RESUMO
The aim of this experiment was to evaluate the relative efficacy of ultrasonic instruments on the removal of posts cast in alloys with different stiffness (nickel-chromium and silver-tin alloys). Sixty roots of extracted bovine incisor teeth were sectioned, prepared and randomly divided into 4 groups (n=15): groups 1 and 2 received posts cast in Ni-Cr and groups 3 and 4 received posts cast in Ag-Sn. All posts were cemented with zinc phosphate cement. Groups 2 and 4 received ultrasonic vibration for 12 minutes. The force necessary for post removal was measured by an Instron universal testing machine. Data were statistically analyzed with ANOVA and when significant differences were found, means were compared using Tukey's intervals with a confidence level of 95%. The mean force (kgf) to remove posts of group 1 (67.84± 15.31) was statistically greater than groups 2 (46.95± 10.85), 3 (48.53± 8.49) and 4 (41.25± 11.40). There was no statistically significant difference among groups 2, 3 and 4. The ultrasonic vibration was effective in reducing the force required to remove posts cast in Ni-Cr (about 31% of reduction), but had little effect on posts cast in Ag-Sn (about 15% of reduction). Posts cast in Ni-Cr were also more retentive then posts cast in Ag-Sn when they were not submitted to ultrasonic vibration.
