Abstract: Objective To explore the clinical efficacy of pulpitis caused by hidden cracks using different root canal preparation instruments and provide clinical reference for choosing the instruments. Methods 60 two-root-canal 1st premolars with pulpitis caused by hidden cracks were randomly divided into three groups: the Protaper group, the MTwo group and the K file group, each with 20 teeth. The teeth in different groups were treated with different root canal preparation instruments and the other root canal treatment steps were the same. Endodontic interappointment pain, postoperative treatment effect in 3 months and the occurrence of vertical root fracture in 1.5 years were observed and analyzed by chi-square test. Results The occurrence rate of endodontic interappointment pain of K file group (50.00%) was significantly higher than that of Protaper group（10.00%，χ2=7.657，P=0.022）and MTwo group (15.00%，χ2=7.958，P=0.019), while there was no significant difference between Protaper group and MTwo group (χ2=2.029，P=0.363). Moreover, there was no significant correlation between root canal preparation method and short-term treatment effect（χ2=1.966，P=0.742）or occurrence of vertical root fracture in 1.5 year (χ2=0.000，P=1.000). Conclusion Compared with K file, rotary NiTi instruments can significantly reduce the occurrence rate of endodontic interappointment pain, have good therapeutic effect and do not significantly increase the risk of vertical root fracture.

Key words: cracked tooth syndrom, root cananl preparation, endodontic interappointment pain, vertical root fracture