Indications

Partial veneer crowns often can be used to restore posterior teeth that have lost moderate amounts of tooth structure, if the buccal wall is intact and well supported by sound tooth structure. They are also commonly used as retainers for an FDP or where restoration or alteration of the occlusal surface is needed. Anterior partial veneers are rarely suitable for restoring damaged teeth, but they can be used as retainers, to reestablish anterior guidance, and to splint teeth. They are particularly suitable for teeth with sufficient bulk because they can accommodate the necessary retentive features.

Contraindications

Partial veneer restorations are contraindicated on teeth that have a short clinical crown because retention may not be adequate. They are also contraindicated as retainers for long-span FDPs. They are rarely suitable for endodontically treated teeth, especially anterior teeth, because insufficient supporting tooth structure remains for the retentive features. Likewise, they should not be used on endodontically treated posterior teeth if the buccal cusps are weakened by the access cavity or on teeth with an extensively damaged crown. As is true of all cast restorations, partial veneer restorations are contraindicated in dentitions with active caries or periodontal disease.

The shape and alignment of teeth are important determinants of the feasibility of partial veneer crowns. The alignment of axial surfaces should be evaluated, and partial veneer crowns should not be placed on teeth that are proximally bulbous. Making the necessary proximal grooves on these teeth is likely to leave unsupported enamel. It may be similarly impossible to prepare adequate grooves on thin teeth of restricted faciolingual dimension.

Partial veneer crowns are usually prepared parallel to the long axis of the tooth, and poorly aligned abutment teeth may not be suitable. When poorly aligned teeth are being prepared for a partial-coverage restoration, problems with unsupported enamel often result.

Advantages

The primary advantage associated with partial veneer crowns is conservation of tooth structure. Another advantage is reduced pulpal and periodontal insult during tooth preparation. Access to supragingival margins is rather easy and allows the operator to perform selected finishing procedures that are more difficult or impossible with complete coverage restorations. Access is also better for oral hygiene. Because less of the margin approximates the soft tissues subgingivally, there is less gingival involvement than with complete coverage.

During cementation of a partial veneer, the luting agent can escape more easily, which produces relatively good seating of the restoration. Because of direct visibility, verification of seating and cement removal are simple. When the restoration is in service, the remaining intact facial or buccal tooth structure permits electric vitality testing.

Disadvantages

Partial veneer restorations have less retention and resistance than do complete cast crowns. Preparing the tooth for this type of coverage is difficult, primarily because only limited adjustments can be made in the path of placement. The preparation of grooves, boxes, and pinholes requires dexterity of the operator. Some metal is displayed in the completed restoration, which may be unacceptable to patients with high cosmetic expectations.

Preparation

The following discussions cover the teeth most commonly prepared for partial veneer restorations. The use of partial veneers on anterior teeth has declined because of the difficulty in achieving an esthetic result. The technique illustrated may be suitable for posterior teeth and, with minimal variation, for other teeth. On both posterior and anterior teeth, meticulous care and precision are required if partial veneer restorations are to be a successful (conservative) alternative to complete-coverage restorations.

Armamentarium

The necessary instruments for a partial veneer crown preparation include the following (Fig. 10-2):

• Narrow (approximately 0.8 mm), round-tipped, tapered diamond (regular or coarse grit)

• Regular-size (approximately 1.2 mm), round-tipped, tapered diamond (fine grit) or carbide

• Football-shaped or wheel-shaped diamond (regular grit)

• Tapered and straight carbide fissure burs

• Small, round carbide bur

• Small-diameter twist drill

• Inverted-cone carbide bur

• Finishing stones

• Mirror

• Explorer and periodontal probe

• Chisels

Fig. 10-2 Armamentarium for a partial veneer crown preparation.

This is the typical armamentarium for a partial veneer crown preparation. Depending on operator preference, additional instruments can be used. The regular- or coarse-grit diamonds are used for bulk reduction, and the fine-grit diamonds or carbides are used for finishing. Pinholes are prepared with the twist drill and finalized with a tapered carbide. The fissure burs are recommended for preparing boxes and ledges, and the inverted-cone carbide is recommended for preparing incisal offsets. Hand instruments can be used to finish proximal flares and bevels. A periodontal probe is invaluable for assessing the direction and dimension of the various steps.

Posterior Partial Veneer Crown Preparations

Maxillary premolar three-quarter crown

The three-quarter crown preparation (Fig. 10-3) derives its name from the number of axial walls involved. Except for a slight bevel or chamfer placed along the bucco-oclusal line angle, the buccal tooth surface remains intact. The other surfaces (including the occlusal surface) are prepared to accommodate a casting in the same manner as a complete crown preparation (see Chapter 8), differing only in the need for axial retention grooves.

Fig. 10-3 The maxillary premolar three-quarter crown. A, Initial depth holes are placed in the mesial and distal fossae approximately 0.8 mm deep. B, They are connected by a guiding groove that extends through the central groove. Additional guiding grooves are placed on the lingual cusp similar to those for a complete cast crown (see Fig. 8-8). The depth cut placed on the triangular ridge of the buccal cusp becomes shallower as it approaches the cusp tip. C, Half the occlusal reduction is completed. Note the functional cusp bevel. The occlusocervical height of the buccal surface is not reduced at this stage. D, Occlusal reduction completed. E, After guiding grooves are placed in the lingual surface of the tooth parallel to the proposed path of placement, the proximoaxial and linguoaxial reductions are initiated. Simultaneously, a smooth and even-width cervical chamfer is created. F, When the axial reduction of the first half is considered acceptable, the other half can begin. G, Proximal grooves are placed perpendicular to the prepared surface, and the buccal wall of each groove is flared to leave no unsupported enamel. The proximal flares are connected with a narrow contrabevel. After rounding of the line angles, the preparation is complete. H, The interproximal clearance relative to adjacent teeth extends cervically as well as near the occlusal aspect of the buccal flares of the proximal grooves.

Occlusal reduction

Upon the completion of occlusal reduction, a clearance of at least 1.5 mm should exist on the functional cusp and at least 1.0 mm on the nonfunctional cusp and in the central groove. Simultaneously, the tooth should be prepared so that the restoration displays a minimum of metal, with preservation of the buccal wall outline.

1. Before any partial veneer crown preparation, mark the proposed location of the margin of the completed preparation on the tooth with a pencil (Fig. 10-4).

2. Place depth grooves for the occlusal reduction. These should be made with a tapered carbide or narrow diamond in the developmental grooves of the mesial and distal fossae and on the crest of the triangular ridge. In the central groove, they should be slightly less (about 0.2 mm) than 1 mm deep to allow for finishing; on the functional (lingual) cusp they should be slightly less than 1.5 mm deep in the location of the occlusal contacts.

