facial and Lingual Smooth Surfaces Affected by Caries, Fractured Cusps, or Other Injury. Requires use of shoulders along with additional features to improve retention and retention form like, skirting and groove extension.

Enhancement of Resistance and Retention Forms. When the tooth crown is short (which is often the case when all cusps are reduced), the operator must strive to maximize retention form in the preparation. Retention features that have already been presented include: 1. Minimal amount of taper (2 degrees per wall) on the vertical walls of the preparation 2. Addition of proximal retention grooves 3. Preparation of facial (or lingual) surface groove extensions In the preparation of a tooth that has been grossly weakened by caries or previous filling material and is judged to be prone to fracture under occlusal loads, the resistance form that cusp capping provides should be augmented by the use of 

Skirts

Collars

Slot

Reverse secondary flare

When properly placed, these features result in onlays that will distribute occlusal forces over most or all of the tooth and not just a portion of it, thus reducing the likelihood of fractures of the teeth. Skirt preparation. •

Skirts are thin extensions of the facial or lingual proximal margins of the cast metal onlay that extend from the primary flare to a termination just past the transitional line angle of the tooth.

A skirt extension is a conservative method of improving both the retention form and the resistance form of the preparation.

56


It is relatively atraumatic to the health of the tooth because it involves removing very little (if any) dentin. Usually the skirt extensions are prepared entirely in enamel.

The preparation of a skirt is done entirely with the slender, flame-shaped, fine-grit diamond instrument. Skirt preparations follow the completion of the proximal gingival bevel and primary flares. However, experienced operators will often prepare the skirt extensions at the same time that the gingival bevel is placed, working from the lingual toward the facial or vice versa.

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 torwards, 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. 57


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 sometimes 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 is 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 to create sufficient retention for the cast material and avoid marginal overhangings.

A disadvantage of skirting is that it increases the display of metal on the facial and lingual surfaces of the tooth. For this reason, skirts are not placed on the mesio-facial margin of maxillary premolars and first molars. Skirting the remaining three line angles of the tooth provides ample retention and resistance form. Collar preparation. •

To increase the retention and resistance forms when preparing a weakened tooth for a MOD onlay capping all cusps, a facial or lingual "collar," or both, may be provided .

Use a No. 271 carbide bur at high speed parallel to the line of draw to prepare a 0.8mm deep shoulder (equivalent to the diameter of the tip end of the bur) around the lingual (or facial) surface to provide for a collar about 2 to 3 mm high occlusogingivally .

To provide for a uniform thickness of metal, the occlusal 1 mm of this reduction should be prepared to follow the original contour of the tooth and should round any undesirable sharp line angle formed by the union of the prepared lingual and occlusal surfaces.

Complete this aspect of the preparation by lightly beveling the gingival margin of the shoulder with the flame-shaped, fine-grit diamond instrument to result in a 30-degree metal angle at the margin.

58


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 be apical to an already lost cuspal element.

Tooth collar

Cuspal collar

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 contraindicated.

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.

59


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.

Slot preparation. •

Occasionally the use of a slot in the dentin is helpful to provide the necessary retention form. An example is the mandibular second molar that has no molar posterior to it and requires a MO onlay restoration capping all of the cusps .

A slot is preferred over preparing a box in the distal because: (1) the former is more conserving of tooth structure and of strength of the tooth crown and (2) the linear extent of marginal outline is less.

To form a slot, use a No. 169L carbide bur whose long axis should parallel the line of draw (this must be reasonably close to a line parallel with the long axis of the tooth)

The slot should have the following approximate dimensions: (1) mesiodistally, the width (diameter) of the bur; (2) faciolingually, 2 mm; and (3) depth, 2 mm gingival of the normally positioned pulpal wall. To be effective, the mesial wall of the slot must be in sound dentin; otherwise insufficient retention form will be obtained.

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 encompass an axial angle for reinforcing and supporting reasons. 60


To add retentive capability of the restoration proximally.

This type of feature is contraindicated in class – IV and V cast materials. Features •

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 to provide obtuse angled tooth structure.

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.

Unlike secondary flare, reverse secondary flare may not involve the entire periphery of the preparation occlusoapically.

