Preparation

The recommended sequence of preparation is illustrated for a maxillary right central incisor (Fig. 9-2); however, the same step-by-step approach can be applied to other teeth (Fig. 9-3). As with all tooth preparations, a systematic and organized approach to tooth reduction will save time.

Armamentarium (Fig. 9-4). The instruments needed to prepare teeth for a metal-ceramic crown include:

• Round-tipped rotary diamonds (regular grit for bulk reduction, fine grit for finishing) or carbides

Preparation Crown Central Incisor

Fig. 9-2. Preparation of a maxillary incisor for a metal-ceramic crown. A, Heavily restored maxillary central incisor. B and C, Rotary instrument aligned with the cervical one third and incisal two thirds to gauge correct planes of reduction. D and E, Guiding grooves placed in the two planes. The cervical groove is made parallel to the path of withdrawal, which usually coincides with the long axis of the tooth. The incisal depth groove is prepared parallel to the facial contour of the tooth. F and G, Incisal guiding grooves are placed. H, Incisal edge reduction. I to K, Facial reduction accomplished in two planes. L, Breaking proximal contact, maintaining a lip of enamel to protect the adjacent tooth from inadvertent damage. M and N, Proximal reduction. O, Placing a 0.5-mm lingual chamfer.

Fig. 9-2. Preparation of a maxillary incisor for a metal-ceramic crown. A, Heavily restored maxillary central incisor. B and C, Rotary instrument aligned with the cervical one third and incisal two thirds to gauge correct planes of reduction. D and E, Guiding grooves placed in the two planes. The cervical groove is made parallel to the path of withdrawal, which usually coincides with the long axis of the tooth. The incisal depth groove is prepared parallel to the facial contour of the tooth. F and G, Incisal guiding grooves are placed. H, Incisal edge reduction. I to K, Facial reduction accomplished in two planes. L, Breaking proximal contact, maintaining a lip of enamel to protect the adjacent tooth from inadvertent damage. M and N, Proximal reduction. O, Placing a 0.5-mm lingual chamfer.

Proximal Reduction

Fig. 9-3. Preparation of a maxillary premolar for a metal-ceramic crown. A, Depth holes. B, Occlusal depth cuts. C, Half of the occlusal reduction is completed. D, Occlusal reduction is complete. Guiding grooves are placed for axial reduction. E and F, Lingual chamfer and facial shoulder are prepared on half the tooth. G, Completed preparation.

Crown Prep Procedure
Fig. 9-4. Armamentarium for the metal-ceramic crown preparation.

I Football- or wheel-shaped diamond (for lingual reduction of anterior teeth) Flat-ended, tapered diamond (for shoulder preparation) Finishing stones Explorer and periodontal probe Hatchet and chisel

The actual sequence of steps can be varied slightly depending on operator preference.

Step-By-Step Procedure. The preparation is divided into five major steps: guiding grooves, in-cisal or occlusal reduction, labial or buccal reduction in the area to be veneered with porcelain, axial reduction of the proximal and lingual surfaces, and final finishing of all prepared surfaces.

Guiding Grooves

1. Place three depth grooves (Fig. 9-5), one in the center of the facial surface and one each in the approximate locations of the mesiofa cial and distofacial line angles (see Fig. 9-2, A to E). These will be in two planes: the cervical portion to parallel the long axis of the tooth, the incisal (occlusal) portion to follow the normal facial contour (see Fig. 9-2, D and E).

2. Perform the facial reduction in the cervical and incisal planes. The cervical plane will determine the path of withdrawal of the completed restoration. The incisal or occlusal plane will provide the space needed for the porcelain veneer; it should be approximately 1.3 mm deep to allow for additional reduction during finishing. The incisal grooves usually extend halfway down the facial surface, although (depending on the shape of the tooth) they may extend to include the incisal two thirds. Cervical grooves are generally made parallel to the long axis of the tooth. However, they can be adjusted slightly to create a more desirable path of withdrawal; in particular, some labial inclination will im-

Resistance Groove Tooth Preparation
Fig. 9-5. Depth grooves in the facial wall are placed in two directions: incisally, parallel to the tooth contour; cervi-cally, parallel to the path of withdrawal. The grooves should be 1.3 mm deep.

prove retention on a tooth with little cingu-lum height. On small teeth it may be advisable to keep the cervical grooves somewhat shallower near the margin.

3. Place three depth grooves (about 1.8 mm deep) in the incisal edge of an anterior tooth. This will provide the needed reduction of 2 mm and allow finishing (see Fig. 9-2, F and G). Verify the depth of these grooves can be verified with a periodontal probe. On posterior teeth where the occlusion is to be established in porcelain, 2 mm of clearance must exist. If the occlusion is to be established in metal, the same minimum clearances are needed as for a complete cast crown. Posterior occlusal reduction incorporates a functional cusp bevel on the lingual cusp, similar to that for a complete cast crown. When initially positioning the diamond for anterior teeth, it may be helpful to observe the long axis of the opposing tooth in the intercuspal position and to orient the instrument perpendicular to that (Fig. 9-6). The grooves must not be too deep; otherwise, an overreduced and undulating surface will result.

Incisal (Occlusal) Reduction. The completed reduction of the incisal edge on an anterior tooth should allow 2 mm for adequate material thickness to permit translucency in the completed restoration. Posterior teeth generally require less (1.5 mm) because esthetics is not as critical. Caution must be used, however, because excessive occlusal reduction shortens the axial walls and thus is a common cause of inadequate retention and resistance form in

The Structure Ofthe Central Incisor
Fig. 9-6. A, Depth grooves 1.8 mm deep placed in the incisal edges to ensure adequate and even reduction. B, Incisal reduction completed on the left central and lateral incisors. Note the angulation of the diamond, perpendicular to the direction of loading by the mandibular anterior teeth.

the completed preparation. This can be particularly problematic on anterior teeth (where as a consequence of tooth form, most of the retention is derived from the proximal walls).

4. Remove the islands of remaining tooth structure. On anterior teeth, access is usually unrestricted, and the thickest portion of the cut ting instrument can be used to maximize cutting efficiency (see Fig. 9-2, H). On posterior teeth, the same pattern is followed as in preparing depth grooves for a complete cast crown (see Chapter 8). This will include the use of a centric cusp bevel, although additional occlusal reduction will be needed where the porcelain is to be applied (see Fig. 9-3, A to C.

Labial (Buccal) Reduction. When completed, the reduction of the facial surface should have produced sufficient space to accommodate the metal substructure and porcelain veneer. A minimum of 1.2 mm is necessary to permit the ceramist to produce a restoration with satisfactory appearance (1.5 mm is preferable). This requires significant tooth reduction. For comparison, the cervical diameter of a maxillary central incisor averages between 6 and 7 mm.

In the cervical area of small teeth, obtaining optimal reduction is not always feasible (see Fig. 7-4.) Often a compromise is made with lesser reduction in the area where the cervical shoulder margin is prepared.

5. Remove the remaining tooth structure between depth grooves (see Fig. 9-2,1 to L), creating a shoulder at the cervical margin (Fig. 9-7). If a restoration with a narrow subgingival metal collar is to be fabricated and sufficient sulcular depth is present, place the shoulder approximately 0.5 mm apical to the crest of the free gingiva at this time. Additional finishing will then result in a margin that is 0.75 to 1 mm subgingival. Use adequate water spray during the entire phase of

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