Metalceramic Restorations

The poor appearance of some metal-ceramic restorations is often due to insufficient porcelain thickness. On the other hand, adequate porcelain thickness is sometimes obtained at the expense of proper axial contour (such overcontoured restorations almost invariably lead to periodontal disease). In addition, the labial margin of a metal-ceramic crown is not always accurately placed. To correct all these deficiencies, certain principles are recommended during tooth preparation that will ensure sufficient room for porcelain and accurate placement of the margins. Otherwise, good appearance would be achievable only at the expense of peri-odontal health.

Facial Tooth Reduction. If there is to be sufficient bulk of porcelain for appearance and metal for strength, adequate reduction of the facial surface is essential. The exact amount of reduction will depend to some extent on the physical properties of the alloy used for the substructure as well as on the manufacturer and the shade of the porcelain. A minimum reduction of 1.5 mm typically is required for optimal appearance. Adequate thickness of porcelain (Fig. 7-46) is needed to create a sense of color depth and translucency. Shade problems are frequently encountered in maxillary incisor crowns at the incisal and cervical thirds of the restoration,

Collarless Metal Ceramic Restorations

Fig. 7-46. Adequate porcelain thickness is essential for preventing direct light reflection from the highly pigmented opaque porcelain. The most critical areas are the gingival and incisal thirds; in practice, opaque modifying stains are often used in these areas.

(Redrawn from McLean JW: The science and art of dental ceramics, vol 1, Chicago, 1979, Quintessence Publishing.)

Fig. 7-46. Adequate porcelain thickness is essential for preventing direct light reflection from the highly pigmented opaque porcelain. The most critical areas are the gingival and incisal thirds; in practice, opaque modifying stains are often used in these areas.

(Redrawn from McLean JW: The science and art of dental ceramics, vol 1, Chicago, 1979, Quintessence Publishing.)

where direct light reflection from the opaque layer can make the restoration appear very noticeable. Because opaque porcelains generally have a different shade from body porcelains, they often need to be modified with special stains in these areas."

With very thin teeth (e.g., mandibular incisors) it may be impossible to achieve adequate tooth reduction without exposing the pulp or leaving a severely weakened tooth preparation. Under these circumstances a less than ideal appearance may have to be accepted.

The labial surfaces of anterior teeth should be prepared for metal-ceramic restorations in two distinct planes (Fig. 7-47). If they are prepared in a single plane, insufficient reduction in either the cervical or the incisal area of the preparation will result.

Incisal Reduction. The incisal edge of a metal-ceramic restoration has no metal backing and can be made with a translucency similar to that of natural tooth structure. An incisal reduction of 2 mm is recommended for good esthetics. Excessive incisal reduction must be avoided because it reduces the resistance and retention form of the preparation.

Proximal Reduction. The extent of proximal reduction is contingent on exact predetermination of the location of the metal-ceramic junction in the completed restoration. The proximal surfaces of anterior teeth will look most natural if they are restored as the incisal edges, without metal backing. This will allow some light to pass through the restoration in a manner similar to what occurs on a natural tooth (Fig. 7-48). Obviously, if the restoration is part of a fixed partial denture, the need for connectors will make this impossible.

Labial Margin Placement. Supragingival margin placement has many biologic advantages. The restorations are easier to prepare properly and eas

Fig. 7-47. Recommended tooth preparation for a metal-ceramic restoration. The facial reduction has two distinct planes.

ier to keep clean. Nevertheless, subgingival margins may be indicated for esthetic reasons, particularly when the patient has a high lip line and the use of a metal collar labial margin is contemplated.

The patient's smile is observed as part of the initial examination (see Chapter 1). It is important to record which teeth and which parts of each tooth are exposed. Patients with a high lip line, which exposes considerable gingival tissue, present the greatest problem if complete crowns are needed. Where the root surface is not discolored, appearance can be restored with a metal-ceramic restoration having a supragingival porcelain labial margin-sometimes called a "collarless" design (see Chapter 24). If the patient has a low lip line, a metal supragingival collar may be placed because the metal is not seen during normal function. Metal margins generally have a more accurate fit than porcelain margins.

However, it cannot be assumed that the patient will be happy with a supragingival metal collar just because the metal is not visible during normal function. Some patients have reservations about exposed metal, and the advantages of such supragin-gival margins must be carefully explained before treatment.

Metal collars can be hidden below the gingival crest, although there will be some discoloration if the gingival tissue is thin. Successful margin placement within the gingival sulcus requires care to ensure that inflammation and/or recession, with resulting metal exposure, are avoided or minimized. The periodontium must be healthy before the tooth is prepared. If periodontal surgery is needed, the sulcular space should not be eliminated completely; rather, a postsurgical depth of about 2 mm should be the objective. Sufficient time should be allowed after surgery for the periodontal tissues to stabilize. Wise found that the gingival crest does not stabilize until 20 weeks after surgery.

Margins should not be placed so far apically that they encroach on the attachment; extension to within 1.5 mm of the alveolar crest will lead to bone resorption. The margin should follow the contour of the free gingiva, being further apical in the middle of the tooth and further incisal interproximally. A common error (Fig. 7-49) is to prepare the tooth so the margin lies almost in one plane, with exposure of the collar labially and irreversible loss of bone and papilla proximally.

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