Prosthodontics: The Metal-Ceramic Crown Preparation PART 1

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. 

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. 

1. Recommended minimum dimensions for a metal-ceramic restoration on an anterior tooth (A) and a posterior tooth (B). Note that 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 will be discussed in later posts. 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.

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 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.

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. If at all possible, a more conservative restorative option such as a composite resin or porcelain laminate veneer or an all-ceramic crown with less reduction 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.

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;

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.

 however, the same step-by-step approach can be applied to other teeth. 

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.

As with all tooth preparations, a systematic and organized approach to tooth reduction saves time.

Armamentarium

The instruments needed to prepare teeth for a metal-ceramic crown 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 (B to D)

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, one in the center of the facial surface and one each in the approximate locations of the mesiofacial and distofacial line angles ( Fig. 2 A to E).

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.

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 (Fig. 2 D 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 (Fig. 2 F 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 the 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. 

6. A: depth grooves 1.8 mm deep placed in the incisal edges to ensure adequate and even reducton. B:  incisal reduction completed on the left central and lateral incisors. Note that angulation of the diamond, perpendicular to the direction of loading by the mandibular anterior teeth.

The grooves must not be too deep; otherwise, an overreduced and undulating surface results. 

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 the anterior teeth (on which, as a consequence of tooth form, most of the retention is derived from 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 (Fig. 2 H). On the posterior teeth, the same protocol is followed as in preparing depth grooves for a complete cast functional. This includes the use of a functional cusp bevel, although additional occlusal reduction is needed where the porcelain is to be applied (Fig. 3 A to C).

Labial (buccal) reduction

When completed, the reduction of the facial surface should have produced sufficient space to accommmodate the metal substructure and porcelain veneer. A minimum of 1.2 mm is necessary for the ceramist to produce a restoration with satisfactory appearance (1.5 mm is preferable). This requires significant 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. 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 (Fig. 2 I to L), creating a shoulder at the cervical margin. 

7. 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.

8. Fig. left: 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. Fig. right: Facial reduction completed. A 6-degree taper has been established between the proximal walls.

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. 

9. A: the facial shoulder preparation should wrap around into the interproximal embrasure and extend at least 1 mm lingual to the promixal 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. 

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. 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.

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 and to undiscolored anterior teeth (in which case a porcelain labial margin is preferred).

10. 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. 

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 cord before finishing. 

11. Gingival displacement cord (under tension) is placed in the interproximal sulcus.

The configuration of the margin is also finalized at this time.

12. 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.

References:

• All information is taken from "Contemporary fixed prosthodontics" Fourth Edition by Stephen F. Rosenstiel BDS MSD, Martin F. Land DDS MSD, Junhei Fujimoto DDS MSD DDSc
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