3. Place three depth grooves on the lingual incline of the buccal cusp. Initially, these should be kept somewhat shallow as they approach the buccal cusp ridge (see Fig. 10-3B). In the area of occlusal contact, the groove should be about 0.8 mm deep so that there will be at least 1 mm of clearance after finishing.

4. Verify groove depth with a periodontal probe. When this is found to be acceptable, remove the islands of tooth structure remaining between the grooves (see Fig. 10-3C and D).

5. Assess the amount of occlusal clearance in maximum intercuspation and in all excursive movements of the mandible (Fig. 10-5). Grinding a small concavity on the incline of the buccal cusp may help obtain sufficient clearance while maintaining the original occlusocervical dimension of the buccal tooth surface (Fig. 10-6).

Fig. 10-4 The anticipated location of the completed preparation is marked with a pencil.

Fig. 10-5 A common error is insufficient reduction of tooth structure in the marginal ridge area (arrow).

Fig. 10-6 Recommended minimum clearances for reduction of a partial veneer crown preparation. Slight hollow grinding of the lingual incline of the buccal cusp results in an acceptable clearance with the least display of metal. Also, the final restoration retains the normal contours of the cuspal ridge, so that incident light is not reflected and the restoration is less evident.

Axial reduction

6. Place grooves for axial alignment in the center of the lingual surface and in the mesiolingual and distolingual transitional line angles. These should be parallel to the long axis of the tooth and should not exceed half the width of the tip of the diamond used to place them.

7. Because the path of placement of a partial veneer is crucial, assess these grooves carefully when correction is still possible. A common error is to incline the path of placement toward the buccal. This either reduces retention or leads to an excessive display of metal. A periodontal probe placed in each groove should be carefully viewed in both planes (mesiodistal and buccolingual). It often helps to pour an irreversible hydrocolloid (alginate) impression in fast-setting plaster and to evaluate the cast with a dental surveyor, particularly if multiple partial veneers are being used as retainers for an FDP.

8. After verifying the alignment, remove tooth structure between the guide grooves (with a smooth continuous motion) and place a cervical chamfer (Fig. 10-7).

9. Carry the diamond into the proximal embrasure and reduce the proximal wall (see Fig. 10-3E and F). For proper reduction of the axial tooth surface, it is important to understand the factors that determine correct positioning of the proximal groove. A proximal groove is placed parallel to the path of placement. Normally, unsupported tooth structure remains on the buccal side of the groove, and this side is flared to remove it. Figure 10-8 illustrates the relationship among the initial axial reduction, groove placement, and location of the cavosurface angle where the flare meets the intact buccal wall. The cavosurface angle is especially significant when a tooth is prepared for a partial veneer that should display a minimum of metal; the farther to the buccal the margin is, the more gold is visible. A subtle but extremely important variable that determines the final location of the cavosurface angle is the apical extension of the preparation. As the cervical chamfer extends closer to the cementoenamel junction, more axial tooth structure is removed. Consequently, the deepest portion of the groove (its pulpal wall) is located slightly closer to the center of the tooth. This results in a flare that can extend farther onto the facial or buccal surface than desirable. Marking the location of the intended facial flare on the tooth with a pencil before initiating the proximoaxial reduction is helpful. The intersection of this mark with the reduced occlusal surface is a convenient reference point.

10. Stop the proximal reduction well short of the pencil mark and usually slightly short of breaking the proximal contact (Fig. 10-9). The resulting flange should be parallel to the linguoaxial preparation, with the chamfer placed sufficiently cervical to provide at least 0.6 mm of clearance with the adjacent tooth and the axial wall allowing for a proximal groove of at least 4 mm of length occlusocervically (see Fig. 10-3F).

Fig. 10-7 Proximal and lingual axial reduction is performed with a round-tipped diamond. The proximal reduction is stopped short of the proposed location of the buccal margin.

Fig. 10-8 A, Upon completion of the proximal axial reduction, a groove is placed perpendicular to the prepared surface. B, Note that some unsupported tooth structure remains at the cavosurface angle. C, After the buccal wall of the proximal groove is flared, no unsupported tooth structure remains. Note: It is important to anticipate in advance the influence of the buccal extent of the proximoaxial reduction (A) on the ultimate location of the margin (C).

Fig. 10-9 The distal proximal reduction is stopped before breaking proximal contact. After groove placement and subsequent flaring, interproximal clearance results.

Groove placement

Preparation of the proximal grooves is best done with a tapered carbide bur.

11. Position the bur against the interproximal flange parallel to the path of placement and make a groove perpendicular to the axial surface. The groove need not be deeper than 1 mm at its cervical end but may be deeper near its occlusal end (Fig. 10-10). During this stage, the bur must be held precisely parallel to the selected path of placement. Allowing it to tip axially results in excessive taper between opposing proximal grooves, which is a commonerror. The criteria that need to be met consist of the following (Fig. 10-11; see also Fig. 10-9):

• The grooves should resist lingual displacement of a periodontal probe or explorer.

• The walls of the grooves should not be undercut in relation to the selected path of placement.

• The walls should be flared toward the intact buccal surface of the tooth (see Fig. 10-3G and H).

Fig. 10-10 Because of the rotary instrument’s taper, the proximal groove is deeper near the occlusal table (A). The floor of the groove should be flat and smooth. Often the proximal chamfer extends slightly cervically to the floor of the groove. If only minimal difference exists, as in B, the cervical margin adjacent to the groove can be beveled. The recommended occlusocervical height for a proximal groove is 4 mm.

Fig. 10-11 The 90-degree angle between the lingual walls of the proximal grooves and the axial walls resists lingual displacement. Because the buccal aspect of the grooves has been adequately flared, no unsupported tooth structure remains.

Depending on available access, it may be feasible to complete the flaring with the same rotary instrument that was used to place the groove (Fig. 10-12). However, removing the last lip of unsupported tooth structure with a chisel is often a better option, because this minimizes the risk of damage to the adjacent tooth.

Fig. 10-12 A, Initial preparation of the mesial proximal groove. Note that the carbide is oriented parallel to the path of placement as dictated by the lingual surface of the tooth. B, Initial flaring has resulted in elimination of most unsupported tooth structure. C, Hand or rotary instruments are used to refine these proximal flares and remove all unsupported enamel.

