Abstract: PURPOSE: To evaluate the clinical outcome of sonic endo-instruments in combination with Mtwo Ni-Ti files in root canal preparation of molars. METHODS: One hundred patients (118 first molars, 428 root canals) were included in this study, they were randomly assigned into 2 groups (n=50).Group 1 was shaped with sonic endo-instruments in combination with Mtwo Ni-Ti files,while group 2 was treated with stainless-steel files, both groups were obturated with lateral condensation technique. The time of root canal shaping, acute post-operative pain, the quality of the root canal filling and the success rate after 1 year were recorded and analyzed using SPSS 11.0 software package. RESULTS: The average preparation time in group 1 was (5.3±0.6)min/canal and (12.7±0.9) min/canal in group 2 (t=100.89,P<0.001). The rate of acute postoperative pain was significantly lower （χ²=4.87, P<0.05）in group 1 (8.20%) than in group 2 (22.81%); in group 1, 92.86% root canals were filled adequately while only 78.92% in group 2, the difference was significant (χ²=17.45, P<0.05);higher success rate (χ²=3.95, P<0.05) after 1 year was achieved in group 1 (91.80%) compared with group 2 (78.95%). CONCLUSIONS: Compared with stainless-steel files, sonic endo instruments in combination with Mtwo Ni-Ti files in root canal preparation of molars seem more effective, cause less postoperative pain and offer higher success rates.

Key words: Sonic, Root canal preparation, Mtwo Ni-Ti fies, Stainless-steel files