[Comparison of dentinal microcracks formation in severely curved root canal with different Ni-Ti instruments]

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.

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.

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).

Compared to WaveOne, ProTaper Next and M3-Pro are more suitable for severely curved canal preparation.