Esthetic inlays and onlays Esthetic inlays and onlays have a number of characteristics in common, whether they are resin, ceramic or fabricated with CAD/CIM technology. Indications •

Esthetics

Large defects or previous restorations

Economic factors

Contraindiations •

Heavy occlusal forces

Inability to maintain a dry field

Deep subgingival preparation

Advantages

61


Improved physical properties

Variety of materials and techniques

Wear resistance

Reduced polymerization shrinkage

Ability to strengthen remaining tooth structure

More precise control of contact and contours

Biocompatibility and good tissue response

Disadvantages •

Increased cost and time

Technique sensitivity

Brittleness of ceramics

Wear of opposing dentition

Resin-to-resin bonding difficulties

Short clinical track record

Low potential for repair

Resin Composite Inlays and Onlays Inlays and onlays made of resin composite are quite popular in Europe, but have not gained wide acceptance in the United States. These restorations may be fabricated intraorally or on a cast. After polymerization, the restoration is bonded in place with a resin luting cement. Resin composite inlays can be highly esthetic and have certain advantages over direct resin composite and bonded ceramic restorations. Advantages Over Direct Resin Composite Restorations •

Contours and contacts can be developed outside of the mouth. If a contact is inadequate, it can easily be corrected prior to cementation.

Several problems associated with direct resin composite restorations are the result of polymerization shrinkage. During polymerization, resin composite shrinks on the order of 2% to 4%, often causing a gap to form at the least retentive marginal interface, which is usually the gingival margin.

Current dentin adhesives have lessened, but not eliminated, the problem. 62


Polymerization shrinkage can also cause cuspal flexure, which is sometimes associated with craze lines in the enamel and postoperative sensitivity.

In theory, polymerization shrinkage should be less of n problem with resin inlays because they are polymerized before cementation.

The only polymerization shrinkage that occurs at the time of cementation is in the thin layer of resin cement.

Resin inlays are reported to have less microleakage and greater strength and hardness, and to result in less postoperative sensitivity than direct resin composite restorations.

Light curing

oven for secondary polymerization

Secondary Polymerization •

Light-cured resin composite inlays undergo initial polymerization, but then are further polymerized in an oven or pressure pot with a combination of intense light, heat, and/or pressure.

The postcure can be performed in a postcure unit specifically made for this purpose, in a toaster oven at approximately 250°F for 7 minutes, or with a curing light or light box.

Studies have shown that the secondary polymerization results in improved physical properties but minimal clinical difference in wear characteristics.

The secondary curing procedures are recommended with all indirect resin systems, although they may not be mentioned in all manufacturers' instructions.

While the in vitro, data suggest that there are significant advantages to the resin inlay compared to the direct resin composite restoration, this is not corroborated by the in vivo data. In a 5-year clinical study and another 8-year clinical study, no statistically sig-

63


nificant difference was found in the success rates for resin inlays compared to directplacement resin composite restorations. Many resin composite inlay systems are available. They are classified as direct (made on the tooth) or indirect (made on a cast). Direct Resin Inlays Inlays can be fabricated directly on the tooth. After preparation, a water-soluble separating medium and a matrix are placed on the tooth. The preparation is bulk filled with resin composite and light cured from all directions. The matrix is removed, and the inlay is teased out of the preparation. Because the resin composite shrinks during polymerization, the inlay is slightly smaller than the preparation and will come out easily if no undercuts are present. The inlay is then postcured. Finally, it is tried in, adjusted, and bonded into the preparation. Direct/Indirect Resin Inlays When the direct/indirect method is used, an impression is made of the prepared tooth and a cast is poured. Because this technique can be done in one appointment, the master cast must be ready to use in a short period of time (5 minutes). Therefore, the products used must be compatible with the technique. The master die can be made from a silicone material (Mach 2, Parkell), or a master cast made from die stone (Snap Stone, Whip Mix). The restoration is fabricated on the die and usually undergoes a primary (light-cured) and secondary (auxiliary curing unit) polymerization. Indirect Resin Inlays and Onlays • •

Resin inlays and onlays are also available through commercial laboratories. They can be constructed from either hybrid resin composite or microfilled resin composite.

However, there is a newer generation of resin materials that has been termed ceromers or ceramic optimized polymers.