Bucco-occlusal contrabevel

12. Connect the mesial and distal flares with a narrow contrabevel that follows the buccal cusp ridges. This can be placed with a diamond, a carbide, or even a hand instrument. Its primary purpose is to remove any unsupported enamel and thereby protect the buccal cusp tip from chipping during function. If group function is planned (as opposed to a mutually protected occlusion), a heavier bevel, chamfer, or occlusal offset is needed, because tooth contact occurs in this area during excursive movement. The bevel should remain within the curvature of the cusp tip rather than extend onto the buccal wall (Fig. 10-13). This results in a convex shape of the restoration, and light is prevented from reflecting back to a casual observer (see Fig. 10-6). Thus, the restoration is less obvious, and the outline form of remaining buccal enamel is perceived as the shape of the tooth.

Fig. 10-13 The buccoclusal contrabevel remains within the curvature of the cusp tip rather than extending onto the buccal surface.

Occlusal offset

If additional bulk is needed to ensure rigidity of the restoration, it can be provided with an occlusal offset. This V-shaped groove extends from the proximal grooves along the buccal cusp. It is not usually necessary for posterior partial veneer crowns but is essential for the structural durability of anterior partial veneer crowns. This is described in detail on p. 301.

Finishing

13. Round all sharp internal line angles to facilitate subsequent procedures. A fine-grit diamond or carbide can be used to blend the surfaces (Fig. 10-14).

14. Reevaluate the flares, paying particular attention to any remaining undercuts, which must be removed. The flares should be straight and smooth, with sufficient clearance between them and the adjacent tooth. A minimum clearance of 0.6 mm is recommended. The mesial flare cannot extend beyond the transitional line angle. However, because the distal margin is less visible, it may extend slightly farther to the buccal margin, allowing better access for oral hygiene.

Fig. 10-14 A and B, A fine-grit diamond in a low-speed contra-angle is used to place the bucco-occlusal contrabevel connecting the mesioproximal and distoproximal flares.

Maxillary molar three-quarter crown

The principles used in a premolar preparation also apply for a maxillary molar (Figs. 10-15 and 10-16). However, some additional leeway may exist for groove placement because more tooth structure is present on molars than on premolars. Also, because of their less prominent position in the dental arch, molars are less visible. As a result, the mesioproximal flare can sometimes be extended onto the buccal surface without spoiling the esthetics.

Fig. 10-15 Three-quarter crown preparation on a maxillary molar. Note that the occlusal reduction follows normal anatomic form.

Fig. 10-16 The three-quarter crown preparation on a maxillary first molar.

Maxillary molar seven-eighths crown

The seven-eighths crown preparation (Fig. 10-17) includes, in addition to the surfaces covered by the three-quarter crown, the distal half of the buccal surface. Therefore, the mesial aspect of this preparation resembles that for a three-quarter crown; the distal aspect resembles that for a complete crown. The mesial half of the buccal tooth surface remains intact and is protected by a narrow contrabevel or chamfer similar to the one used in the three-quarter crown preparation. A distal groove may be placed, although generally this is not necessary. A groove in the middle of the buccal surface is placed parallel to the path of placement. Distal to this groove, the buccal surface is reduced in two planes, cervical and occlusal; the cervical plane parallels the path of placement, and the occlusal plane follows the normal anatomic contour. The lingual surface of the tooth also is reduced in two planes, and functional cusp bevels are incorporated.

Fig. 10-17 The maxillary molar seven-eighths crown preparation. A, Occlusal depth grooves. On the lingual of the mesiobuccal cusp, they are identical to grooves for any functional cusp. On the buccal, note their difference from grooves placed on the triangular ridges. The mesial groove becomes shallower as it approaches the cuspal ridge; the distal extends through the cuspal ridge. B, Mesial half of the occlusal reduction is completed. Normal occlusal form can be recognized in the reduced area. C, Occlusal reduction completed. D, Distal half of the axial reduction completed. This is comparable to the preparation for a complete cast crown. The rotary instrument is moved parallel to the guiding grooves placed in the lingual tooth surface. E, Mesial half of the axial reduction completed and a proximal groove placed. F, The buccal groove, with flaring of the mesial groove. Note the monoplane of the flare, extending from the deepest portion of the groove to the cavosurface angle. G, A contrabevel connects the mesial flare with the buccal groove. The mesial wall of the buccal groove is smooth and has a 90-degree cavosurface angle, leaving no unsupported enamel.

Occlusal reduction

Upon completion of the occlusal reduction, adequate clearance should exist in all excursive movements of the mandible. Minimum measurements are the same as for the three-quarter crown preparation.

1. Place depth grooves in the central and developmental grooves, as well as on the crests of the triangular ridges. To delineate the extent of the lingual functional cusp bevel, they should extend onto the lingual surface of the tooth. On the lingual incline of the mesiobuccal cusp, they resemble depth cuts for the three-quarter crown preparation. On the distobuccal cusp, they should be approximately 0.8 mm deep to provide sufficient occlusal clearance for this nonfunctional cusp (see Fig. 10-17A).

2. Remove the tooth structure between the depth grooves. Concave shaping of the resulting mesiobuccal incline may again prove useful because it enables the occlusocervical height of the cusp to be maintained. When completed, this bevel should provide 1.5 mm of clearance in maximum intercuspation, as well as throughout all excursive movements of the mandible (see Fig. 10-17B and C).

Axial reduction

In principle, the steps for axial reduction follow those for occlusal reduction.

3. Place three alignment grooves in the lingual wall and transfer the selected path of placement to the distobuccal transitional line angle area, where a fourth alignment groove can be placed.

4. Start the reduction in the middle of the lingual surface. The mesial half is prepared in the same way as for a three-quarter crown and the distal half as for a complete crown (see Fig. 10-17D).

5. Carry the facial reduction sufficiently mesial to include the buccal groove. Occlusally the buccal surface of maxillary molars is rather flat, therefore some additional reduction may be necessary in the occlusal third. This follows the normal anatomic configuration of the tooth and often resembles a small version of the functional cusp bevel. If correctly performed, the reduction allows for contouring of the restoration so that when viewed from the mesial aspect, the distal half of the restoration is hidden behind the mesiobuccal cusp. A frequent error is to over-taper the buccal wall segment, with resulting loss of retention.

Groove placement, flaring, and contrabevel

6. Prepare the mesial groove in the same way as for the three-quarter crown (see Fig. 10-17E and F).

7. Place the buccal groove parallel to the mesial groove and perpendicular to the buccoaxial wall. It is often not necessary to flare the buccal groove because the flat configuration of this area of the tooth precludes any unsupported enamel after the groove is placed. The buccal groove should resist mesiodistal displacement of a probe.

8. Connect the two grooves with a smooth contrabevel that follows the ridge of the mesiobuccal cusp (see Fig. 10-17G). This bevel should meet the same criteria as described in the three-quarter crown preparation. Adequate clearance must be established interproximally upon completion (Fig. 10-18). All surfaces are finished to the same specifications as the preceding preparations (Fig. 10-19).

