Armamentarium

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

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.

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.

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.

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 b/>