Cusp fracture of endodontically treated posterior teeth restored with amalgam. Teeth restored in Denmark before 1975 versus after 1979.

The aim of the present study was to analyze the frequency and the severity of cuspal fracture for posterior teeth endodontically treated by 91 Danish dentists and restored with amalgam either before 1975 or after 1979. A database from a previous study was analyzed (1584 teeth with an MO, a DO, or an MOD amalgam restoration without cuspal overlays). Teeth restored before 1975 had a significantly lower frequency of cusp fracture than teeth restored after 1979. Moreover, when comparing the frequency of subcrestal fractures, that of the second group (after 1979) was more than twice as high as that of the first group (before 1975). Two factors with a bearing on these findings changed in the time period 1975-1979; the high-copper amalgams took over in Denmark, and the use of Gates-Glidden burs to achieve straight-line access to the root canal was introduced. It is suggested that the weakening of the coronal part of the root, caused by the use of Gates-Glidden burs, and the expansion and low creep of high-copper amalgams may be two of the reasons for the increased frequency and severity of cuspal fracture found in the past decade in Denmark.