Fracture resistance of amalgam coronal-radicular restorations

The effect of pulp chamber depth and extension into the root canal space on fracture resistance was examined on endodontically treated teeth with coronal-radicular amalgam restorations. Six groups of 10 mandibular molars were mounted in acrylic resin, and crowns were ground apically until the wall height of the pulp chamber was 2, 4, or 6 mm. Three millimeters of gutta-percha was removed from the three canals of one-half the teeth and amalgam was condensed into the canal space to a height 7.5 mm above the cementoenamel junction (CEJ). The remaining teeth had amalgam condensed from the floor of the chamber to 7.5 mm above the CEJ. The amalgam restorations were loaded with an Instron instrument (Instron Corp., Canton, Mass.) until failure. Amalgam extension into the root canal space contributed minimally to the fracture resistance of the amalgam coronal-radicular restoration with four or more millimeters of chamber wall. If less than 4 mm of chamber wall height remained, however, the fracture load was substantially increased. Amalgam extension into the root canal space should be confined to teeth with limited remaining pulp chambers.