## 4 Discussion  
The mechanical properties of the NiTi files were assessed using four tests. The bending resistance test evaluated the flexibility of the NiTi file, with lower bending resistance indicating superior flexibility. The CM-wire NiTi files, HyFlex CM and V Taper 2H, demonstrated superior flexibility, which is consistent with previous research findings [9; 19; 26]. Among the tested NiTi files, the CM-wire NiTi files exhibited no superelasticity, while the gold-wire NiTi files exhibited lower superelasticity. After bending the CM-wire NiTi files to 45\({}^{\circ}\), the residual angle was nearly 45\({}^{\circ}\) even after the external force was removed. By contrast, the gold-wire NiTi files displayed spring-back behavior after the removal of the bending force. In addition, the larger cross-sectional area of ProTaper Gold and ProfaTaper Gold contributed to their higher bending resistances than those of HyFlex CM and V Taper 2H. The buckling resistance of a file is its capacity to resist sideways deflection when the compressive load exceeds the resistance of the file [37]. However, there is no standard for testing the buckling resistance in NiTi files. [PERSON] et al. [37] measured buckling resistance as the force generated by a 1 mm lateral elastic displacement. In our study, we measured buckling resistance as the force generated by a 1 mm vertical (downward) displacement, because the lateral displacement could not be measured. Herein, ProFile exhibited the highest buckling resistance. The DSC results showed that ProFile is composed of austenite at 22 \({}^{\circ}\)C (Figure 6A), which explains its buckling behavior because the stiffness and hardness of austenite are higher than those of martensite [10; 11]; therefore, an austenite-rich NiTi file more effectively resists sideways deflection by a compressive load. Buckling resistance can be used to assess the penetrating ability of NiTi files. The orifice of the root canal is often narrow and calcified, and any instrument used must be capable of penetrating it [38]. Furthermore, during re-treatment, gutta-percha and root canal sealers should be removed. NiTi files with a high buckling resistance are suitable for penetrating any previous root canal filling materials that may be present. ProFile comprises three radial lands and U-shaped flutes, whereas the cross-sectional shape of ProTaper Gold is convex and triangular without radial lands (Figure 4A,B). Thus, the ProFile shape facilitates more efficient removal of old filling materials and debris toward the orifice of the root canal. According to the results of the Pearson correlation analysis, the bending and buckling resistances were moderately positively correlated. HyFlex CM and V Taper 2H exhibited low bending and buckling resistances (Figure 2A,B). Therefore, these flexible NiTi files are ideal for operating on curved root canals [39; 40]. Meanwhile, HyFlex CM had the lowest UTS among the tested NiTi files, and the stiffness of V Taper 2H and HyFlex CM was lower than that of other NiTi files because of their high martensite content. Therefore, extreme caution should be taken when using these files to shape a narrow, calcified canal.