Fig. 10-18 The seven-eighths crown preparation. Note that adequate clearance has been established. From this perspective, it is evident why little or no flaring is necessary for the buccal groove, as opposed to the considerable flaring needed for the mesial groove.

Fig. 10-19 A, Three-quarter crown restoring maxillary premolar and seven-eighths crown restoring maxillary molar. B and C, Fixed dental prosthetic retainers: a seven-eighths crown as the distal and a three-quarter crown as the mesial.

Mandibular premolar modified three-quarter crown

Mandibular partial veneer preparations (Fig. 10-20) are made more often on premolars than on molars. They differ from maxillary molar three-quarter crown preparations in two respects: (1) Additional retention is required because of the shorter crown lengths of mandibular teeth. This can be obtained by extending the preparation buccally, although because of their rather prominent position in the dental arch, these teeth should be modified only distal to their height of contour (Fig. 10-21). (2) The axial surface that is not prepared (the buccal) includes the functional cusp. This means that additional tooth structure must be removed to provide sufficient bulk of metal for strength.

Fig. 10-20 The mandibular premolar modified three-quarter crown preparation. A, Depth holes placed in the mesial and distal fossae approximately 0.8 mm deep. B, The holes are connected by a guiding groove that extends through the central groove and the mesial and distal marginal ridges. Guiding grooves are also placed in the buccal and lingual triangular ridges, extending through the cuspal ridges on both sides. C, Half the occlusal reduction is completed. D, Occlusal reduction and mesial half of the axial reduction are completed. E, Axial reduction is completed. The proximal grooves have been placed. Note that the distal groove is close to the buccolingual center of the tooth. This enables retention of considerable tooth structure in the area of the distobuccal line angle, enhancing the resistance form of the preparation. F, The mesial groove has been flared and the functional cusp chamfer placed. G, Facial view. There is considerable width of the chamfer on the functional cusp. Note that the distobuccal cervical margin angles occlusally as it progresses mesially. This enables a more conservative tooth preparation in the area of the distobuccal modification that is placed to improve resistance form.

Fig. 10-21 Modified three-quarter crown restoring a mandibular second premolar. A, B, and C, The three-quarter crown is serving as the anterior retainer for a three-unit fixed dental prosthesis. Because the distobuccal modification remains in the distal fourth of the buccal preparation, it is hidden behind the normal height of contour of the buccal tooth surface. Note the considerable thickness of gold that protects the buccal cusp.

Occlusal reduction

1. Place 0.8-mm depth grooves on the buccal inclines of the lingual cusp and 1.3-mm grooves on the lingual inclines of the buccal cusp (see Fig. 10-20A and B). These guiding grooves are once again placed to follow the basic groove and fissure pattern of the occlusal surface. Only one depth cut needs to be placed to accommodate the functional cusp bevel on the distal aspect of the distal ridge.

2. Reduce the occlusal surface by removing the tooth structure between the grooves (see Fig. 10-20C).

Axial reduction

3. Place guiding grooves on the lingual surface to parallel the proposed path of placement and the long axis of the tooth.

4. Prepare the mesial half as already described for the three-quarter and seven-eighths crown (see Fig. 10-20D).

5. Reduce the distal surface as for a complete crown, extending the preparation to the transitional line angle and onto the buccal surface. However, it should not extend mesially beyond the middle of the distal half of the buccal surface, and the chamfer should not extend too far cervically; otherwise, the distobuccal line angle is unnecessarily reduced, which would decrease the resistance form (see Fig. 10-20E).

Finishing

The modified three-quarter crown preparation can include two or three grooves.

6. Place the mesial and buccal grooves as described for the seven-eighths crown (see Fig. 10-20F). Another distal groove may be placed. In general, to gain as much length as possible, the grooves of the three-quarter crown should be slightly buccal. Care must be taken so that the distal groove is slightly closer to the center of the distal wall (so that the distobuccal line angle is not undermined).

7. Connect the mesial and buccal grooves with a functional cusp chamfer after the grooves and mesial flare have been placed and evaluated. The chamfer must be heavy enough to allow 1.5 mm of clearance in the area of occlusal contact (see Fig. 10-20G). A regular or thick diamond is used to place the chamfer, which should connect the grooves and provide a protective “staple” linkage of alloy in the completed restoration. Insufficient tooth reduction where this chamfer meets the mesial flare is a common error. Finally, all prepared surfaces are smoothed and the internal line angles rounded.

Anterior Partial Veneer Crown Preparations

As stated, with the advent of metal-ceramic restorations, the use of partial veneer restorations on anterior teeth has become rare. Nevertheless, two anterior partial veneer crown preparations, the maxillary canine three-quarter crown and the pinledge, are worthy of consideration (Figs. 10-22 and 10-23).

Fig. 10-22 A, Deficient anterior guidance resulting from years of parafunctional activity. B, An anterior partial veneer crown has reestablished guidance, allowing the intact sound labial tooth structure to be retained as a conservative alternative to a metal-ceramic restoration.

Fig. 10-23 A, Caries-free canine and lateral incisor of adequate bulk: excellent candidates for anterior partial veneer crowns. B, The canine restored with a three-quarter crown, serving as the anterior retainer for a three-unit fixed dental prosthesis (FDP) to replace the first premolar. The lateral incisor has been restored with a modified pinledge that serves as a retainer for an anterior four-unit FDP. C, Satisfactory esthetics with minimal display of metal are apparent.

Maxillary canine three-quarter crown

The three-quarter crown on a maxillary canine (Figs. 10-24 and 10-25) is probably one of the most demanding of all tooth preparations. As with such preparations on other teeth, on a maxillary canine it involves the proximal and lingual surfaces and leaves the facial surface intact. However, the greater degree of difficulty stems from the different shape of the canine tooth. Unless the placement of grooves is determined very precisely in advance, there is an undesirable display of metal in the interproximal embrasures (see Fig. 10-25A and B). The relatively short proximal walls do not allow much correction after initial groove placement. Similarly, the greater degree of curvature in each proximal wall immediately adjacent to the contact area significantly influences the location of the preparation’s facial margin.

Fig. 10-24 The maxillary canine three-quarter crown preparation. A, A guiding groove is placed on the lingual surface. B, Half the lingual surface is reduced. Clearance is verified before reduction of the other half. C, Lingual reduction is completed, with an incisal bevel placed. No significant change has occurred in the incisocervical height. D, After an alignment groove is placed in the center of the cingulum wall, half the axial reduction is complete. Note that the path of placement parallels the incisal or middle third of the labial surface. As a result, the lingual chamfer is quite wide, perhaps even resembling a shoulder. This permits paralleling of the cingulum wall, with the proximal grooves and pinhole providing additional retention. E, Axial reduction is completed. Any final modification of the path of placement is done at this time before groove placement. F, Proximal grooves. The visible mesial groove has been flared, but unsupported enamel remains on both grooves where they meet the incisal bevel. G, Completed preparation. The lingual pinhole is surrounded by adequate dentin. Note the horizontal ledge prepared before pinhole placement.

