Mechanical Requirements

Function. The greatest stresses in a provisional restoration are likely to occur during chewing. Unless the patient avoids contacting the prosthesis when eating, internal stresses will be similar to those occurring in the definitive restoration. The strength of poly (methyl methacrylate) resin is about one-twentieth that of metal-ceramic alloys,5 making fracture of the provisional restoration much more likely. Fracture is not usually a problem with a complete crown as long as the tooth has been adequately reduced. Breakage occurs more frequently with partial-coverage restorations and fixed partial dentures. Partial-coverage restorations are inherently weaker because they do not completely encircle the tooth.

An FPD must function as a beam in which substantial occlusal forces are transmitted to the abutments. This creates high stresses in the connectors,' which are often the site of failure. To reduce the risk of failure, connector size must be increased in the provisional compared to the definitive restoration (Fig. 15-7). Greater strength is achieved by reducing

Fig. 15-8. In this mesiodistal section, an overcontoured connector crowds the gingiva. Pressure ischemia and poor access for plaque removal promote gingivitis.

Indications for High-Strength Provisional Restorations box i 5-1

A tong-spon posterior FPD

Prolonged treatment time

Patient unable to avoid excessive forces prosthesis

Above-average masticatory muscle strength History of frequent breakage the the depth and sharpness of the embrasures. This increases the cross-sectional area of the connector while reducing the stress concentration associated with sharp internal line angles. The biologic and sometimes the esthetic requirements place limits on just how much larger connectors can be made. To avoid jeopardizing periodontal health, they should not be overcontoured near the gingiva (Fig. 15-8). Good access for plaque control must have high priority.

In some instances high-strength provisionals (e.g., cast metal, fiber reinforced or heat-processed resin) can spare the practitioner and the patient inconvenience, lost time, and the expense of remaking a restoration (Box 15-1).

Displacement. To avoid irritation to the pulp and tooth movement, a displaced provisional must be recemented promptly. An additional office visit is usually required, resulting in considerable inconvenience to the patient and the dentist. Displacement is best prevented through proper tooth preparation and a provisional with a closely adapted internal surface. Excessive space between the restoration and the tooth places greater demands on the luting agent, which has lower strength than regular cement and thus cannot tolerate the added force. For this and for biologic reasons, unlined preformed crowns should be avoided.

Removal for Reuse. Provisional restorations often need to be reused and therefore should not be damaged when removed from the teeth. In most in stances, if the cement is sufficiently weak and the provisional has been well fabricated, it will not break when removed.

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