Abstract: Objective:?To study the fatigue strength of different generations of precurved NiTi instruments, and the shaping ability of precurved instruments in the curved canals. Methods:?Six kinds of NiTi rotary instruments were divided respectively into precurved group and non-precurved group (n=10): Hyflex CM (HCM), Protaper Universal (PTU) S1, S2, Protaper Next (PTN) X1, Mtwo 10/.04, 15/.05..Fatigue tests were carried out by using stainless steel blocks simulating curved canals . The whole process was recorded through a high-definition digital camera.The rotation time and the number of revolutions of each group were calculated.48 curved root canals of permanent teeth with angles of curvature ranging from 20~60°were randomly distributed to each group of PTUS2、PTNX1（15/.04） and Mtwo（15/.05） (n=8). Micro CT was used to scan and record the root canal morphology. Root canal transportation (CT) of each measuring point was calculated. Results:?The results of rotational fatigue measurement showed that there was no significant difference in the anti-fatigue strength between precurved group and non-precurved group of Mtwo, PTU and HCM instruments (P＞0.05). However, the difference of two groups of PTN was statistically significant (P＜0.05). In general, the anti-fatigue strength of HCM is significantly higher than that of the other three NiTi instruments in not only precurved group but also non-precurved group (P＜0.05). The results of root canal deviation showed that there was no significant difference between precurved group and non-precurved group after preparation for Mtwo, PTU and PTN (P＞0.05). Conclusions:?The risk of fracture of precurved PTN in root canal preparation is so high that avoiding precurved operation is recommended. The fatigue strength of Mtwo and PTU is relatively less affected by precurved operation, so that it can be properly performed. HCM fatigue strength is recommended for precurved technology. In clinical practice, NiTi instruments with tip diameter less than 0.15 mm may permanently deformed after precurved operation, and root canal deviation maybe of a low probability.

Key words: precurved technology, curved root canal, root canal preparation, NiTi instruments, anti-fatigue strength