## 4 Discussion  
In the present study, there was a significant difference in the mechanical properties of the NiTi rotary instruments according to their design. The XP-endo Shaper showed the lowest bending and buckling resistances, UTS, and EM. Spring Endo and Spring H files exhibited higher bending and buckling resistances, UTS, and EM than TruNatomy Prime and XP-endo Shaper files. Although the tip sizes of TruNatomy Prime (#26) and XP-endo Shaper files are larger than those of Spring Endo (#25) and Spring H (#25) files, the overall mass of the instrument was the lowest for XP-endo Shaper files, followed by TruNatomy Prime files, due to a smaller taper. SEM images of the fractured instruments from the torsional and cyclic fatigue resistance tests showed cross-sectional images at 3 and 5 mm from the apex, respectively (Figures 3 and 4). The cross-sectional areas of Spring Endo and Spring H files were larger than those of TruNatomy Prime and XP-endo Shaper files. Among the mechanical test results, bending and buckling resistances, UTS, and EM had strong positive correlations with each other (Table 3) because they are all strongly affected by the cross-sectional area [19]. Our results are in agreement with those of previous studies, according to which NiTi files with larger cross-sectional areas had higher bending and buckling resistances and UTS [19; 20; 21]. The XP-endo Shaper presented the highest cyclic fatigue resistance, followed by TruNatomy Prime (Table 2, Figure 1). In this study, the cyclic fatigue resistance test of NiTi files were performed at 22 \({}^{\circ}\)C. According to DSC curves, the XP-endo Shaper and TruNatomy Prime files consisted of austenite, martensite, and R-phase at 22 \({}^{\circ}\)C (Figure 5A,B), whereas Spring Endo consisted of austenite and Spring H was composed of martensite at 22 \({}^{\circ}\)C (Figure 5C,D). The R-phase is a transitional phase between martensite and austenite [7]. The EM of R-phase is lower than those of martensite or austenite phases, and the transformation strain of the R-phase is less than 10% of martensitic transformation [22]. Therefore, the presence of the R-phase can explain the higher NCF of TruNatomy Prime and XP-endo Shaper files. The NCF of XP-endo Shaper was significantly higher than that of the other instruments. It was presumed that geometric designs, such as taper and cross-sectional configurations, as well as the Max-Wire property, influenced the NCF. XP-endo Shaper had the smallest cross-sectional area at D3. Our results are consistent with those of previous studies showing that NiTi files with smaller cross-sectional areas presented superior resistance to cyclic fatigue [23; 24]. Further, XP-endo Shaper has a unique design, with a curved shape, booster tip, and six cutting edges [11]. The contact time between the surface of the instrument and the artificial root canal during rotation was less with XP-endo Shaper than with other instruments. This may have lowered the stress developing due to repetitive bending during the cyclic fatigue resistance test. TruNatomy Prime had the second highest NCF among all the instruments tested. This could be attributed to several factors, including thermomechanical processing, special heat treatment, off-centered parallelogram cross-sectional design, and slim NiTi wire (0.8 mm) [11; 14]. those of Spring Endo, whereas the EM was higher for Spring Endo (Table 2, Figure 1). Spring H consisted of martensite at body temperature (Figure 5D), although the type of heat treatment has not been revealed. The increased ARF and decreased EM after heat treatment are consistent with the findings of previous studies [25; 26]. Buckling is a sudden sideways deflection that occurs during vertical compressive loading [27]. [PERSON] et al. measured the buckling resistance as the force generated by a 1-mm lateral elastic displacement of the NiTi file [27]. In the present study, we measured the buckling resistance as the maximum load during a 1-mm axial movement of the file because lateral displacement could not be measured.