INDICATIONS

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

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.

ADVANTAGES

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.

DISADVANTAGES

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.

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 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.

Armamentarium

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

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

• Football- or wheel-shaped diamond (for lingual reduction of anterior teeth)

• Flat-ended, tapered diamond (for shoulder preparation)

• Finishing stones

• Explorer and periodontal probe

• Off-angle hatchets (see Fig. 9-4B to D)

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 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, 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.

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 mesiofacial and distofacial line angles (see Fig. 9-2A to E). These are in two planes: the cervical portion to parallel the long axis of the tooth and the incisal (occlusal) portion to follow the normal facial contour (see Fig. 9-2D and E).

2. Perform the facial reduction in the cervical and incisal planes. The cervical plane determines the path of placement of the completed restoration. The incisal or occlusal plane provides the space needed for the porcelain veneer; it should be approximately 1.3 mm deep to allow for additional reduction during finishing. The secondary facial 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 placement; in particular, some labial inclination improves retention on a tooth with little cingulum 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. These provide the needed reduction of 2 mm and allow finishing (see Fig. 9-2F and G). Verify the depth of these grooves with a periodontal probe. On posterior teeth on which 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 the diamond is initially positioned for anterior teeth, it may be helpful to observe the long axis of the opposing tooth in maximum intercuspation and to orient the instrument perpendicular to that (Fig. 9-6). The grooves must not be too deep; otherwise, an overreduced and undulating surface results.

Fig. 9-5 Depth grooves in the facial wall are placed in two directions: incisally, parallel to the tooth contour, and cervically, parallel to the path of placment. The grooves should be 1.3 mm deep.

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.

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 reduction (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 completed preparation. This can be particularly problematic on anterior teeth (on which, 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 cutting instrument can be used to maximize cutting efficiency (see Fig. 9-2H). On posterior teeth, the same protocol is followed as in preparing depth grooves for a complete cast functional (see Chapter 8). This includes the use of a functional cusp bevel, although additional occlusal reduction is needed where the porcelain is to be applied (see Fig. 9-3A 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 for the ceramist to produce a restoration with satisfactory appear-ance (1.5 mm is preferable). This requires signifi-cant 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.) A compromise is often 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. 9-2I 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 then results in a margin that is 0.75 to 1 mm subgingival. Use adequate water spray during the entire phase of preparation, because a significant amount of tooth structure is being removed and copious irrigation (along with intermittent strokes) expedites the preparation process. Such a cautious approach prevents unnecessary trauma to the pulp. The resulting shoulder should be approximately 1 mm wide and should extend well into the proximal embrasures when viewed from the incisal (occlusal) side (Fig. 9-8). Where access allows, establishing this shoulder from the proximal gingival crest toward the middle of the facial wall is preferred. This minimizes placement of the initial shoulder preparation too close to the epithelial attachment. If the margin is established from facial to proximal, a tendency exists to “bury” the instrument and encroach on the epithelial attachment. Proper margin position must be maintained in relation to the crest of the free gingiva (see Fig. 7-51). The location and specific configuration of the facial margin depend on several factors: the type of metal-ceramic restoration selected, the cosmetic expectations of the patient, and operator preference.

Fig. 9-7 A, The cervical shoulder is established as the tooth structure between the depth grooves is removed. The rotary instrument is moved parallel to the intended path of placement during this procedure. B, The facial reduction should be completed in two phases; initially, one half is maintained intact for assessment of the adequacy of reduction. Note the two distinct planes of reduction on the facial. The proximal aspect parallels the cervical reduction on the facial wall. C, Facial reduction completed. A 6-degree taper has been established between the proximal walls.

Fig. 9-8 A, The facial shoulder preparation should wrap around into the interproximal embrasure and extend at least 1 mm lingual to the proximal contact. B, The shoulder preparation extends adequately to the lingual side of the proximal contact. Note that on the mesial (visible) side, the preparation extends slightly farther than on the distal (cosmetically less critical) side.

From a periodontal point of view, a supragingival margin is always preferred. Its application is restricted, however, because patients often object to a visible metal collar or discolored root surface. Such objections are common, even when the gingival margin is not visible during normal function, as in patients with a low lip line, and generally limit the use of supragingival margins to posterior teeth (Fig. 9-9) and to undiscolored anterior teeth (in which case a porcelain labial margin is preferred; see Chapter 24). The optimum location of the margin should be carefully determined with the full cooperation of the patient. Where a subgingival margin is to be placed, careful tissue manipulation is essential; otherwise, there will be damage that leads to permanent gingival recession and subsequent exposure of the metal collar. This is most effectively avoided through meticulous gingival displacement with a cord before finishing (Fig. 9-10). The configuration of the margin is also finalized at this time (Fig. 9-11).

