Advantages

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.

Disadvantages

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).

Fig. 11-2 A sloping shoulder is not recommended for the all-ceramic crown. It does not support the porcelain. Incisal loading leads to tensile stresses near the margin.

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).

Indications

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.

Contraindications

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.

Fig. 11-5 Unfavorable occlusal loading such as this edge-to-edge relationship on the lateral incisor is a contraindication to the all-ceramic crown, particularly in view of the parafunctional activity of this patient.

Preparation

Armamentarium

The instruments needed for preparing an all-ceramic crown (Fig. 11-6) include the following:

• Narrow, round-tipped, tapered diamonds, regular and coarse grit (0.8 mm)

• Square-tipped, tapered diamond, regular grit (1.0 mm)

• Football-shaped diamond

• Finishing stones and carbides

• Mirror

• Periodontal probe

• Explorer

• Chisels and hatchets

• High- and low-speed handpieces

Fig. 11-6 Armamentarium for an all-ceramic crown preparation.

Step-by-step procedure

The preparation sequence for a ceramic crown (Fig. 11-7) is similar to that for a metal-ceramic crown; the principal difference is the need for a 1-mm-wide chamfer circumferentially (Fig. 11-8).

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.

Fig. 11-8 Note the uniform chamfer width of 1 mm on this all-ceramic crown preparation.

Incisal (occlusal) reduction

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.

1. Place three depth grooves in the incisal edge, initially keeping them approximately 1.3 mm deep to allow for additional loss of tooth structure during finishing. The grooves are oriented perpendicular to the long axis of the opposing tooth to provide adequate support for the porcelain crown.

2. Complete the incisal reduction, reducing half the surface at a time, and verify its adequacy upon completion.

Facial reduction

3. After placing depth grooves, reduce the facial or buccal surface and verify that adequate clearance exists for 1 mm of porcelain thickness. One depth groove is placed in the middle of the facial wall, and one each in the mesiofacial and distofacial transitional line angles. The reduction is then performed with a cervical component parallel to the proposed path of placement and an incisal component parallel to the original contour of the tooth. The depth of these grooves should be approximately 0.8 mm to allow finishing. The reduction is performed on half of the facial surface at a time.

4. Accomplish the bulk reduction with the round-tipped tapered diamond (which results in a heavy chamfer margin). Be sure to maintain copious irrigation throughout.

Lingual reduction

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