Fig. 10-25 A, Proposed margin location outlined on the tooth with a pencil. B, Careful assessment of the anticipated outline from as many directions as possible is valuable at this time. C, Preparing the incisal bevel. Typically a lingually tilted bevel is prepared at a 45-degree angle to the long axis of the tooth. D, The lingual surface is reduced with a wheel- or football-shaped diamond.

Incisal and lingual reduction

1. Remove enough enamel to allow 1 mm of metal thickness. The design of the incisal bevel should prevent contact between opposing teeth and the incisal margin. However, the original configuration of the facial surface should be preserved without significant incisal reduction of the tooth. Outlining the anticipated location of the margin with a pencil can be helpful.

2. Place depth grooves for both the incisal bevel and the lingual reduction (see Fig. 10-24A). The direction of the bevel may vary somewhat, depending on the configuration of the tooth. In general, it makes an angle of approximately 45 degrees with the long axis of the tooth.

3. After the depth has been verified, perform the reduction. A football- or wheel-shaped diamond is used to reduce the concave lingual wall (see Figs. 10-24B and 10-25D). The lingual reduction should not extend onto the cingulum itself, which will be prepared as part of the axial reduction. (The completed reduction is shown in Fig. 10-24C.)

Axial reduction and groove placement

The path of placement of the restoration must be accurately determined before axial reduction. Mesiodistally, it should parallel the long axis of the tooth; buccolingually, it should parallel the middle third or incisal two thirds of the facial surface. This allows the preparation of proximal grooves of optimum length in an area of the tooth where sufficient bulk is present.

4. To enhance the retention and resistance form of the preparation, place a slightly exaggerated chamfer on the lingual aspect of the tooth (see Fig. 10-24D) and a guiding groove in the middle of the lingual wall. When alignment has been verified, the axial reduction can be performed in the same manner as the other preparations (Fig. 10-26). It is important to understand the difference between this phase of the preparation on a canine, with little bulk of lingual tooth structure as opposed to a premolar or molar. After completion, a proximal flange should result; this guides the rotary instrument during groove placement (see Figs. 10-24E and 10-26B). The technical aspects of the preparation of proximal grooves are like those described for the other partial veneer preparations (Figs. 10-27 and 10-28). Theprimary difference is the direction in which the groove is prepared. Because the groove is placed perpendicular to the proximal wall, its deepest portion is slightly labial to the proximal flange that results when proximoaxial reduction is completed. As a result, the proximal flares extend slightly farther onto the facial surface. This is even more accentuated by the curvature of the proximal wall (Fig. 10-29). Meticulous assessment of the needed extent of the initial axial reduction is a prerequisite for successful preparation(Fig. 10-30; see also Fig. 10-24F). (The required interproximal clearance is illustrated in Fig. 10-31.)

Fig. 10-26 A, A regular-grit diamond is used to complete the axial reduction. Mesiodistally, the diamond is oriented parallel to the long axis of the tooth. B, When the reduction is completed, a mesial and distal flange results; this serves as a guide during preparation of the proximal groove.

Fig. 10-27 A, Because the groove is prepared perpendicular to the proximal surface of the tooth, its deepest portion is slightly buccal to where axial reduction was halted. B, The dotted line indicates the proposed flare. Note that the curvature of the tooth causes the final margin to be located a considerable distance facial to where the initial axial reduction stopped. C, Completed flares.

Fig. 10-28 A, A tapered carbide is used to place the proximal groove. B, Initial groove preparation is completed. C, The carbide is moved parallel to itself. D, Mesial and distal grooves must be prepared in strict alignment.

Fig. 10-29 Differences between the proximal flares on premolars and canines. A designates where the initial proximal reduction is halted. Because a facial component is present in the direction of groove placement on the canine, as opposed to the premolar, the starting point (B) for the flare is located farther to the facial aspect. In conjunction with the greater degree of proximal curvature of canines, it is crucial that the initial axial reduction not be carried too far facially; otherwise, the final margin will extend too far onto the labial surface of the tooth and result in excessive display of metal.

Fig. 10-30 A, Unsupported enamel remaining after initial groove placement. B, A carbide bur can be used to flare the grooves. C, The flared groove. Note the irregularity of the margin near the cervical aspect of the groove. D, After the flaring. Note that a mesial box, rather than a groove, has been prepared. This restoration is designed to contain an intracoronal partial removable dental prosthesis rest; hence, the box. Nevertheless, there is adequate resistance to lingual displacement. E, A special mandrel is placed in the box to ensure that it fits within its confines. It is identical in size to the male attachment of the partial removable dental prosthesis.

Fig. 10-31 Completed three-quarter crown preparation. Note the location of the facial margin in relation to the adjacent teeth. Sufficient interproximal clearance has been established, but unnecessary display of metal is avoided.

Incisal offset and lingual pinhole

Anterior partial veneer crowns require a means of reinforcement for preserving the casting’s integrity. Posterior three-quarter crowns usually do not need as much additional reinforcement because the solid “corrugated” occlusal surface provides rigidity. For an anterior tooth, an incisal offset or groove is needed to create a band of thicker metal to provide a “staple” configuration. This provides additional rigidity and resistance against bending of the casting.

5. Connect the mesial and distal grooves with an incisal offset. It should improve the general resistance form of the preparation against lingual displacement and should have a V configuration. Sufficient dentin must be preserved facially to the offset to prevent the metal from being visible through the translucent tooth enamel. This is most effectively accomplished with an offset that is slightly narrower labiolingually than incisocervically. The offset should follow the normal configuration of the incisal edge, and its transition into the proximal flares should be smooth and continuous. An inverted-cone diamond or carbide (Fig. 10-32) can be used to prepare the offset.

6. Place a pinhole in the cingulum area slightly off center to improve the retention and resistance form of this preparation. The pinhole is prepared in five stages: first, a small horizontal ledge is made with a large, tapered carbide bur; second, a slight “dimple” is created with a round bur at the intended pinhole location; third, a pilot hole is prepared with a small-diameter twist drill^* (itmust be parallel to the precise path of placement of the restoration); fourth, the preparation is completed with a tapered carbide bur to a pinhole depth of approximately 2 mm; finally, a larger, round bur is used to countersink or bevel the junction between pinhole and ledge.

