Ultrasonic debonding of composite-retained restorations

Ultrasonic chisels are used clinically to remove composite-retained bridges from one or both abutment teeth. Displacement amplitude measurements at the tip of the two designs available showed that the curved chisel was twice as powerful as the straight chisel. Investigations were undertaken to assess the efficiency of these chisels in vitro. Cast metal retainers were made, which incorporated particulate roughening of the fitting surface and these were bonded with a composite luting cement to extracted premolar teeth. Ten clinicians were asked to remove the retainers from the teeth and their application forces were measured. Clinicians were able to apply the highest force with the straight chisel design. The time taken to remove the retained restorations under increasing application forces and different power settings of the two chisels was then measured. Optimal time for removal of the retainers (ie with 5-10 minutes' exposure) required the use of high displacement amplitudes of the chisel, together with high application forces. Scanning electron microscopy of the failed bond revealed that composite was removed in those areas where the chisel was applied to the tooth. However, there was a disproportionate amount of tooth surface damage compared to that inflicted upon the metal retainer.