Abstract: PURPOSE: To explore the mechanism of failure of ProTaper NiTi handfiles during root canal preparation. METHODS: A total of 2061 discarded ProTaper handfiles were collected after clinical use. The files were studied under stero-microscope to determine the type of defect. The fractured files were photographed digitally and the pictures were analyzed by software Image-Pro Plus to measure the distance from the break point to the tip. The data were statistically analyzed by using SPSS 11.0 software package. RESULTS: Plastic deformation was observed in 166 files (8.2%), and the types of defects included unwinding, winding flutes, and plastic bending. Instrument separation occurred in 581 files (28.2%), and the fracture rate was the highest in the F2 file group (41.7%). The SX file group had the highest facture level, and the mean distance from the breakpoint to the file tip was (4.4±1.1) mm. CONCLUSIONS： For shaping files, especially at the apical level, the main cause of mental failure is overload of torque, whereas for finishing files, especially at the higher level, is cycle fatigue. Understanding the defect types of ProTaper handfiles is useful for avoiding file breakage during root canal therapy.

Key words: Instrument separation, ProTaper, Fatigue failure