The removal of cemented dowels by ultrasonics

It is said that ultrasonic energy is effective in reducing retentive forces of dowels cemented to root canals. However, it is still not clear to what degree the retentive forces actually become reduced. Therefore, we started to measure retentive forces of dowels after applying of ultrasonic vibration. In this study, four kinds of cement, zinc-phosphate cement, carboxylate cement, glassionomer cement, adhesive resin cement, were used.Results were as follows:1. The retentive forces of dowels become 1/3-1/2 values after 3min. Ultrasonic vibration, except when the adhesive resin cement was used. Glassionomer cement showed the highest reduction rate, and zinc-phosphate cement and carboxylate cement followed it. Adhesive resin cement indicated the lowest reduction rate.2. Until the first 1.5min. vibration, the reduction rate of glassionomer cement was actually lower than that of zinc-phosphate cement and carboxylate cement. Glassionomer cement could be considerably resist until the occurrence of the initial crack of cement, but after that the destruction would progress very quickly.3. Dowels cemented with zinc-phosphate cement could be easily removed by adding 5-10 min. vibration with an aid of some auxiliary tools.