Posterior Partial Veneer Crown Preparations

Maxillary Premolar Three-quarter Crown (Fig. 10-3). The three-quarter crown preparation derives its name from the number of axial walls involved.

Except for a slight bevel or chamfer placed along the buccocclusal 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.

Occlusal Reduction. Upon the completion of occlusal reduction, a clearance of at least 1.5 mm should exist on the centric cusp and at least 1.0 mm

Partial Veneer Crown

Fig. 10-3. The maxillary premotar 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 centric 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 withdrawal, 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.

Fig. 10-3. The maxillary premotar 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 centric 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 withdrawal, 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.

on the noncentric 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 develop mental 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 centric (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 (Fig. 10-3, B). 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 (Fig. 10-3, C and D).

5. Assess the amount of occlusal clearance in the intercuspal position 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 oc-clusocervical dimension of the buccal tooth surface (Fig. 10-6).

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

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