Background. Investigations of cuspal-coverage amalgam restorations suggest that tooth fracture is the leading cause of failure, while for Class I and II restorations, the leading cause is caries. In this study, the authors evaluated the causes of failure for a large number of cuspal-coverage restorations. Methods. The causes of failure for 706 cuspal-coverage amalgam restorations were determined through the use of a questionnaire. Dentists from a variety of dental schools; Army, Navy, Ar Force, Public Health and Veterans Affairs dental clinics; and private practice were asked to record pertinent information regarding patients and restoration failures from choices provided on a survey form. Results. The survey documented 706 failed restorations. Mandibular first molars accounted for 36.25 percent of all failures. The majority of failures were caused by fractured teeth (24.3 percent), caries (20 percent) and fractured restorations (17.1 percent). Among all of the failed restorations, 82.15 percent were restorable, 9.35 percent were repairable and 8.50 percent were nonrestorable. Among the fractured teeth, 80 percent were restorable, 14.5 percent were nonrestorable and 5.5 percent were repairable. Among the carious teeth, 84 percent were restorable, 8 percent were nonrestorable and 8 percent were repairable. A chi(2) analysis revealed that tooth fracture was more likely to be associated with nonrestorability than either caries (chi(2) = 5.013, P <.05) or restoration fracture (chi(2) = 6.202, P <.05). Conclusions. The leading cause of failure among the 706 restorations was tooth fracture, which resulted in significantly greater numbers of nonrestorable teeth than either caries or fractured restorations. Clinical Implications. Tooth fracture creates a greater risk of nonrestorability than any other cause of failure. Replacement or coverage of fracture-prone cusps is likely to improve the life expectancy of complex amalgam restorations.