Fig. 9-9 Supragingival margins on the maxillary premolars. They were possible because of a favorable lip line hiding the cervical aspect of these posterior teeth. The subgingival margins on the mandibular premolars were prepared only because of previously existing restorations.

Fig. 9-10 A, Gingival displacement cord (under tension) is placed in the interproximal sulcus. B, A second instrument can be used to prevent the cord from rebounding from the sulcus after it has been packed.

Fig. 9-11 A, After tissue displacement, the facial margin is extended apically. Caution is needed, because if the diamond inadvertently grabs the cord, it may be ripped out of the sulcus and traumatize the epithelial attachment. B, Note the additional apical extension of the shoulder on the distal aspect. C, The entire facial shoulder is placed at a level that will be subgingival after the tissue rebounds. D, The facial margin has been prepared to the level of the previously placed cord.

Axial reduction of the proximal and lingual surfaces

Sufficient tooth structure must be removed to provide a distinct, smooth chamfer of about 0.5 mm width (see Fig. 9-2M to P).

6. Reduce the proximoaxial and linguoaxial surfaces with the diamond held parallel to the intended path of placment of the restoration. These walls should converge slightly from cervical to incisal or occlusal. A taper of approximately 6 degrees, as measured as the angle between opposing axial walls, is recommended. On anterior teeth, a lingual concavity is prepared for adequate clearance for the restorative materials. Typically, 1 mm is required if the centric contacts in the completed restoration are to be located on metal. When contact is on porcelain, additional reduction is necessary. For anterior teeth, usually only one groove is placed, in the center of the lingual surface. For molars, three grooves can be placed in a manner similar to that described for the all-metal complete cast crown (see Chapter 8).

7. Make a lingual alignment groove by positioning the diamond parallel to the cervical plane of the facial reduction. When the round-tipped diamond of appropriate size and shape is aligned properly, it is submerged almost halfway into the tooth structure. Verify the alignment of the groove, and carry the axial reduction from the groove along the lingual surface into the proximal; maintain the originally selected alignment of the diamond at all times.

8. As the lingual chamfer is developed, extend it buccally into the proximal to blend with the interproximal shoulder placed earlier (Fig. 9-12). Alternatively, a facial approach may be used. Although this is slightly more difficult initially, after some practice it should be easy to eliminate the lingual guiding groove and to perform the proximal and lingual axial reduction in one step; however, this requires that the diamond be held freehand parallel to the path of placement. The proximal flange that resulted from the shoulder preparation can be used as a reference for judging alignment of the rotary instrument (Fig. 9-13). The interproximal margin should not be inadvertently placed too far gingivally and thereby infringe on the attachment apparatus. It must follow the soft tissue contour (see p. 190). On posterior teeth, the lingual wall reduction blends into the functional cusp bevel placed during the occlusal reduction. Anterior teeth require an additional step: After preparation of the cingulum wall, one or more depth grooves are placed in the lingual surface. These are approximately 1 mm deep.

9. Use a football-shaped diamond to reduce the lingual surface of anterior teeth (see Fig. 9-2P). It is helpful to stop when half this reduction has been completed, to evaluate clearance in maximum intercuspation and all excursions. The remaining intact tooth structure can serve as a reference.

Fig. 9-12 A lingual chamfer is prepared to allow adequate space for metal. A smooth transition from interproximal shoulder to chamfer is essential.

Fig. 9-13 A, Proximal reduction of the flange with a facial approach. B, Once sufficient tooth structure has been removed, the cervical chamfer is prepared simultaneously with the lingual axial surface. After the distolingual preparation has been completed, the mesial chamfer is blended into a smooth transition with the shoulder.

Finishing

The margin must provide distinct resistance to vertical displacement of an explorer tip, and it must be smooth and continuous circumferentially. (A properly finished margin should feel like smooth glass slab.) All other line angles should be rounded, and the completed preparation should have a satin finish free from obvious diamond scratch marks. Tissue displacement is particularly helpful when subgingival margins are being finished (Fig. 9-14). Sometimes this step is postponed until just before impression making after tissue displacement.

Fig. 9-14 Controlled tissue displacement can be helpful when finishing the margin with a fine-grit diamond or another rotary instrument.

