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

5. Use the football-shaped diamond for lingual reduction after placing depth grooves approximately 0.8 mm deep. The lingual reduction is done in the same way as the other anterior tooth preparations (see Chapters 9 and 10) until a clearance of 1 mm in all mandibular excursive movements has been obtained. Adequate space must exist for the porcelain in all load-bearing areas.

6. After the selected path of placement has been transferred from the cervical wall of the facial preparation, place a depth groove in the middle of the cingulum wall.

7. Repeat the shoulder preparation, this time from the center of the cingulum wall into the proximal aspect, until the lingual shoulder meets the facial shoulder. This margin should follow the free gingival crest and should not extend too far subgingivally.

Chamfer preparation

For subgingival margins, displace the tissue with cord before proceeding with the chamfer preparation. The ultimate objective is to direct stresses optimally in the completed porcelain restoration. This is accomplished when the chamfer or rounded shoulder margin completely supports the crown; any forces exerted on the crown are then in a direction parallel to its path of placement. A sloping shoulder results in unfavorable loading of the porcelain, with a greater likelihood of tensile failure. A 90-degree cavosurface angle is optimal. Care must be taken, however, that no residual unsupported enamel is overlooked, because it easily chips off.

The completed chamfer should be 1 mm wide, smooth, continuous, and free of any irregularities.

Finishing

8. Finish the prepared surfaces to a final smoothness as described for the other tooth preparations. Be sure to round any remaining sharp line angles to prevent a wedging action, which can cause fracture.

9. Perform any additional margin refinement as needed, using either the diamond or a carbide rotary instrument of choice.

Indications

A ceramic inlay can be used instead of amalgam or a gold inlay for patients with a low caries rate who require a Class II restoration and wish to restore the tooth to its original appearance. It is the most conservative ceramic restoration and enables most of the remaining enamel to be preserved.

Contraindications

Because these restorations are time consuming and expensive, they are contraindicated in patients with poor oral hygiene or active caries. Because of their brittle nature, ceramic restorations may be contraindicated in patients with excessive occlusal loading, such as those with bruxism.

Advantages

Ceramic inlays and onlays can be extremely esthetic restorations. The restoration wear associated with posterior composite restorations is not a problem with the ceramic restorations. Marginal leakage associated with polymerization shrinkage and high thermal coefficient of expansion of the resin is reduced, because the luting layer is very thin.

Disadvantages

Accurate occlusion can be difficult to achieve with ceramic inlays and onlays. Because they are fragile, intraoral occlusal adjustment is impractical before they are bonded in place. Therefore, any areas of adjustment need careful finishing and polishing, which is a time-consuming procedure. Rough porcelain is extremely abrasive of the opposing enamel. Castable glass-ceramic restorations (see Chapter 25) are less abrasive than the traditional feldspathic porcelain. Wear of the composite resin-luting agent can be a problem, leading to marginal gaps. These eventually allow chipping or recurrent caries. Accuracy is important with these restorations, because accurately fitting restorations (marginal gaps less than 100 μm) have been shown to reduce this problem significantly. Finishing of the margins can be difficult in the less accessible interproximal areas. Resin flash or overhangs are difficult to detect and can initiate periodontal disease.

Bonded ceramic inlays are a relatively new concept, and long-term clinical performance is hard to judge. The patient should always be made aware that unforeseen problems may surface over time when a newer procedure is used.

Preparation

Armamentarium

As for metal inlays, carbide burs are used in the preparation (Figs. 11-9 and 11-10), but diamonds may be substituted:

• Tapered carbide burs

• Round carbide burs

• Cylindrical carbide burs

• Finishing stones

• Mirror

• Explorer and periodontal probe

• Chisels

• Gingival margin trimmers

• Excavators

• High- and low-speed handpieces

• Articulating film

Fig. 11-9 Maxillary first molar preparation for a mesio-occlusal-distal (MOD) ceramic inlay. A, Defective restoration. B, The restoration and caries removed. C, Unsupported enamel removed and glass ionomer base placed. D, The completed ceramic restoration.

(Courtesy of Dr. R. Seghi.)

Fig. 11-10 Armamentarium for the ceramic inlay preparation.

Advantages and Indications

The main advantage of laminate veneers is that they are conservative of tooth structure. Typically, only about 0.5 mm of facial reduction is needed. Because this is confined to the enamel layer, local anesthesia is not usually required. The main disadvantage of the procedure relates to difficulty in obtaining restorations that are not excessively contoured. This is almost inevitable in the gingival area if enamel is left for bonding. Little has been reported about the effect of the restorations on long-term gingival health and whether or how often they need replacement over a patient’s lifetime.

Esthetic veneers should always be considered as a conservative alternative to cemented crowns. In many practices, they have largely replaced metal-ceramic crowns for the treatment of multiple discolored but otherwise sound teeth.

Preparation

Step-by-step procedure

The gingival third and proximal line angles are often overcontoured with these restorations (Fig. 11-12). Therefore, maximum reduction should be achieved with minimum penetration into the dentin.

Fig. 11-12 Porcelain facial veneer preparation. A, The proximal contact areas and incisal edge are preserved, and the preparation is limited to enamel. Normally, a reduction depth of about 0.5 mm is recommended, but making a series of depth holes with a round bur guards against penetrating thin enamel. B, Tetracycline-stained teeth. Composite resin veneers were placed earlier but failed to mask the discoloration satisfactorily. Six maxillary porcelain labial veneers will be provided. C and D, Completed tooth preparations. E, Interim restorations made directly with composite resin, which are retained by etching small areas of enamel (see Chapter 15).

1. Make a series of depth holes with a round bur to avoid penetrating abnormally thin enamel. The amount of reduction required depends somewhat on the extent of discoloration. A minimum of 0.5 mm is usually adequate. The reduction should follow the anatomic contours of the tooth.

2. Place the “long chamfer” margin (Fig. 11-13). This design has an obtuse cavosurface angle, which exposes the enamel prism ends at the margin for better etching. The margin should closely follow the gingival crest so that all discolored enamel prisms are veneered without undue encroachment on the gingival sulcus.

3. Wherever possible, place the preparation margin labial to the proximal contact area to preserve it in enamel. However, slight clearance for separating the definitive cast and for accessing the proximal margins for finishing and polishing is essential. A diamond finishing strip helps create the necessary clearance. Sometimes the proximal margins are extended lingually to include existing restorations. This can necessitate considerabletooth reduction to avoid creating an undercut. Some authorities advocate placing the ceramic margin on composite material rather than extending the preparation to enamel, but this is not recommended. Extensive existing restorations are a contraindication for porcelain laminate veneers.

4. If possible, do not reduce the incisal edge (Fig. 11-14); this helps support the porcelain and makes chipping less likely. If the incisal edge length is to be increased, the preparation should extend to the lingual aspect. Care is needed to avoid undercuts with this modification. Visualizing the path of placement of the restoration is important, because an undercut prevents placement of the veneer.

5. To prevent areas of stress concentration in the porcelain, be sure that all prepared surfaces are rounded (see Fig. 11-12C and D).

Fig. 11-13 The recommended margin (“long chamfer”) for facial veneers has an obtuse cavosurface angle, and so the ends of the enamel prisms are exposed for differential etching.

Fig. 11-14 The preferred design for porcelain laminate veneers maintains part of the incisal edge in enamel. If the edge is to be lengthened, a modified preparation with lingual extension is needed (dotted line).

ALL-CERAMIC CROWN PREPARATION

CERAMIC INLAY AND ONLAY PREPARATION

PORCELAIN LAMINATE VENEERS