Cuspal Fracture

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Protrusive Interference
Fig. 7-24. A, Nonreplacement of missing teeth has led to supraocclusion and a protrusive interference (arrow). B, Teeth reduced with the help of trial tooth preparations and diagnostic waxing. C, Restorations with anterior guidance.

A 120-degree sloped shoulder margin (Fig. 7-17, F) is used as an alternative to the 90-degree shoulder for the facial margin of a metal-ceramic crown. The sloped shoulder reduces the possibility of leaving unsupported enamel and yet leaves sufficient bulk to allow thinning of the metal framework to a knife-edge for acceptable esthetics.

A beveled shoulder margin (Fig. 7-17, G) is often recommended for the facial surface of a metal-ceramic restoration where a metal collar (as opposed to a porcelain labial margin) is used. The beveling removes unsupported enamel and may allow some finishing of the metal. However, a shoulder or sloped shoulder is preferred for biologic and esthetic reasons. This allows improved esthetics because the metal margin can be thinned to a knife edge and hidden in the sulcus without the need for positioning the margin closer to the epithelial attachment (Fig. 7-23, B).

Occlusal Considerations. A satisfactory tooth preparation should allow sufficient space for developing a functional occlusal scheme in the finished restoration. Sometimes a patient's occlusion is disrupted by supraerupted or tilted teeth (Fig. 7-24). When these teeth are prepared for restoration, the eventual occlusal plane must be carefully analyzed and the teeth reduced accordingly. Often considerable reduction is needed to compensate for the supraeruption of abutment teeth.

Sometimes even endodontic treatment is necessary to make enough room. However, under these circumstances, violating the principle of conservation of tooth structure is preferable to the potential harm from a traumatic occlusal scheme. Obviously, careful judgment is needed, and diagnostic tooth preparations and waxing procedures are essential to determining the exact amount of reduction required to develop an optimum occlusion.

Preventing Tooth Fracture. No tooth is unbreakable. If teeth are smashed together (as in an automobile accident, sport injury, or biting on a hard object unexpectedly), a cusp may break. Cus-pal fracture also can occur from parafunctional habits such as bruxism.

The likelihood that a restored tooth will fracture can be lessened if the tooth preparation is designed to minimize potentially destructive stresses (Fig. 7-25). For example, an intracoronal cast restoration (inlay) has a greater potential for fracture because when occlusal forces are applied to the restoration,

Cuspal Coverage

Fig. 7-25. A, An intracoronal cast restoration (inlay) can act as a wedge during cementation or function. If the cusps are weakened, fracture will occur. B, A cuspal-coverage onlay provides better protection but often lacks retention. C, A complete crown provides the best protection against fracture. It also has the best retention, but it can be associated with periodontal disease and poor esthetics. (Redrawn from Rosenstiel SF: In Rayne J, editor: General dental treatment, London, 1983, Kluwer Publishing.)

Fig. 7-25. A, An intracoronal cast restoration (inlay) can act as a wedge during cementation or function. If the cusps are weakened, fracture will occur. B, A cuspal-coverage onlay provides better protection but often lacks retention. C, A complete crown provides the best protection against fracture. It also has the best retention, but it can be associated with periodontal disease and poor esthetics. (Redrawn from Rosenstiel SF: In Rayne J, editor: General dental treatment, London, 1983, Kluwer Publishing.)

it tends to wedge opposing walls of the tooth apart. This wedging must be resisted by the remaining tooth structure; if the structure is thin (as with a wide preparation isthmus), the tooth may fracture during function. Providing a cuspal coverage restoration (onlay) rather than an inlay lessens the chance of such fracture. However, although not conservative of tooth structure, a complete crown is often a better solution, because it offers the greatest protection against tooth fracture, tending to "hold" the cusps of the tooth together.

r MECHANICAL CONSIDERATIONS

The design of tooth preparations for fixed prostho-dontics must adhere to certain mechanical principles; otherwise, the restoration may become dislodged or may distort or fracture during service. These principles have evolved from theoretical and clinical observations and are supported by experimental studies.

Mechanical considerations can be divided into three categories:

1. Providing retention form

2. Providing resistance form

3. Preventing deformation of the restoration

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  • annukka
    What is a cuspal fracture?
    7 months ago
  • Alfie
    What is intracoronal restoration?
    2 months ago

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