Currently, four ceromer products are widely used: 1. Artglass (Heraeus Kulzer), 2. BelleGlass HP (Kerr),

64


3. Targis (Ivoclar), and 4. Skulptur FibreKor (Jeneric/Pentron). These materials are reported to have greater durability, fracture toughness, wear resistance, esthetics, and repairability. However, in one laboratory study, repaired ceromers were found to be 30% to 60% weaker than the parent material. There is minimal independent laboratory data available on the physical properties of the ceromer products. Ferracane compared Artglass to the traditional hybrid composite, Charisma (Heraeus Kulzer). The fracture toughness of Artglass was higher than with Charisma; however, the flexural modulus and the hardness were higher with Charisma. Similarly, the physical properties were evaluated for Artglass, Targis, and a traditional hybrid resin composite,-Z100 (3M).The ceromer material may be combined with a fiber-reinforced material, which significantly increases fracture resistance. However, flexural strength of fiberreinforced ceromer has been shown to significantly decrease after storage in water. Ceromer restorations are bonded in the same manner as other indirect resin inlays/onlays. Posterior Bonded Porcelain Restorations •

Ceramic inlays were introduced in 1913, but did not become popular because of difficulties in fabrication and a high failure rate. In the 1980s, the development of compatible refractory materials made fabrication easier, and the development of adhesive resin cements greatly improved clinical success rates.

The modern generation of bonded porcelain restorations was first described in 1983. When it became clear that the technique had merit in anterior applications, interest developed in the use of bonded porcelain for posterior applications.

In 1986, Red-ford and Jensen described the strengthening effect of porcelain inlays on the fracture resistance of natural teeth. In 1988, Jensen reported excellent clinical success in a 2-year in vivo study.

The technique has since been refined to the point that porcelain inlays and onlays are now an accepted operative modality.

Indications

65


•

The indications for posterior bonded porcelain restorations overlap those for direct and indirect posterior resin composite restorations, which have been described.

•

These restorations are indicated when there is an overriding desire for esthetics and all margins can be placed on enamel.

Materials 1. Feldspathic porcelain 2. Hot pressed ceramics 3. Machinable ceramics for CAD/CAM system Feldspathic Porcelain Inlays and Onlays These porcelains are partially crystalline minerals (feldspar, silica, alumina) dispersed in a glass matrix. Porcelain restorations are made from finely ground ceramic powders that are mixed with distilled water or a special liquid, shaped into the desired form, then fired and fused together to form a translucent, material that looks like tooth structure. Currently, many ceramic inlays and onlays are fabricated in the dental laboratory by firing dental porcelains on refractory dies. The fabrication steps for fired ceramic inlays and onlays can be summarized as follows: 1. After tooth preparation, an impression is made and a "master" working cast is poured of die stone. 2. The die is duplicated and poured with a refractory investment capable of withstanding porcelain firing temperatures. The duplication method must result in the master die and the refractory die being accurately interchangeable. 3. Porcelain is added into the preparation area of the refractory die and fired in an oven. Multiple increments and firings are necessary to compensate for sintering shrinkage . 4. The ceramic restoration is recovered from the refractory die, cleaned of all investment, and seated on the master die and working cast for final adjustments and finishing. The major disadvantage of this technique is its technique sensitivity. Although some technicians can routinely fabricate these restorations with excellent marginal integrity, many dentists complain of problems with fit and strength. Inlays and onlays fabricated

66


with this technique must be handled very gently during try-in to avoid fracture. Even after cementation, the incidence of fracture is rather high for this type of ceramic restoration. M Hayashi et al found that the in vivo longevity of fired ceramic inlay restorations dropped from 92% to 80% between the sixth and eighth year after restoration because bulk fracture occurred in five cases (11%) during the two years. (Operative Dentistry 2000) Hot Pressed Glass Ceramics. •

In 1968, it was discovered that certain glasses could be modified with nucleating agents and, upon heat treatment, be changed into ceramics with organized crystalline forms. Such "glass-ceramics" were stronger, had a higher melting point than noncrystalline glass, and had variable coefficients of thermal expansion.

At first, these glass-ceramics were primarily developed for cookware and other heatresistant products.

In 1984, the glass-ceramic material Dicor (Dentsply International, York, Pennsylvania) was patented and rapidly became a popular ceramic for dental restorations.

A major disadvantage of Dicor was its translucency, which necessitated external application of all shading.Dicor restorations were made using a lost wax , centrifugal casting process.

Newer Leucite-reinforced glass ceramic system also use the lost-wax method but the material is heated to high temperature pneumatically pressed, rather than centrifuged, into a mold.

Hot pressed ceramic appear to be stronger than fires Feldspathic porcelain clinically.

