ULTRASONIC FATIGUE BEHAVIOR OF NICKEL-TITANIUM ENDODONTIC INSTRUMENTS UNDER FLEXURAL CONDITIONS

摘要

Nickel-titanium (NiTi) endodontic instruments with superelasticity are very popular since they could avoid or limit the failure risk, especially in curved canals. In this study, the ultrasonic fatigue behaviors of NiTi endodontic instruments with different sizes under ultrasonic conditions were investigated using a self-designed apparatus, and the flexural constraints were constructed by simulated canals made of ethoxyline. The ultrasonic fatigue life, that is, the numbers of vibrationto- failure for NiTi instruments are beyond 107 cycles. To make more sense of the ultrasonicfatigue mechanism, the bending test and finite element analysis (FEA) method were used, as well as the fractograhic analysis of the ultrasonic fatigue fractured surfaces by scanning electron microscope (SEM). The results of ultrasonic fatigue tests demonstrate that the numbers of vibration-to-failure for NiTi instruments are influenced by the flexural conditions which are characterized by curvature angle and curvature radius. The NiTi instruments with larger diameters exhibit longer duration time than these with smaller diameters when tested under the same flexural condition; while for the same-sized instruments, the smaller of the curvature angle or the greater of the curvature radius, the longer the duration time is. The qualitative analysis of the bending and FEA results indicates that the ultrasonic fatigue lives for NiTi instruments are controlled by the combination of flexural conditions and ultrasonic oscillations, and the positions where the maximum stress produced by these two factors tends to initiate the fatigue cracks. SEM analysis results indicate that the ultrasonic fatigue cracks of NiTi endodontic files under flexural conditions originate from the outer surface of curved segment of instruments. There are clear fatigue striations and dimples existing in the propagation region of the fatigue cracks, which indicate the excellent ductility and the resistance to the propagation of fatigue cracks of nickel-titanium endodontic files.