Torsional Strength and Toughness of Nickel-Titanium Rotary Files

Introduction: The aim of this investigation was to compare torsional strength, distortion angle, and toughness of various nickel-titanium (NiTi) rotary files. Methods: Five NiTi rotary instruments with different cross-sectional geometries were selected: TF and RaCe with equilateral triangle, Pro Taper with convex-triangle, Pro File with U-shape, and Mtwo with S-shape. The size 25/.06 taper of IF, RaCe, Pro File, and Mtwo and the Pro Taper F1 files were tested, all with the same diameter at D5. A metal mounting block with a cubical hole was constructed in which 5 mm of the file tip was rigidly held in place by filling the mold with a resin composite. The files were subjected to clockwise rotation at 2 rpm in a torsion tester. The torque and angular distortion were monitored until the file failed. The data were compared statistically for the yield and ultimate strengths, plastic hardening period, and toughness. Results: TF and RaCe had significantly lower yield strength than other systems. TF had a significantly lower ultimate strength than other files, whereas Mtwo showed the greatest. Pro File showed the highest distortion angle at break, followed by IF. Pro File also showed the highest toughness value, whereas TF and RaCe both showed a lower toughness value than the others (P < .05). Fractographic examination revealed typical pattern of torsional fracture for all brands, characterized by circular abrasion marks and skewed dimples near the center of rotation. Conclusions: Under the limitations of the present study, the 5 tested NiTi rotary files showed a similar mechanical behavior under torsional load, with a period of plastic deformation before actual torsional breakage but with unequal strength and toughness value. (J Endod 2011;37:382-386)