Fig. 10-32 An inverted-cone diamond or carbide can be used to prepare the incisal offset. Note the faciolingual inclination of the rotary instrument.

The technical aspects of pinhole preparation are described in the ensuing paragraphs. The completed preparation (Fig. 10-33) is carefully assessed for any remaining undercuts. The flares are a common area for undercuts, and all surfaces should be smoothed as previously described.

Fig. 10-33 A, Completed three-quarter crown preparation on a maxillary canine. B, The contralateral canine. C, A three-quarter crown serves as the anterior retainer for a three-unit fixed dental prosthesis (FDP); its female intracoronal rest is incorporated in the mesial box. D, Note the connector and the open embrasures on the contralateral side. E to G, Labial views of the cemented FDP. H, The definitive RDP.

Pinledge Preparations

A pinledge (Fig. 10-34) is occasionally used as a single restoration, generally to reestablish anterior guidance, in which case only the lingual surface is prepared. More commonly, however, it is used as a retainer for an FDP (Fig. 10-35) or to splint periodontally compromised teeth (Fig. 10-36). In these cases, one or more of the proximal surfaces are included in the preparation design to accommodate the required connector or connectors. Retention and resistance are provided primarily by pins that extend to a depth of 2 mm into dentin. In comparison with other retainers, the pinledge preparation is very conservative of tooth structure.

Fig. 10-34 The pinledge preparation on a maxillary central incisor. A, Guiding grooves placed for lingual reduction. B, The lingual reduction completed and an incisal bevel placed. C, Incisal and cervical ledges prepared. D, Indentations have been made. Note the spacing of the ledges in relation to each other and to the pulp. All pinholes will be in sound dentin. E, Pinholes prepared to a depth of 2 mm. The junction between the ledge and the pinholes has been countersunk.

Fig. 10-35 A, Modified pinledge serving as a retainer for a four-unit fixed dental prosthesis (FDP). An additional pinhole was placed in the cingulum and in the cervical aspect of the proximal groove; in the latter instance, this was done because insufficient tooth structure remained to provide resistance against lingual displacement. B, The FDP on the definitive cast. C, A four-unit FDP consisting of a modified pinledge, two metal-ceramic pontics, and a metal-ceramic crown.

Fig. 10-36 A, Periodontally compromised but caries-free teeth of adequate buccolingual width are excellent candidates for a pinledge retained fixed splint. B, The definitive cast. C, Pinledge splint consisting of six separate castings that were soldered together and seated. D, A minimum display of metal results. The pinledge preparations allow retention of the intact labial enamel of all six anterior teeth.

The preparation steps themselves are not difficult, but advance planning and a thorough understanding of the various steps are prerequisites to success. Diagnostic preparation on an accurate cast is particularly useful during the planning phase. Preparation of a number of parallel pinholes with a common path of placement can be intimidating. With some practice, however, this can be accomplished freehand by most operators, especially when a tapered bur is used. Paralleling devices are available for practitioners who do not feel comfortable preparing multiple pinholes. In general, pinledges are highly esthetic restorations. Plaque control after treatment is easier because of short margin length and largely supragingival margin location.

Contraindications

Patients with poor oral hygiene or a high caries rate are not good candidates for this type of restoration. Young patients with large pulps generally are better served by a resin-retained FDP (see Chapter 26). It is often not possible to place pinholes of adequate size and length in teeth that are thin labiolingually (Fig. 10-37). Pinledges are contraindicated on nonvital teeth and when the alignment of the abutment conflicts with the proposed path of placement of the FDP. Because less surface area is involved in the preparation, pinledges are not as retentive as their less conservative counterparts. Therefore, they should not be used when optimum retention is needed.

Fig. 10-37 Where incisors are thin labiolingually and insufficient dentin remains facial to the casting, appearance is compromised by a pinledge restoration.

Maxillary central incisor pinledge

Three designs of pinledge preparations are discussed here: the conventional pinledge (see Fig. 10-34), involving only the lingual surface of the tooth; the pinledge with a proximal slice (Fig. 10-38); and the pinledge with a proximal groove (Fig. 10-39A). The latter two can serve equally well as retainers for an FDP; the choice of one over the other depends primarily on tooth configuration and the presence or absence of caries. A tooth with a slight proximal convexity can often be prepared successfully with a proximal slice, whereas one with a small carious lesion often lends itself better to the proximal groove variation. The pinledge preparation with proximal slice is described first.

Fig. 10-38 Pinledge preparation with a proximal slice. The slice provides room for a fixed dental prosthesis connector. Sufficient tooth structure should remain between the slice and the pinhole adjacent to it. Note that the junction between pinhole and ledge has been beveled or countersunk.

Fig. 10-39 A, Modified pinledge preparation with a proximal groove. The path of placement of this groove is compatible with the preparation as well as with the pinholes. B, A similar preparation on a maxillary canine. Note two similarities with the three-quarter crown: the heavy lingual chamfer and the incisal offset blending into the proximal groove to provide additional bulk for reinforcement.

Design

1. Draw the outline of the proposed preparation onto the tooth (Fig. 10-40D). A line is marked along the height of contour of the incisal edge and on the proximal wall to include the area needed for a connector. The lingual chamfer is placed immediately adjacent to the crest of the marginal ridge. The cervical extent of the margin is on the height of contour of the cingulum, but it may be extended farther cervically at a later stage to blend into the proximal aspect of the preparation.

Fig. 10-40 A, Although periodontally compromised and malpositioned, these six caries-free anterior teeth are excellent for pinledge preparations. B, Orthodontic repositioning of the teeth. C, Stabilization after the repositioning. D, Outline of the proposed preparations drawn on the teeth.

Proximal reduction

2. Prepare the proximal slice with a tapered diamond. (Disks may be preferred by some operators.) The diamond is either held parallel to the path of placement or given a slight lingual inclination. The primary purpose of this step is to provide sufficient reduction to allow adequate metal in the area for a subsequent connector. The proximal reduction includes the proximal contact area, but care must be taken not to extend the reduction too far facially, because this alters the outline form of the tooth. For esthetic reasons, the reduction must not extend onto the labial surface.

Incisal and lingual reduction

3. Prepare the incisal bevel with the diamond inclined slightly toward the lingual aspect. It extends just beyond the previously placed pencil line on the crest of the incisal edge, but it must remain within the curvature of the incisal edge to minimize display of metal. Sufficient clearance provides functional contact on metal rather than on the junction between metal and tooth structure. The desired metal thickness is 1 mm, except in the area close to the margin.

4. Perform the lingual reduction with a football- or wheel-shaped diamond after placing reduction grooves, as has been described in other anterior preparations. Metal thickness of 1 mm is required in maximum intercuspation and throughout excursive movements. The reduction follows the lingual marginal ridge and continues its chamfer configuration cervically until it runs into the proximal reduction. To facilitate subsequent stages of the preparation, care must be taken to maintain as much tooth structure as possible in the incisal third.

