## 4 Discussion
Sufficient buckling resistance can enable exploration of canal orifices and negotiation of narrow canals [28]. Instruments with low buckling resistances, such as the XP-endo Shaper, require the prior use of glide path instruments. Spring Endo and Spring H files demonstrated a higher buckling resistance, suggesting that instruments with larger diameters have a higher buckling resistance, which agrees with the findings of a previous study [29]. ARF represents the ability to sustain deformation under a twisting force before fracture and is related to ductility. In the present study, ARF and NCF had a strong positive correlation (Table 3) because both were affected by the flexibility of the NiTi file. In the present study, NiTi files with a small cross-sectional diameter around D3 (XP-endo Shaper, TruNatomy Prime) demonstrated a higher ARF. The bending resistances of the XP-endo Shaper and TruNatomy Prime files were the lowest, indicating that they were the most flexible. In this study, the Spring Endo was not compared to spring-free controls with the same cross-sectional shape, tip size, and taper. According to a finite element study by [PERSON] et al., a lower torque is required to axially rotate NiTi files with spring shafts compared with NiTi file with identical design without spring [15]. According to the FE-SEM results, only TruNatomy Prime had a parallelogram cross-section, whereas the other instruments had a triangular cross-section (Figure 4). In this study, there was no significant difference in the UTS of TruNatomy Prime with a parallelogram cross-section and XP-endo Shaper with a triangular cross-section. There has been no study to compare torsional fracture resistances of TruNatomy Prime and XP-endo Shaper. XP-endo Shaper demonstrated lower torsional resistance compared to TRUShape #30/06, ProFile Vortex #30/04, and FlexMaster #30/04 with modified triangular cross-sectional designs [8; 30], which were conducted at 37 \({}^{\circ}\)C. Conversely, the torsional resistance test was performed at 22 \({}^{\circ}\)C in this study. Experimental temperature and alloy characteristics seem to collectively affect the torsional resistance. During canal shaping, XP-endo Shaper removed more of the artificial canal wall from the resin block (Table 4). These files can expand and reach areas that conventional files cannot access [31]; therefore, the cutting efficiency of XP-endo Shaper files was superior. The maximum clockwise torque generated by XP-endo Shaper files was greater than that generated by TruNatomy Prime and Spring Endo files, and was comparable to that of Spring H files. A previous study reported that NiTi files with a larger taper caused a higher torque and screw-in force [32]. Although XP-Shaper files have a minimal taper, abundant cutting of the resin canal wall during instrumentation generated greater clockwise torque. A parallelogram cross-section with an off-centered design, as well as a regressive taper and small radius, contributed to the reduced torque and stress. The limitation of this study was that the mechanical tests and simulated canal shaping were performed at 22 \({}^{\circ}\)C. The main phase of XP-endo Shaper changes from martensite to austenite on heating from room temperature to body temperature. XP-endo Shaper, which is composed of Max-Wire NiTi alloy, display optimal properties only at higher temperatures (37 \({}^{\circ}\)C) [34]. Therefore, experiments at body temperature are necessary to observe the mechanical properties of the XP-endo Shaper files used in clinical practice.