9 THE METAL-CERAMIC CROWN PREPARATION
In many dental practices, the metal-ceramic crown is one of the most widely used fixed restorations. This has resulted in part from technologic improvements in the fabrication of this restoration by dental laboratories and in part from the growing amount of cosmetic demands that challenge dentists today.
The restoration consists of a complete-coverage cast metal crown (or substructure) that is veneered with a layer of fused porcelain to mimic the appearance of a natural tooth. The extent of the veneer can vary.
To be successful, a metal-ceramic crown preparation requires considerable tooth reduction wherever the metal substructure is to be veneered with dental porcelain. Only with sufficient thickness can the darker color of the metal substructure be masked and the veneer duplicate the appearance of a natural tooth. The porcelain veneer must have a certain minimum thickness for esthetics. Consequently, much tooth reduction is necessary, and the metal-ceramic preparation is one of the least conservative of tooth structures (Fig. 9-1).
Fig. 9-1 Recommended minimum dimensions for a metal-ceramic restoration on an anterior tooth (A) and a posterior tooth (B). Note the significant reduction needed compared to that for a complete cast or partial veneer crown.
Historically, attempts to veneer metal restorations with porcelain had several problems. A major challenge was the development of an alloy and a ceramic material with compatible physical properties that would provide adequate bond strength. In addition, it was initially difficult to obtain a natural appearance.
The technical aspects of the fabrication of this restoration are discussed further in Chapter 24. For now, only a brief description is provided. The metal substructure is waxed and then cast in a special metal-ceramic alloy that has a higher fusing range and a lower thermal expansion than do conventional gold alloys. After preparatory finishing procedures, this substructure, or framework, is veneered with dental porcelain. The porcelain is fused onto the framework in much the same manner as household articles are enameled. Modern dental porcelains fuse at a temperature of about 960° C (1760° F). Because conventional gold alloys would melt at this temperature, the special alloys are necessary.
The metal-ceramic crown is indicated on teeth that require complete coverage and for which significant esthetic demands are placed on the dentist (e.g., the anterior teeth). It should be recognized, however, that, if esthetic considerations are paramount, an all-ceramic crown (see Chapters 11 and 25) has distinct cosmetic advantages over the metal-ceramic restoration; nevertheless, the metal-ceramic crown is more durable than the all-ceramic crown and generally has superior marginal fit. Furthermore, it can serve as a retainer for a fixed dental prosthesis because its metal substructure can accommodate cast or soldered connectors. Whereas the all-ceramic restoration cannot accommodate a rest for a removable prosthesis, the metal-ceramic crown may be successfully modified to incorporate occlusal and cingulum rests as well as milled proximal and reciprocal guide planes in its metal substructure (see Chapter 21).
Typical indications are similar to those for all-metal complete crowns: extensive tooth destruction as a result of caries, trauma, or existing previous restorations that precludes the use of a more conservative restoration; the need for superior retention and strength; an endodontically treated tooth in conjunction with a suitable supporting structure (a post and core); and the need to recontour axial surfaces or correct minor malinclinations. Within certain limits, this restoration can also be used to correct the occlusal plane.
Contraindications for the metal-ceramic crown, as for all fixed restorations, include patients with active caries or untreated periodontal disease. In young patients with large pulp chambers, the metal-ceramic crown is also contraindicated because of the high risk of pulp exposure (see Fig. 7-4). If at all possible, a more conservative restorative option such as a composite resin or porcelain laminate veneer (see Chapter 25) or an all-ceramic crown with less reduction (see Chapter 11) is preferred.
A metal-ceramic restoration should not be considered whenever a more conservative retainer is feasible, unless maximum retention is needed, as for a long-span fixed dental prosthesis. If the facial wall is intact, the practitioner should decide whether it is truly necessary to involve all axial surfaces of the tooth in the proposed restoration. Although perhaps technically more demanding and time consuming, a more conservative solution that satisfies the patient’s needs and may provide superior long-term service can usually be found.
The metal-ceramic restoration combines, to a large degree, the strength of cast metal with the esthetics of an all-ceramic crown. The underlying principle is to reinforce a brittle, more cosmetically pleasing material through support derived from the stronger metal substructure. Natural appearance can be closely matched by good technique and, if desired, through characterization of the restoration with internally or externally applied stains. Retentive qualities are excellent because all axial walls are included in the preparation, and it is usually quite easy to ensure adequate resistance form during tooth preparation. The complete-coverage aspect of the restoration permits easy correction of axial form. In addition, the required preparation is often much less demanding than for partial-coverage retainers. In general, the degree of difficulty of a metal-ceramic preparation is comparable to that of preparing a posterior tooth for a complete cast crown.
The preparation for a metal-ceramic crown requires significant tooth reduction to provide sufficient space for the restorative materials. To achieve better esthetics, the facial margin of an anterior restoration is often placed subgingivally, which increases the potential for periodontal disease. However, a supragingival margin can be used if significant cosmetic concerns do not preclude it or if the restoration incorporates a porcelain labial margin (see Chapter 24).
In comparison with an all-ceramic restoration, the metal-ceramic crown may have slightly inferior esthetics, but it can be used in higher stress situations or on teeth that would not provide adequate support for an all-ceramic restoration.
Because of the glasslike nature of the veneering material, a metal-ceramic crown is subject to brittle fracture (although such failure can usually be attributed to poor design or fabrication of the restoration). A frequent problem is the difficulty of accurate shade selection and of communicating it to the dental ceramist. This is often underestimated by the novice. Because many procedural steps are required for both metal casting and porcelain application, laboratory costs generally render the metal-ceramic restoration among the more expensive of dental procedures.
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 saves time.
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 placement, which usually coincides with the long axis of the tooth. The secondary facial 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. P, A football-shaped diamond is recommended for lingual reduction of anterior teeth. Q to S, Finishing the preparation with a fine-grit diamond. T, The completed preparation.
Fig. 9-3 Preparation of a maxillary premolar for a metal-ceramic crown. A, Depth holes. B, Occlusal depth cuts. C, Completed occlusal reduction. D and E, Lingual chamfer and facial shoulder are prepared on half the tooth (lingual view [D] and facial view [E]. F, Completed preparation.
Fig. 9-4 Armamentarium for the metal-ceramic crown preparation. A, Diamond rotary instrument. B to D, Off-angle hatchets. These are useful for smoothing the shoulder margins of metal-ceramic crown preparations.
The preparation is divided into five major steps: guiding grooves, incisal 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.