11 TOOTH PREPARATION FOR ALL-CERAMIC RESTORATIONS
All-ceramic inlays, onlays, veneers, and crowns are some of the most esthetically pleasing prosthodontic restorations. Because there is no metal to block light transmission, they can resemble natural tooth structure better in terms of color and translucency than can any other restorative option. Their chief disadvantage is their susceptibility to fracture, although this is lessened by use of the resin-bonded technique.
The restorations may be fabricated in several ways. The technique (first developed more than 100 years ago) originally called for a platinum foil matrix to be intimately adapted to a die. This supported the porcelain during firing and prevented distortion. The foil was removed before cementation of the restoration.
Complete ceramic crowns should have relatively even thickness circumferentially. For the hot-pressed ceramic crown (IPS Empress* or OPC†) (Fig. 11-1), usually about 1 to 1.5 mm is needed to create an esthetically pleasing restoration. Incisally, a greater ceramic thickness may be required.
Only minor differences in tooth preparation design exist among the restorations fabricated with the various techniques. Therefore, the hot-pressed crown preparation is described in detail, and the necessary variations are discussed when pertinent.
The advantages of a complete ceramic crown include its superior esthetics, its excellent translucency (similar to that of natural tooth structure), and its generally good tissue response. Lack of reinforcement by a metal substructure enables slightly more conservative reduction of the facial surface than is possible with the metal-ceramic crown, although the lingual surface needs additional reduction for strength. The appearance of the completed restoration can be influenced and modified by selecting different colors of luting agent. However, changing cement color under restorations that rely on an opaque core for strength, such as the slip-cast alumina core system (In-Ceram‡), is ineffective.
The disadvantages of a complete ceramic crown include reduced strength of the restoration because of the absence of a reinforcing metal substructure. Because of the need for a shoulder-type margin circumferentially, significant tooth reduction is necessary on the proximal and lingual aspects. Porcelain brittleness, when combined with the lack of a reinforcing substructure, requires the incorporation of a circumferential support with a shoulder. Thus, by comparison, the proximal and lingual reductions are less conservative than those needed for a metal-ceramic crown.
Difficulties may be associated with obtaining a well-fitting margin when certain techniques are used. The “unforgiving” nature of porcelain, if an inadequate tooth preparation goes uncorrected, can result in fracture.
Proper preparation design is critical to ensuring mechanical success. A 90-degree cavosurface angleis needed to prevent unfavorable distribution of stresses and to minimize the risk of fracture (Fig. 11-2). The preparation should provide support for the porcelain along its entire incisal edge, unless a ceramic system that includes a high-strength core is chosen (see Chapter 25).
All-ceramic restorations are not effective as retainers for a fixed dental prosthesis, although the strongest of the slip-cast materials (In-Ceram Zirconia§) and the higher-strength pressed systems (IPS Empress 2¶) may be suitable for anterior applications. The brittle nature of porcelain necessitates that connectors of large, cross-sectional dimension (a minimum of 4 × 4 mm is recommended) be incorporated in the fixed dental prosthesis design. This typically leads to impingement on the interdental papilla by the connector, with increased potential for periodontal failure.
Wear has been observed on the functional surfaces of natural teeth that oppose porcelain restorations. This also applies to teeth opposed by metal-ceramic restorations, especially the mandibular incisors, which can exhibit significant wear over time (see Fig. 19-1).
The complete ceramic crown is indicated in areas with a high esthetic requirement where a more conservative restoration would be inadequate (Fig. 11-3). Usually such a tooth has proximal and/or facial caries that can no longer be effectively restored with composite resin.
Fig. 11-3 A, Inadequately fitting all-ceramic crowns have led to recurrent caries and gingival recession around these central incisors. The patient was a professional model and therefore had exceptionally high esthetic requirements. B, The gingival defect was corrected by minor periodontal recontouring, the teeth were reprepared, and new all-ceramic crowns were provided.
The tooth should be relatively intact with sufficient coronal structure to support the restoration, particularly in the incisal area, where it is important not to exceed a maximum porcelain thickness of 2 mm; otherwise, failure of the brittle material will occur.
Because of the relative weakness of the restoration, the occlusal load should be favorably distributed (Fig. 11-4). In general, this means that centric contact must be in an area where the porcelain is supported by tooth structure (e.g., in the middle third of the lingual wall).
Fig. 11-4 The design of the occlusion on an all-ceramic crown is crucial to avoid fracture. Centric contacts are best confined to the middle third of the lingual surface. Anterior guidance should be smooth and consistent with contact on the adjacent teeth. Leaving the restoration out of contact is not recommended. Future eruption may lead to protrusive interferences, precipitating fracture.
The ceramic crown is contraindicated when a more conservative restoration can be used. Rarely is it recommended for molar teeth. Because of the increased occlusal load and the reduced esthetic demand, metal-ceramic restorations are the treatment of choice. If occlusal loading is unfavorable (Fig. 11-5) or if it is not possible to provide adequate support or an even shoulder width of at least1 mm circumferentially, a metal-ceramic restoration should be considered instead.
The instruments needed for preparing an all-ceramic crown (Fig. 11-6) include the following:
Fig. 11-7 All-ceramic crown preparation. A, Labial view. B, Lingual view. To prevent stress concentrations in the ceramic, all internal line angles should be rounded. The shoulder should be as smooth as possible to facilitate the technical aspects of fabrication.
The completed reduction of the incisal edge should provide 1.5 to 2 mm of clearance for porcelain in all excursive movements of the mandible. This enables fabrication of a cosmetically pleasing restoration with adequate strength. If the restoration is used for posterior teeth (rare), 1.5 to 2 mm of clearance is needed on all cusps.