The fabrication steps for one type of leucite-reinforced pressed ceramic restoration are summarized as follows: 1. After tooth preparation, an impression is made and a master working cast is poured of die stone. A wax pattern of the restoration is made using conventional techniques . 2. After spruing , investing, and wax pattern burnout, a shaded ceramic ingot and aluminum oxide plunger are placed into a special furnace . 3. At approximately 1100° C, the ceramic ingot becomes plastic and is slowly pressed into the mold by an automated mechanism.

67


4. The restoration is seated on the master die and working cast for final adjustments and finishing 5. To accurately reproduce the tooth shade, a heavily pigmented surface stain is typically applied. R Frankenberger et al investigated the six-year clinical performance of adhesively-luted IPS Empress (Leucite-reinforced) ceramic inlays and onlays.After six years IPS Empress restorations revealed a 7% failure rate with 94% of the remaining restorations having marginal deficiencies. ( Operative Dentistry 2000) The ceramic ingots are relatively translucent and available in a variety of shades, so staining for inlay and onlay restorations is typically rather minimal. The advantages of hot pressed ceramics are their: a. similarity to traditional "wax-up" processes, b. excellent marginal fit, and c. relatively high strength. The surface hardness and occlusal wear of these ceramics are very similar to enamel. Although pressed ceramic inlays are stronger than porcelain inlays made on refractory dies, they are still quite fragile until cemented. The incidence of postcementation fracture for pressed ceramic inlays is expected to be lower than that for ceramic inlays fired on refractory dies, but higher than for inlays made with CAD/CAM systems. However, little information is available comparing the various systems. CAD/CAM. Rapid improvements in technology have spawned several computerized devices thai can fabricate ceramic inlays and onlays from high-quality ceramics in a matter of minutes. •

Some CAD/CAM systems are very expensive laboratory-based units requiring the submission of an impression or working cast of the prepared tooth.

•

The CEREC system was the first commercially available CAD/CAM system developed for the rapid chairside design and fabrication of ceramic restorations.

•

The 2001 versions of this device are the CEREC 2 and CEREC 3 (Sirona USA, Charlotte, North Carolina)

68


Generation of a CEREC restoration begins after the dentist prepares the tooth and uses a scanning device to collect information about the shape of the preparation and its relationship with the surrounding structures. This step is termed an optical impression. •

A videoimage of the prepared tooth is displayed to ensure proper positioning of the scanning device. The CEREC systems use the optical techniques of Moire fringe displacement and active triangulation to measure the height and depth of the preparation.

The system projects an image of the preparation and surrounding structures on a monitor, allowing the dentist or auxiliary personnel to use the CAD portion of the system to design the restoration.

The operator must input and/or confirm some of the boundaries of the restoration, such as the position of the gingival margins.

Once the restoration has been designed, the computer directs a micromilling device (CAM portion of the system), which mills the restoration out of a block of high-quality ceramic or composite in a matter of minute.

Then the restoration is removed from the milling device, ready for Try-in and cementation.

The major disadvantages of CAD/CAM systems are high cost and the need for extra training. However, CAD/CAM technology is changing rapidly, with each new generation of devices having more capability, accuracy, and ease of use. CLINICAL PROCEDURES Many of the clinical procedures described are common to both laboratory-fabricated and CAD/CAM restorations. TOOTH PREPARATION •

Preparations for specific types of indirect tooth-colored inlays and onlays may vary because of differences in fabrication steps for each commercial system and variations in the physical properties of the restorative materials.

As a first clinical step, the patient should be anesthetized and the area isolated with rubber dam preferentially.

Preparations for indirect tooth-colored inlays and onlays basically are meant to provide adequate thickness for the restorative material and at the same time a passive insertion pattern with rounded internal angles and well-defined margins.

69


All margins should have a 90°-degree butt-joint cavosurface angle to ensure marginal strength of the restoration. All line and point angles internal and external, should be rounded to avoid stress concentrations in the restoration and tooth, thereby reducing the potential for fractures.

The carbide bur or diamond used for tooth preparation should be a tapering instrument that creates occlusally divergent facial and lingual walls.

Gingival-occlusal divergence allows for passive insertion and removal of the restoration.

The junction of the sides and tip of the cutting instrument should have a rounded design to avoid creating sharp, stress-inducing internal angles in the preparation.

Although the optimal gingival-occlusal divergence of the preparation is unknown, it should be greater than the 2° to 5° per wall recommended for cast metal inlays and onlays. Divergence can be increased because the tooth-colored restoration will be adhesively bonded and because very little pressure can be applied during try-in and cementation.

