## 1 Introduction  
Most nickel-titanium (NiTi) rotary file systems recommend a crown-down approach. The enlargement of the coronal region precedes the shaping of the middle and apical region of the root canal. Coronal preflaring reduces the initial coronal curvature of the root canal,and prevents iatrogenic problems such as ledge, canal blockages or transportation [1, 2]. Coronal preflaring facilitates the tactile detection of the apical constriction and increases the accuracy of working length determination [3, 4, 5]. The coronal space prepared by a preflaring instrument functions as a reservoir for root canal irrigant to coronally flush the debris resulting from root canal shaping, and reduces the amount of debris extruded apically [6, 7]. NiTi rotary files used for coronal preflaring should possess a proper cutting ability and hardness as well as resistance to fracture in the coronal curvature of the root canal.  
By applying heat treatment to the manufacturing process, NiTi files have become more flexible and more resistant to cyclic fatigue fracture [8, 9]. HyFlex CM (Coltene/Whaledent, Alstatten, Switzerland) and HyFlex EDM (Coltene/Whaledent) are made of controlled memory wire (CM wire; DS Dental, Johnson City, TN, USA). Unlike the conventional superelastic NiTi file, CM wire instruments do not rebound to their original shape when the external load is removed [10]. The HyFlex CM #25/08 instrument, used as a cervical preflaring instrument, has lower bending resistance than conventional Ni-Ti files (ProFile Oriface Shaper #30/06, BioRaCe #25/08, ProTaper universal Sx), while the cutting efficiency of HyFlex CM #25/08 is superior. [11].  
HyFlex EDM is manufactured by the electrical discharge machining (EDM) process of CM wire. The EDM process hardens the surface of the NiTi file, resulting in superior resistance to fracture and low degradation is observed after multiple root canal shapings [12]. A number of previous studies have demonstrated the superior resistance to cyclic fatigue fracture of HyFlex EDM compared to a conventional NiTi file, M-wire, gold heat-treated or blue heat-treated NiTi files [13, 14, 15]. When compared to HyFlex CM, HyFlex EDM exhibited greater hardness, maximum torque to fracture and cyclic fatigue resistance [12, 13, 16, 17, 18]. The HyFlex EDM orifice opener (#25/12) was released for the preparation of the root canal orifice prior to glide path file, yet little is known about its mechanical properties [19].  
OneFlare (MicroMega, Besancon, France) was manufactured through T-wire heat treatment after the grinding process. [PERSON] et al. [20] compared the mechanical properties of OneFlare with a NiTi file which had identical design but was made with conventional NiTi wire. OneFlare exhibited superior cyclic fatigue resistance and lower bending resistance.  
A lot of studies have been conducted on the cutting efficiency and dentinal crack formation of cervical preflaring NiTi files; however, a few studies have assessed flexibility and cyclic fatigue resistance [11, 20, 21, 22, 23]. Additionally, no study has compared the cyclic fatigue resistance of HyFlex CM #25/08, HyFlex EDM orifice opener, and OneFlare. The objective of this study was to compare the bending resistance and cyclic fatigue resistance of three coronal preflaring NiTi files with different heat treatments.