Fracture resistance of electropolished rotary nickel-titanium endodontic instruments

The purpose of this study was to investigate the effect of electropolishing on cyclic flexural fatigue and torsional strength of rotary nickel-titanium endodontic instruments. Electropolished and nonelectropolished ISO size 30 (0.04 taper) EndoWave (J Morita Corporation, Osaka, Japan), ProFile (Dentsply Maillefer, Ballaigues, Switzerland), and RaCe (FKG, La-Chaux De Fonds, Switzerland) instruments from the same manufacturing batches were investigated. The number of rotations to fracture and torque at fracture were determined and compared among the instruments tested. Instruments were viewed under a scanning electron microscope (SEM) to assess the degree and quality of electropolishing. Overall, electropolished instruments performed significantly better than nonelectropolished instruments in cyclic fatigue testing and, to a lesser extent, in static torsional loading. When viewing electropolished instruments with the SEM, milling grooves, cracks, pits, and areas of metal rollover were observed, although they were more evident in the nonelectropolished instruments. Electropolishing may have beneficial effects in prolonging the fatigue life of rotary NiTi endodontic instruments. The benefits of electropolishing are likely to be caused by a reduction in surface irregularities that serve as points for stress concentration and crack initiation.