5. Smooth the incisal and lingual reduction with fine-grit diamonds and stones before preparing the ledges and pinholes.

Ledges and indentations

Two ledges are prepared across the reduced lingual surface. They provide room for sufficient bulk of metal to ensure rigidity. The restoration would otherwise not be very strong because it would consist of only a thin sheet of metal.

The ledges are prepared parallel to the incisal edge of the tooth, as viewed from the lingual aspect, and parallel to one another, as viewed from the incisal aspect. In selected areas, they are widened to provide indentations of sufficient size to accommodate the pinholes. The determination of the incisocervical location of the ledges depends on the configuration of the pulp and the available bulk of tooth structure (Fig. 10-41). The incisal ledge is usually prepared 2 to 2.5 mm cervical to the incisal edge, or one fourth of the total height of the preparation from the incisal edge. The cervical ledge is placed on the crest of the cingulum at the center of the cervical one fourth of the preparation.

Fig. 10-41 Proximal lingual views of the location of ledges in relation to the height of the crown. The incisal ledge is placed so that its floor is one fourth of the preparation’s height from the incisal edge. The cervical ledge is placed so its floor bisects the cervical fourth. Note that the path of insertion is parallel to the incisal two thirds of the labial wall. Adequate offset of the cervical pinhole either mesially or distally is needed to prevent pulpal exposure.

6. Prepare two ledges with a cylindrical carbide bur. The recommended minimum width for the ledge is 0.7 mm. Drawing the proposed location of the ledges on the lingual surface of the tooth is helpful. The design of the ledges must be compatible with the path of placement of the restoration, which is parallel to the incisal two thirds of the labial surface of the tooth.

7. Make indentations in the left and right sides of the incisal ledge and slightly off center in the cervical ledge to prevent subsequent pulp exposure when the pinholes are placed. These incisal indentations are as widely spaced as possible to retain as much dentin as possible between the pinholes and the pulp. Because the completed pinhole must be surrounded by sound dentin, it is not possible to place holes in the extreme corners because of the tooth’s structure. However, every effort should be made to prepare the indentations so that the pinholes are surrounded by dentin and away from the pulp. This is particularly important for younger patients. The relationship between recommended pinhole locations and the pulp is illustrated in Figure 10-42. In general, this means that the indentations are just within the mesial and distal marginal ridges, about 1.5 mm inside the external tooth contour (Fig. 10-43). The same carbide bur can be used to prepare the indentations. When the indentations are completed, their configuration should resemble a half cylinder. Again, their orientation is parallel to the selected path of placement, and their floor should be smooth and continuous with the floor of the ledges. When combined, they should provide a flat area 1 to 1.2 mm wide buccolingually.

Fig. 10-42 Relationship between pinhole placement and pulp configuration. A to C, Lingual views. D to F, Cross-sections through incisal pinholes. G to I, Cross-sections through cervical pinholes. Dotted lines show the mean pulp chamber size of various age groups.

(Data from Ohashi Y: Shikagakuho 68:726, 1968.)

Fig. 10-43 A, Proposed location of the ledges marked on the teeth with a pencil. Note the orientation of the carbide in relation to the long axis of the tooth. B, Ledge preparation completed on one side. Pilot holes for some pinholes have been placed.

Pinhole preparation

8. Sink pilot channels with either a small, round bur or a small twist drill. The shallow indentations prevent skating of the selected bur. The depth of the completed pinhole should be at least 2 mm but can be as much as 3 mm when the placement and orientation of the pilot channels are satisfactory.

9. Enlarge and deepen the pilot channels with a tapered carbide bur when their placement and orientation are satisfactory. At this stage, any small corrections in orientation can be made. Less experienced operators may spend a great deal of time attempting to determine the correct alignment of the bur. However, it should be remembered that the design and location of the pinholes have already been determined by the placement of the ledges and indentations; therefore, the only remaining concern should be verification of the position of the rotary instrument and attainment of the minimum depth of the pinholes. Some operators find it helpful to place a second bur in a prepared pinhole to help transfer the path of placement, although precautions must be taken to prevent its being swallowed or inhaled. Preparing multiple pinholes a little at a time may also be helpful, moving from one to the next and gradually deepening each. This enables alignment verification as the pinholes are prepared.

10. Bevel the junction between pinhole and indentation with a round bur slightly larger than the largest diameter of the pinhole (Fig. 10-44). (The required interproximal clearance is illustrated in Fig. 10-45.)

11. Inspect all surfaces of the preparation for smoothness and evaluate the margin. Correct any area that requires more distinct delineation (Fig. 10-46).

Fig. 10-44 Note the relation among the ledge, the indentation, and the pinhole. Recommended dimensions are given in the buccolingual cross section on the right. a, Ledge; b, indentation; c, pinhole; d, countersink.

Fig. 10-45 Modified pinledge preparation with a proximal groove. Adequate interproximal clearance has resulted from the proximal flare.

Fig. 10-46 A, Ledges and indentations prepared. B, Pinhole preparation with low-speed handpiece. C, The completed pinledge preparations. Utility wax has been placed over the brackets for impression making.

Indications

An inlay can be used instead of amalgam for patients with a low caries rate who require a small Class II restoration in a tooth with ample supporting dentin. It is among the least complicated cast restorations to make and can be very durable when it is carefully done. An onlay allows the damaged occlusal surface to be restored with a casting in the most conservative manner. It should be considered in the restoration of a severely worn dentition when the teeth are otherwise minimally damaged or for the replacement of a mesio-occlusal-distal (MOD) amalgam restoration when sufficient tooth structure remains for retention and resistance form.

Contraindications

Because these restorations rely on intracoronal (wedging) retention, inlays and onlays are contraindicated unless there is sufficient bulk to provide resistance and retention form. MOD inlays may increase the risk of cusp fracture and are generally not recommended. Extensive onlays, required where caries or existing restorations extend beyond the facial or lingual line angles, are contraindicated unless pins are used to supplement retention and resistance.

Advantages

Cast inlays and onlays can prove to be extremely long-lived restorations because of the excellent mechanical properties of the gold alloy. Low creep and corrosion mean that if inlay or onlay margins are accurately cast and finished, they will not deteriorate. The lack of corrosion may be an esthetic advantage. Gold does not lead to the tooth discoloration sometimes associated with dental amalgam. Unlike an inlay or amalgam, an onlay can support cusps, reducing the risk of tooth fracture.