Throughout preparation, the cutting instruments used to develop vertical walls are oriented to a single path of draw, usually the long axis of the tooth crown.

The occlusal step should be prepared 1.5 - 2 mm in depth.

70


Most composite and ceramic systems require that any isthmus and any groove extension be at least 1.5 mm wide to decrease the possibility of fracture of the restoration.

Facial and lingual walls should be extended to sound tooth structure and should go around the cusps in smooth curves.

Ideally, there should be no undercuts that would prevent the insertion or removal of the restoration.

Small undercuts, if present, can be blocked out using a glass-ionomer liner. The pulpal floor should be smooth and relatively flat. Following removal of extensive caries or previous restorative material from any internal wall, the wall is restored to more nearly ideal form with a light-cured glass-ionomer liner/base.

The facial, lingual, and gingival margins of the proximal boxes should be extended to clear the adjacent tooth by at least 0.5 mm. These clearances will provide adequate access to the margins for impression material and for finishing and polishing instruments.

For all walls, a 90-degree cavosurface margin is desired because composite and ceramic inlays are fragile in thin sections.

The gingival margin should be extended as minimally as possible because margins in enamel are greatly preferred for bonding and because deep gingival margins are difficult to impress and to isolate properly during cementation.

When a portion of the facial or lingual surface is affected by caries or other defect, it may be necessary to extend the preparation (with a gingival shoulder) around the transitional line angle to include the defect.

The axial wall of the shouldered extension should be prepared to allow for adequate restoration thickness.

When extending through or along cuspal inclines to reach sound tooth structure, a cusp usually should be capped if the extension is two thirds or greater than the distance from any primary groove to the cusp tip.

If cusps must be capped, they should be reduced 1.5 to 2 mm and should have a 90-degree cavosurface angle.

It is important to round all line angle and point angles, internal and external , to avoid stress concentration.

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CLINICAL PROCEDURES FOR CAD/CAM INLAYS AND ONLAYS •

When delivering a CAD/CAM inlay, more adjustments are usually necessary when trying-in, finishing, and polishing. The original CEREC system milled the occlusal surface relatively flat without any significant surface detail, and did not take into account the opposing occlusion.

The newer CEREC 2 and 3 systems are able to mill in occlusal contours in a variety of manners. They can extrapolate existing contours beyond the cavosurface margin to the central groove, or they can build the surface up to the level of a scanned wax bite. The neighboring teeth, in particular the marginal ridges and cusp heights, also can be used as references for the design of the occlusal surface of a CAD/CAM restoration.

If the preoperative contours of the tooth were satisfactory, the system can reproduce them in the restoration. When adjusting the occlusion of a CAD/CAM inlay, it may be necessary to use medium-grit diamonds with air-water spray coolant for initial contouring of the occlusal surface, followed by the instrumentation previously discussed for finishing and polishing.

CONCLUSION Cast metal inlays and onlays offer excellent restorations that may be underutilized in dentistry. Even though the technique requires multiple patient visit and excellent laboratory support, the resulting restorations are long lasting. The popularity of cast gold restorations has lessened over the past 20 years because of increased emphasis on esthetics. Advances in ceramic, composite, and adhesive technology have resulted in the development of a variety of tooth-colored indirect restorations. These offer an excellent alternative to direct composite restorations, especially for large restorations, and are more conservative than full coverage restorations. However because of their clinical procedures are relatively technique-sensitive, proper case selection, operator skill, and attention to detail are critical to success.

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REFERENCES: 1. Theodore M Roberson, Harald O.Heymann and Edward J. Swift, Jr. Sturdevant’s Art and Science of Operative Dentistry. 4th edition. 2. M A Marzouk, A L Simonton and R D Gross. Operative dentistry. Modern theory and practice. 1st edition 3. Gerald T. Charbeau. Principles and practice of Operative Dentistry. 3rd edition 4. James B Summitt. Fundamentals of operative dentistry. 2nd edition. 5. Frankenberger, A Petschelt and N Kramer. Leucite-Reinforced Glass Ceramic Inlays and Onlays after Six years: Clinical behaviour. Oper Dent 2000;25:459-465 6. M Hayashi, Y Tsuchitani, Y Kawamura, M Miura, F Takeshige and S Ebisu. Eightyear clinical evaluation of fired Ceramic Inlays. Oper Dent 2000; 25:473-481

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