10. Finish the margins with diamonds, hand instruments, such as the off-angle hatchets (see Fig. 9-4B) or carbides (see Fig. 9-2Q and R). All internal line angles should be rounded to facilitate the impression-making and die-pouring steps (see Fig. 9-2S). The finishing steps for the facial margin depend on the design of margin chosen (Fig. 9-15; see also Table 7-2). A porcelain labial margin requires proper support for the porcelain. A shoulder with a 90-degree cavosurface angle is recommended. This type of shoulder can also be used for a crown with a conventional metal collar and offers the advantage of allowing the collar to be kept narrow. However, there is then the risk of leaving unsupported enamel. For this reason, the margin is often beveled or sloped to create a more obtuse cavosurface angle (Fig. 9-16). A flat-ended diamond in a low-speed handpiece creates the 90-degree shoulder. Any unsupported enamel must be removed subsequently by careful planing with a sharp chisel. Care must also be taken to orient the rotary instrument as it moves around the tooth if inadvertent undercuts are to be avoided. When a metal-collar design of ceramic restoration is planned, the need for a 90-degree shoulder is less crucial. A sloping shoulder has been advocated to ensure the elimination of unsupported enamel and to minimize marginal gap width (see Chapter 7). Such a shoulder (cavosurface angle of about 120 degrees) can be accomplished with a flat-ended diamond by changing its alignment, with particular attention to the configuration of the tooth structure cervical to the margin. Alternatively, a hatchet can be used to plane the margin to the correct angulation. Again, be careful to avoid undercutting the axial wall of the preparation where it meets the shoulder during finishing. A shoulder-bevel margin is most effectively achieved with a flame-shaped carbide bur or hand instrument, depending on the length of bevel required (Fig. 9-17). In general, a short bevel with a cavosurface angle of 135 degrees is advocated, although longer bevels have been recommended for improved marginal fit. Special care must be exerted where the bevel meets the interproximal chamfer. The chamfer and bevel should be continuous with each other. Care must be taken not to damage the epithelial attachment during beveling; tissue displacement before preparation of subgingival bevels is recommended.

11. After a satisfactory facial margin has been obtained, round all sharp line angles within the preparation (see Fig. 9-2S). This facilitates surface wetting and expedites subsequent procedures (impression making, pouring of casts, waxing, and investing). A fine-grit diamond operating at low speed is particularly useful. However, where access allows, a slightly larger tapered diamond may be preferred because the greater diameter of its tip prevents “ditching” of the chamfer. Blend all surfaces together, and remove any sharp transitions (Figs. 9-18 and 9-19; see also Fig. 9-2T).

Fig. 9-15 A, Completed preparation. Note that the transition from incisal to axial walls is rounded, and a distinct 90-degree or slightly sloping shoulder has been established. B, An even chamfer width and a smooth transition between lingual and axial surfaces. The chamfer is distinct and blends smoothly into the facial shoulder.

Fig. 9-16 A, 90-degree shoulder. B, 120-degree shoulder. C, Shoulder bevel.

Fig. 9-17 The shoulder bevel.

Fig. 9-18 Facial (A) and lingual (B) views of metal-ceramic preparations.

Fig. 9-19 The “wingless” variation does not exhibit the defined transition from chamfer to shoulder seen in Fig. 9-15. Rather, the shoulder gradually narrows toward the lingual side. Interproximally, the same criteria for minimum extension of the shoulder apply as for the wing-type or flange preparation.

Evaluation

Areas often missed during finishing are the incisal edges of anterior preparations and the transition from occlusal to axial wall of posterior preparations. The completed chamfer should provide 0.5 mm of space for the restoration at the margin. The chamfer must be smooth and continuous, and when it is evaluated, a distinct resistance to vertical displacement of the tip of an explorer or periodontal probe should be felt. The chamfer should be continuous with the interproximal shoulder or beveled shoulder. The cavosurface angle of the chamfer should be slightly obtuse or 90 degrees. Under no circumstances should any unsupported tooth structure remain, especially at the facial margin. Care is also needed to avoid creating an undercut between the facial and lingual walls. This aspect of the preparation should be thoroughly evaluated. Excessive convergence should also be avoided, because this may lead to pulpal exposure. All residual debris is removed with thorough irrigation. (Various examples of metal-ceramic preparations are shown in Figs. 9-20 and 9-21.)

Fig. 9-20 A to F, Metal-ceramic crowns used to restore maxillary incisor teeth.

Fig. 9-21 A, Metal-ceramic preparations on the maxillary premolars in conjunction with more conservative preparations on the molars. B, Buccal view of the preparations. Note that, by comparison, considerable tooth reduction was needed on the premolars to accommodate metal-ceramic restorations. C, Except for the molars, all remaining teeth in this patient have been prepared for metal-ceramic restorations. Note the subtle variations and modifications of the same underlying theme: wing-type preparations on the anterior teeth, wingless on the premolars. D, Mandibular arch of the same patient. Many of the smaller mandibular teeth were prepared with wingless restorations. Because of previously existing restorations, excessively heavy shoulder-like chamfers resulted on some of the posterior teeth.

SUMMARY CHART