Objective This study aims to investigate the influence of K file preflaring and torque on the failure of rotary nickel-titanium ProTaper instruments. Methods An in vitro method was carried out to calculate the mean number of S-shaped simulated root canals before file fracture or deformation under different conditions. Results Group A1(S1 on simulators with no preflaring) shaped 13 blocks, and Group B1(S1 on manually preflared simulators) shaped 36 blocks. Group A2(S2 with low torque) shaped 17 blocks, and Group B2(S2 with high torque) shaped 18 blocks. Group A3 (F1 with low torque) shaped 14 blocks, and Group B3(F1 with high torque) shaped 14 blocks. Group A4 (F2 with low torque) shaped 6 blocks, and Group B4(F2 with high torque) shaped 6 blocks. Conclusion K file preflaring is a major determinant in reducing the failure of rotary nickel–titanium ProTaper instruments, whereas torque exhibiting no influence on the failure of rotary nickel-titanium ProTaper instruments.
