Abstract: Objective To observe and analyze defects in Mtwo instruments after clinical use. Methods A total of 419 Mtwo instruments after clinical use were collected at Nanjing Stomatology Hospital from March 2013 to January 2014. The instruments were examined under a stereomicroscope at 30× magnification. The instruments were divided into three groups:① without defects, intact; ② with defects; ③ fractured or separated. A scanning electron microscope was used to examine some typical changes of the fractured instruments to analyze the separation types. Results 63.5% of the Mtwo instruments showed defects. The incidence of instrument separation was 21.2%. The incidences of instrument separation of four types of Mtwo instruments were respectively #10.04：31.6%，#15.05：13.8%，#20.06：21.0%，#25.06：17.7%. #10.04 was the most frequently fractured instrument (31.6%). Deformations were mostly observed in #15.05 instruments. Cyclic fatigue was the main mechanism for instrument separation of Mtwo. Torsional fracture was observed mostly in #10.04 instruments. According to the SEM, fatigue striations were observed in the instruments fractured because of cyclic fatigue. Dimpled rupture and microvoids could be found in the instruments as a result of torsional stress. Conclusion Cyclic fatigue seems to be an important mechanism for instrument separation of Mtwo rotary instruments. Its proportion in instrument separation is different from that of torsional stress. The rotary file size is correlated with the incidence and mechanism of instrument separation.

Key words: Mtwo instruments, instrument separation, cyclic fatigue, torsional stress