Modal testing of nickel-titanium rotary instruments using finite element method

Objectives: Nondestructive detection on bulk structure properties of an object has been wildly used for industrial manufacture. Recently, a nondestructive detection method, resonance frequency (RF) technique, was introduced for dental implant stability detection. Mechanically, natural frequency of an object is associated not only with altering its boundary stability, but also its bulk elastic properties. Accordingly, in this study, RF analysis was carried out to test the structural status of endodontic of Ni-Ti rotary instruments which were subjected to various degrees of cyclic loadings. Methods: A three-dimensional solid model of nickel titanium rotary file was established by reconstructing its 3D geometric shape using a computer aided design software. Then finite element (FE) model of the instrument was obtained by meshing the pre-established solid model. To validate the model, natural frequency of the rotary instruments modal with various clamping situations were calculated and compared to the analogs values obtained from in vitro modal testing experiments. Then the changes in RF value of the model with various degrees of fatigue fracture were computed. Results: Our finite element simulation showed that increasing of nickel-titanium rotary instrument cycling fatigue fracture level causes a significant decrease in the natural frequency, and a strong relationship was found (P<0.05) between the frequency and the fracture level. Conclusion: Our results suggested that natural frequency is an important parameter for assessing the cycling fatigue fracture of an endodontic of Ni-Ti rotary instrument.