Nondestructive Test of Endodontic NiTi Rotary Instruments with Resonance Frequency

Objectives: Ni-Ti alloy instruments are extensively used in the endodontic treatment due to their outstanding mechanical properties. Unfortunately, the Ni-Ti rotary file separates unexpectedly during the treatment procedure. There is no proper method to detect the instrument fracture in advance so far. The purpose of this study is to develop a method for detecting the fatigue status of the instrument. Methods: In this investigation, modal testing was performed by a commoditized device, IMPLOMATES®, to monitor the resonance frequency responses of the profiles used clinically. Results: Our results demonstrated that the cyclic loading on the profile results in decreasing the natural frequency. The frequency significantly decreased with a value of about 600 Hz ~ 800 Hz when the profiles preceded 70 % to 85 % of total life spam. Our in vivo experiments also revealed that the resonance frequencies of the three-month used Ni-Ti rotary files (8800 Hz~ 9300 Hz) are significantly lower than the new files (10000 Hz ~11000 Hz, p < 0.05). Conclusions: These results showed that resonance frequency can be treated as an effective parameter in the evaluation of the fracture status of Ni-Ti rotary instruments.