Mechanical Properties of Various Heat-treated Nickel-Titanium Rotary Instruments Hye-Jin Goo, DDS, MS,* Sang Won Kwak, DDS, MS,* Jung-Hong Ha, DDS, MS, PhD, † Eugenio Pedull  a, DDS, MS, PhD, ‡ and Hyeon-Cheol Kim, DDS, MS, PhD* Abstract Aim: The purpose of this study was to compare the bending stiffness, cyclic fatigue, and torsional fracture resistances of heat-treated and conventional nickel- titanium rotary instruments. Methods: V-Taper 2 (VT2; #25/.08), V-Taper 2H (V2H; #25/.06), Hyflex CM (HCM; #25/.06), HyFlex EDM (HDM; #25/variable taper), and ProTaper Next X2 (PTN; #25/variable taper) were tested. The bending stiffness was measured with the customized device (AEndoS), and the files (n = 15) were fixed at 3 mm from the tip and bent at 45  with respect to their long axis. Cyclic fatigue resistance was tested by pecking and rotating instruments (n = 15) in artificial canal with a 7.8-mm radius and 35  angle of curvature until fracture. The ultimate torsional strength and toughness were estimated by using AEndoS. The file tip of 5 mm was fixed between resin blocks and driven clockwise at 20 rpm until fracture. The results were analyzed by using one-way analysis of variance and Duncan post hoc comparison. The fracture surfaces and longitudinal aspect of each group were examined under the scanning electron microscope. Results: CM-wire instruments had lower bending stiffness than others. HDM showed the highest cyclic fatigue resis- tance, followed by VTH and HCM (P < .05). VT2 showed the highest ultimate strength, followed by HDM, VTH- PTN, and HCM. HDM and VT2 showed significantly higher toughness than VTH, HCM, and PTN (P < .05). Scanning electron microscope analysis showed typical fractographic features of cyclic fatigue and torsional fractures. Conclusions: CM-wire instruments showed higher flexibility and cyclic fatigue resistance than M- wire and conventional nickel-titanium instruments. Large cross-sectional area and conventional nickel- titanium showed high torsional resistance. (J Endod 2017;-:1–6) Key Words Bending stiffness, CM-wire, cyclic fatigue, electro discharge machining, NiTi rotary file, torsional fracture resistance N ickel-titanium (NiTi) files have advantages such as higher flexibility, fewer canal deviations, and shorter procedural time than stainless steel files (1). However, NiTi files are likely to be vulner- able to fracture in clinical use. Instrument separation is the major problem with NiTi rotary instrumentation techniques. Cyclic and torsional fatigues are 2 main mechanisms that may lead to instrument separation (2). When the instrument rotates in a curved canal, it generates repetitive tension/compression cycles in the region of maximum flexure; then cyclic fatigue occurs (2). During root canal shaping procedures, a part of the instrument binds to the dentin, and the rest of the file continues to rotate, resulting in torsional fracture (3). To overcome these drawbacks, manufacturers have been trying to make NiTi files of superior mechanical properties by using heat treatments, different cross-sectional designs, and new manufacturing processes (4–6). There are 3 kinds of heat-treated NiTi alloys used for endodontic instruments: M-wire, R-phase, and CM-wire. M-wire has been developed through thermomechanical processing and contains 3 crystalline phases, including deformed and micro-twinned martensite, R-phase, and austenite (7, 8). M-wire showed contrasting findings about cyclic fatigue in the literature. Some articles reported significantly improved cyclic fatigue resistance of M-wire files such as ProTaper Next (Dentsply Sirona, Ballaigues, Switzerland) in comparison with conventional NiTi instruments (7, 9); instead, other literature showed no difference in cyclic fatigue of M-wire and conventional NiTi files (10, 11). R-phase instrument has greater flexibility and increased resistance to cyclic fatigue than conventional NiTi files (12). However, R-phase instrument was reported to have a lower torsional strength than conventional NiTi files (13, 14). Recently, brand-new NiTi rotary instruments made from a controlled memory wire (CM-wire; DS Dental, Johnson City, TN) have been introduced. When the conventional NiTi alloy gets a certain range of mechanical load, austenite is transformed to stress-induced martensite (15). Martensite phase is unstable at temperatures above austenite finishing temperature (Af) and returns to austenite by a reverse transformation when the load is From the *Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Pusan National University, Yangsan; † Department of Conservative Dentistry, School of Dentistry, Kyungpook National University, Daegu, Korea; and ‡ Department of General Surgery and Surgical-Medical Specialties, University of Cata- nia, Catania, Italy. Address requests for reprints to Dr Hyeon-Cheol Kim, Department of Conservative Dentistry, School of Dentistry, Pusan National University, Geumo-ro 20, Mulgeum, Yangsan, Gyeongnam 50612, Korea. E-mail address: golddent@pusan.ac.kr 0099-2399/$ - see front matter Copyright ª 2017 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2017.05.025 Significance CM-wire instruments showed higher flexibility and cyclic fatigue resistance than M-wire and conven- tional NiTi instruments. These characteristics of CM-wire instruments may improve the quality of root canal preparations. Basic Research—Technology JOE — Volume -, Number -, - 2017 Heat-treated Nickel-Titanium Rotary Instruments 1