## 4 Discussion  
This study evaluated the bending resistance and cyclic fatigue resistance of three kinds of orifice preflaring NiTi files manufactured via different heat treatments. In the present study, EDM presented the highest bending resistance at both experimental temperatures, and the highest NCF at BT. The cyclic fatigue resistance of the HyFlex EDM Orifice Opener has not been yet investigated. Nonetheless, the outstanding superior cyclic fatigue resistance of HyFlex EDM instruments has been proven in previous studies [13; 14; 15; 31], which is attributed to its unique manufacturing process. EDM is a noncontact machining procedure which uses a spark that occurs when a discharge occurs between two electrodes [12; 15]. EDM does not require direct contact between workpieces; therefore, it prevents mechanical stress such as machining grooves and milling marks produced in the traditional grinding process [10; 31]. Figure 4: XRD results of HyFlex CM (**A**), HyFlex EDM (**B**), and OneFlare (**C**). Peaks for austenite (*), martensite (arrow) and R-phase (arrowhead) are matched. The higher bending resistance of EDM was due to the bigger dimensions at 3 mm away from tip. The taper of CM, EDM and OF was 8%, 12% and 9%, respectively. When the bending resistance was tested, the vertical blade was positioned at the same position, i.e., on the 3 mm point from the tip. The cross-sectional area at the 3 mm point was the largest in EDM, which resulted in a higher bending resistance of EDM. Besides, the higher hardness of EDM and lack of machining flaws resulted in higher bending resistance [16]. [PERSON] et al. reported the higher bending stiffness of HyFlex EDM compared with HyFlex CM, both #25/06 size [13]. The NCF of EDM was not significantly affected by the experimental temperature, while the NCFs at BT of CM and OF were significantly lower than those measured at RT. This is in agreement with [PERSON] et al. [32], who reported that the NCF of four different NiTi files were reduced, including Hyflex CM when the cyclic fatigue was tested in water with BT. The reduced NCF at BT could be due to the austenite finish temperature of the tested instruments being higher than the BT. However, when martensite transforms into austenite, the enthalpy change of CM and OF was much lower compared with EDM (Figure 3). It is speculated that the phase transition of martensite to austenite occurs more easily and the austenite phase will be the primary state at BT. According to the XRD results CM, EDM, and OF contained a martensite and R-phase as well as an austenite phase (Figure 4). The three instruments tested were manufactured with different heat treatments; the R-phase was generated during the proprietary heat treatments [10]. EDM exhibited two distinct peaks in the cooling cycle (Figure 3B): the first one is the R-phase and the second one is the martensite peak. CM and OF had two indistinct peaks in the cooling curve (Figure 3A,C), and the presence of the R-phase was supported by the XRD results. Major peaks for austenite were quite similar among three instruments; however, the peak for martensite was lower for OF compared to CM and EDM (Figure 4). Previous studies reported that cyclic fatigue resistance is reduced by a larger cross-sectional area [33; 34]. Since EDM had a 12% taper and CM had an 8% taper, the cross-sectional area was smaller for CM. The smaller cross-sectional area of CM contributed to superior cyclic fatigue resistance compared to EDM at RT. Nevertheless, EDM had a higher NCF than CM at BT, and the alloy properties of the NiTi file seem to have a more decisive effect on cyclic fatigue resistance than the cross-sectional area. According to the manufacturer, this controlled memory effect helps clinician reduce the preparation errors. However, according to [PERSON] et al. [35] there was no significant difference in the straightening of the curved root canal between HyFlex CM and Mtwo (VDW GmbH, Munich, Germany), a conventional superelastic NiTi file. [PERSON] et al.