Effects of Surface Finishing On the Fatigue Resistance of NiTi Endodontic Instruments

Although NiTi endodontic instrument have 2-3 times the elastic flexibility in bending and torsion, as well as superior resistance to torsional fracturing when compared to similar stainless steel instruments, NiTi instrument fatigue fracture occurs during root dental treatment. The combined action of torsional stress and cyclic loading inside the canal is responsible for the breakage of instruments in use. Factors affecting the incidence and mode of failures include the dimension and surface condition of the instrument, rotation rate, radius of canal curvature, and the presence of a straight-line access to the apical portion of the canal. The present study evaluated the influence of electropolishing surface treatment on fatigue fracture NiTi instruments. Methods: endodontic instruments with or without electropolishing surface treatment were used in an artificial curved canal under rotational speed of 300 rpm until fracture. The number of rotations to fracture and torque at fracture were determined. Fractured surfaces and the helical shafts of instruments were analyzed by SEM. Results: Electropolishing process alters the surface morphology of the instrument and renders a more homogeneous surface, with less defects. Electropolished instruments performed significantly better than nonelectropolished instruments in cyclic fatigue testing (P < .001). Actually, the number of cycles to fracture of a polished instrument was 124% higher than that of a nonpolished instrument. SEM analysis showed that the fractured surface of both polished and nonpolished instruments had ductile morphologic characteristics. Evaluation of the separated fragments after cyclic fatigue testing showed the presence of microcracks near the fracture surface. Polished instruments exhibited fine cracks that assumed an irregular path (zigzag crack pattern), whereas nonpolished instruments showed cracks running along the machining grooves. Conclusions: Electropolishing surface treatment of NiTi instruments significantly increased the cyclic fatigue resistance. The electropolishing process reduced surface irregularities that serve as points for stress concentration and crack initiation.