Abstract: PURPOSE: This study was aimed to compare the incidence of dentinal microcracks produced by 3 kinds of Ni-Ti instruments during root canal procedures in severely curved canals. METHODS: Two hundred and forty extracted human molars with mesial roots of 25° to 40° curvatures were selected and divided into A, B, C group, with 80 teeth in each group according to root curvature of 25°-30° （excluding 25° ）, 30°-35° ,35°-40°. Each of them was prepared with K file to 15#. Then, each group was divided into 4 sub-groups (n=20), one was as control, the others were prepared with WaveOne, ProTaper Next and M3-Pro, respectively. After preparation, all roots were stained with 1% methylene blue for 24 hours. The roots were then sectioned at the most curved plane and 2mm below and above the most curved plane with alow-speed saw under cold water. Stereomicroscope was used to inspect dentinal microcracks and differences between each group were analyzed using Chi-square test with SPSS 20.0 software package. RESULTS: Microcracks were observed in the group of WaveOne, ProTaper Next and M3-Pro. WaveOne system induced more dentinal microcracks compared with ProTaper Next and M3-Pro system (P<0.05), and there was no significant difference between ProTaper Next and M3-Pro system (P>0.05). The number of dentinal microcracks in WaveOne, ProTaper Next and M3-Pro group increased with the increase of root curvature. Except Waveone in group A (25°-30°) and group C (35°-40°), the occurrence of dentinal microcracks in two groups had significant difference (P<0.05), there was no significant difference among other groups (P>0.05). CONCLUSIONS: Compared to WaveOne, ProTaper Next and M3-Pro are more suitable for severely curved canal preparation.

Key words: Ni-Ti instruments, Root canal preparation, Curved canal, Root curvature, Dentinal microcracks