###### Abstract  
Although nickel-titanium (NiTi) rotary files are invaluable tools in dentistry, the relationship between their properties and performance has not been elucidated. Herein, the performance of gold heat-treated (ProPaper Gold and ProfaTaper Gold), controlled memory (CM)-wire (HyFlex CM and V Taper 2H), T-wire (2 Shape), and R-phase heat-treated (K3 XF) NiTi files was evaluated; the non-heat-treated ProFile was used as the control. The bending, buckling, cyclic fatigue, and torsional resistances of the NiTi files were determined, and their phase transformation behavior was studied through differential scanning calorimetry (DSC). The angle of rotation until fracture (ARF), ultimate torsional strength (UTS), and stiffness were evaluated via torsional resistance testing. One-way analysis of variance (ANOVA) and post hoc analyses were conducted using the Games-Howell test and Tukey's test. ProFile displayed the highest buckling resistance (8 N), CM-wire NiTi files exhibited the lowest bending resistance (0.660-0.758 N cm), and HyFlex CM displayed the highest cyclic fatigue resistance. ProPaper Gold exhibited high UTS and low ARF. K3 XF demonstrated high bending resistance and the lowest cyclic fatigue resistance. The CM-wire NiTi files were the most suitable for use in curved canals, while the gold-wire NiTi files were ideal for constricted canals. ProFile was recommended for use in re-treatment cases. b +
Footnote †: journal: Applied Sciences  
1
Footnote 1: Department of Conservative Dentistry, College of Dentistry, Kyung Hee University Medical Center, Kyung Hee University, 23 Kyungheedae-ro, Seoul 02447, Republic of Korea 1  
Footnote 2: Department of Conservative Dentistry, College of Dentistry, Kyung Hee University Medical Center, Kyung Hee University Medical Center, Kyung Hee University, 23 Kyungheedae-ro, Seoul 02447, Republic of Korea 1  
Footnote 3: Department of Conservative Dentistry, College of Dentistry, Kyung Hee University Medical Center, Kyung Hee  
micro-twinned martensite, R-phase, and austenite. The R-phase is an intermediate phase between austenite and martensite; it has a lower elastic modulus than martensite [8; 12]. M-wire exhibits superelasticity and shape memory effects similar to those of conventional NiTi alloys but is more flexible and resistant to cyclic fatigue fracture [8; 13; 14]. In 2008, R-phase heat treatment was first conducted by Sybron Endo (Orange, CA, USA) for the manufacture of Twisted File (SybronEndo). Twisted File was fabricated by twisting R-phase wires instead of grinding them, which led to enhanced flexibility and increased cyclic fatigue resistance in relation to those of a conventional NiTi file (ProFile; Dentsply Maillefer, Ballaigues, Switzerland) [15]. Unlike Twisted File, K3 XF (Kerr Dental, Orange, CA, USA) is manufactured via a traditional grinding process, followed by post-machining R-phase heat treatment [10]. K3 XF exhibits higher flexibility and resistance to cyclic fatigue than K3 [16], which has an identical design but does not undergo R-phase heat treatment. K3 presents one endothermic peak during austenitic transformation, whereas K3 XF displays two peaks and has a higher A\({}_{\text{f}}\)[17]. According to differential scanning calorimetry (DSC) results, Twisted File and K3 XF consist of austenite, martensite, and R-phase at 22 \({}^{\circ}\)C [17; 18]. R-phase NiTi files display superelasticity and shape memory effects similar to those of M-wire NiTi files [8]. Controlled memory (CM)-wires, introduced in 2010, are thermomechanically treated NiTi wires with a lower nickel content (52 wt.%) than that of conventional NiTi files [19; 20; 21]. In contrast to conventional NiTi, M-wire, and R-phase alloys, CM-wires do not display superelasticity. Thus, when an external force bends CM-wire NiTi files, they do not return to their original position when the force is removed [7]. The A\({}_{\text{f}}\) of CM-wires is 31-55 \({}^{\circ}\)C [9; 19; 22; 23]. At room temperature, CM-wires contain a higher martensite content than M-wire or R-phase alloys [15].