RF analysis of Ni-Ti rotary instruments subjected to cyclic fatigue

Objectives: In the endodontic treatment, Ni-Ti alloy instruments are extensively used due to their outstanding mechanical property. Unfortunately, the nickel titanium rotary file separates unexpectedly during the treatment procedure. There is no proper method to detect the instrument fracture in advance so far. Resonance frequency analysis (RFA) is a method wildly used for nondestructive test on dental implant. It has been reported that RF value of an object can be changed by alerting its structure properties. The purpose of this study is to test the possibility for using RFA as a tool to check the fatigue status of the endodontic Ni-Ti rotary instrument. Methods: Life spam of the tested objects was obtained by averaging the results from cyclic fatigue tests on ten unused profiles. Then modal testing was performed to monitor the vibrational frequency responses of the cyclic-loaded profiles. In addition, the RF values of profiles with three-month used clinically were also tested and compared to the readings of unused ones. Results: Our results demonstrated that the cyclic loading on the profiles tend to decrease their natural frequency. The frequency significantly decreased with a value of about 400 Hz ~ 950 Hz (558+146 Hz) when the test profiles preceded 77 % to 85 % of total life spam. In addition, tests on clinical used profiles also revealed that the mean RF value of the three-month used nickel titanium rotary files (9271+220 Hz) was significantly lower ( p < 0.05)than the RF value of unused files (10054+252 Hz). Conclusion: Our results showed that resonance frequency has potential to become an effective parameter in the fracture status evaluation of nickel titanium rotary instruments subjected to a series of cyclic loadings.