Effect of R-phase heat treatment on torsional resistance and cyclic fatigue fracture

Introduction: The purpose of this study was to investigate whether the resistance to torsional stress and cyclic fatigue would be improved by the novel R-phase heat treatment by using instruments with identical geometric characteristics. Methods: K3XF and K3 files with sizes of #30/.04 and #30/.06 were selected for the study (n = 10 per group). To evaluate the torsional resistance, the torsional load and distortion angle at yield, ultimate, and fracture points were recorded. The toughness until fracture was calculated as well. The number of cycles to failure for each instrument was determined to evaluate the cyclic fatigue resistance. Statistical analysis was performed by using the nonparametric Mann-Whitney U test (for torsional resistance) and independent t test (for cyclic fatigue). After torsional and cyclic fatigue tests, all fractured fragments were observed under a scanning electron microscope to evaluate the topographic features of the fractured surfaces. Results: Although K3XF showed superior cyclic fatigue resistance to K3 (P <.000), there was no significant difference in torsional resistance in terms of ultimate strength, fracture angle, and toughness between the 2 brands of K3XF and K3 regardless of the instruments' taper. The fractured cross-sectional surfaces showed typical features of cyclic fatigue and torsional fractures. K3XF had a distinctive surface with numerous irregular micropores. Conclusions: The heat-treated K3XF instruments that used R-phase heat treatment technology showed improved cyclic fatigue resistance without decline of torsional strength.