Effect of prolonged ultrasonic instrumentation on the retention of cemented cast crowns

Occasionally the need arises to remove restorations that have been cemented. Several ultrasonic tips have been developed to aid in removing cemented restorations. Earlier studies of the effects of ultrasonic vibration reveal detrimental effects on amalgam surface and marginal integrity and, at short instrumentation periods (2.5 minutes) in vitro, no significant effect on the retention of cemented cast crowns. Twenty identically machined metal dies had nonprecious crowns waxed, cast, and fitted. Each casting was cemented twice with glass ionomer and zinc phosphate cement. Each cement was tested for the effects of vibration on retention. The cements were mixed according to manufacturer's specifications, placed under 5 kg load, then placed in 37 degrees C water and contained in a humidor. Retention was tested with a 4202 Instron machine to determine the forces needed to overcome retention. Retention ranged from a high of 125.36 kg for glass ionomer without vibration to a low of 19.11 kg for zinc phosphate vibrated for 12 minutes. The use of vibration is considered an advantageous adjunct to other cast restoration removal devices.