Disadvantages

In the restoration of a small carious lesion, an inlay is not very conservative of tooth structure. This is because additional tooth removal is necessary after minimal proximal extension to achieve a cavity preparation without undercuts and to enable access for impression making. This extension may lead to additional display of metal and gingival encroachment, which is undesirable for periodontal health. Because they do not encircle the tooth, inlays rely on the bulk of the buccal and lingual cusps for resistance and retention form. There is concern that high occlusal force will lead to cusp fracture as a result of wedging from the inlay.

Preparation

Armamentarium

Carbide burs are usually used for inlay or onlay preparations (Fig. 10-47), but diamonds can be substituted if preferred:

• Tapered carbide burs

• Round carbide burs

• Cylindrical carbide burs

• Finishing stones

• Mirror

• Explorer and periodontal probe

• Chisels

• Hatchet

• Gingival margin trimmers

• Excavators

• High- and low-speed handpieces

• Articulating film

Fig. 10-47 Armamentarium for inlays and onlays.

Class II Inlay Preparation (Fig. 10-48)

Occlusal analysis

1. Carefully assess the occlusal contact relationship and mark it with articulating film. The margins of the restoration should not be too close (≥1.0 mm) to a centric contact; otherwise, there will be damaging stresses at the gold-enamel junction.

2. Apply rubber dam. Because good visibility and moisture control are essential during tooth preparation and caries excavation, the use of a rubber dam is strongly recommended.

Fig. 10-48 The MO inlay preparation. A, An occlusal outline is prepared following the central groove and extended proximally. B, Gingival extension undermines the marginal ridge while removing caries. C, Unsupported enamel is removed, and the walls of the proximal box are defined. This is easily accomplished with hand instruments. D, An occlusal bevel or chamfer completes the preparation. E, Occlusal view of the completed preparation.

Outline form

3. Penetrate the central groove just to the depth of the dentin (typically about 1.8 mm) with a small round or tapered carbide bur held in the path of placement of the inlay. In general, this is perpendicular to an imaginary line connecting the buccal and lingual cusps, not necessarily perpendicular to the occlusal plane. For example, on mandibular premolars, it is angled toward the lingual aspect.

4. Extend the occlusal outline through the central groove with the tapered carbide. The bur should be held in the same path of placement and kept at the same depth—just into dentin. The buccolingual extension should be as conservative as possible to preserve the bulk of the buccal and lingual cusps. Resistance to proximal displacement is achieved with a small occlusal dovetail or pinhole. The outline should avoid the occlusal contacts.

5. Extend the outline proximally, undermining the marginal ridge, and stop it at the height of contour of the ridge (Fig. 10-49A).

6. Advance the bur cervically to the carious lesion and then lingually and buccally, taking care to hold it in the precise path of placement. There should be a thin layer of enamel remaining between the side of the bur and the adjacent tooth (Fig. 10-49B). This prevents accidental damage. The bur should move parallel to the original unprepared proximal surface, creating a convex axial wall in the box. The opposing buccal and lingual walls contribute significantly to retention, so great care must be taken not to tilt the bur during this step. It should be held in the path of placement throughout. The width of the gingival floor of the box should be about 1.0 mm (mesiodistally). Correct cervical, lingual, and buccal extension at this stage is just beyond the proximal contact area. The completed inlay will require a minimum of 0.6 mm of proximal clearance to allow an impression to be made, but some of this will be achieved with the proximal flares and gingival bevels. Sharp line angles between the occlusal outline and proximal box are rounded at this time (Fig. 10-49C).

Fig. 10-49 Preparation of a mandibular premolar tooth for a disto-occlusal inlay. A, Occlusal outline. B, Proximal box initiated. C, Proximal box extended to remove contact. D, Completed preparation.

(Courtesy of Dr. H. Bowman.)

Mesio-Occlusal-Distal Onlay Preparation

The occlusal outline and proximal boxes of an onlay preparation (Fig. 10-50) are similar to those of an inlay. The additional steps are the occlusal reduction and a functional (centric) cusp ledge.

Fig. 10-50 The mesio-occlusal-distal (MOD) onlay preparation. A, An occlusal outline is prepared to follow the central fossa. B, The marginal ridges are undermined. C and D, The proximal boxes are refined. They should extend just beyond the proximal contact area. E, Depth grooves are placed for occlusal reduction: 0.8 mm on the nonfunctional cusp and 1.3 mm on the functional cusp. F, Note the buccal functional cusp bevel as part of the completed occlusal reduction. A buccal shoulder is prepared, approximately at the level of the pulpal floor. G, A continuous bevel completes the preparation. The bevel on the buccal shoulder makes a smooth transition into the proximal bevel of the box. A small contrabevel is placed on the lingual cavosurface margin.

Outline form

1. Prepare the occlusal outline with a tapered carbide bur just beyond the enamel-dentin junction (approximately 1.8 mm deep) and extend it through the central groove, incorporating any deep buccal or lingual grooves. Existing amalgam restorations are removed as part of this step (Fig. 10-51A).

2. Extend the outline both mesially and distally to the height of contour of the marginal ridge. As with an inlay, the boxes with an MOD onlay are prepared by advancing the bur gingivally and then buccally and lingually, always holding it in the precise path of placement of the preparation. If a thin section of proximal enamel remains as the bur advances, damage to the adjacent tooth will be prevented (Fig. 10-51B). Correct gingival, buccal, and lingual extension of the preparation normally depends on the contact area with the adjacent tooth. A minimum clearance of 0.6 mm is needed for impression making. Sometimes existing restorations or caries necessitate that a box be extended beyond optimal. However, if a box requires extension beyond the transitional line angle, the preparation will have little resistance form, and an alternative restoration such as a complete crown should be considered. Preparing the boxes is a key step when an onlay is fabricated (Fig. 10-51C and D). The tapered bur should be held precisely in the planned path of placement throughout. Tilting, often caused by trying to advance the bur too quickly, is commonly done and is difficult to correct.

3. Round sharp line angles between the occlusal outline and proximal boxes.

Fig. 10-51 Preparation of a mandibular molar tooth for a mesio-occlusal-distal (MOD) onlay. A, Preparation outline. B, Proximal boxes extended to remove contacts. C, Unsupported enamel removed with hand instruments. D, Proximal boxes are extended to form a 90-degree cavosurface angle. E, Occlusal reduction grooves. F, Functional cusp ledge placed for distal half. G and H, Completed preparation.

(Courtesy of Dr. H. Bowman.)

PARTIAL VENEER CROWN PREPARATION, POSTERIOR TEETH

PARTIAL VENEER CROWN PREPARATION, ANTERIOR TEETH

PINLEDGE PREPARATION

CLASS II INLAY PREPARATION

MOD ONLAY PREPARATION