## 4 Discussion
ProFile displayed the second largest ARF after V Taper 2H, with the values differing to a small degree (Figure 2D). The small cross-sectional area resulting from the U-shaped flute is considered to contribute to the larger ARF of ProFile than that of most other NiTi files. ProTaper Gold exhibited the highest UTS and lowest ARF values (Figure 2D,E). The larger the ARF of a NiTi file, the greater its elastic and plastic deformation before separation [44]. The UTS and ARF are possibly affected by the core mass of NiTi files. ProTaper Gold presents an 8% taper at the 3 mm tip region and has a convex triangular cross-section, granting it more metal mass in the 3-4 mm tip region than any other NiTi file tested in this study. This finding is in agreement with the previously reported results, which confirmed that a larger cross-sectional area contributes to a higher bending resistance and UTS [26; 45; 46]. NiTi files with low ARF values, such as ProTaper Gold and ProfaTaper Gold, may not provide sufficient warning signs, such as permanent deformation, before fracture [44]. Based on these results, ProTaper Gold is recommended for use in calcified and/or straight canals. It is suggested to discard the ProTaper Gold file after use for a severely calcified root canal rather than reuse it in the remaining root canal of a tooth. The bending and buckling resistances of ProfaTaper Gold were lower than those of ProTaper Gold (Figure 2A,B). ProfaTaper Gold exhibited higher cyclic fatigue resistance than ProTaper Gold, as represented by the NCF value (Figure 2C). The ARFs of ProfaTaper Gold and ProTaper Gold were not significantly different (Figure 2D), but ProfaTaper Gold exhibited a lower UTS than ProTaper Gold (Figure 2E). Although ProfaTaper Gold is a counterfeit version of ProTaper Gold, the cross-sectional shapes of the two files differ. ProTaper Gold has a more convex cross-section with a larger cross-sectional area than ProfaTaper Gold (Figures 4B1,C1 and 5B1,C1), which contribute to its higher bending and buckling resistances, higher UTS, and lower cyclic fatigue resistance. Additionally, according to the heating curve in the DSC profiles of ProfaTaper Gold (Figure 6C), a mixture of austenite, martensite, and R-phase exists at 22 \({}^{\circ}\)C. Because the elastic modulus of the R-phase is lower than those of austenite and martensite [47], NiTi files comprising the R-phase are more flexible and less resistant to bending and buckling. The stiffness values of V Taper 2H and HyFlex CM were the lowest, differing only marginally (Figure 2F). A low stiffness indicates lower resistance to continuous clockwise rotation. Stiffness was found to be positively correlated with the bending resistance (\(p<0.001\), Pearson's r = 0.829), which is a measure of the resistance to bending in the direction perpendicular to the long axis of the NiTi file. V Taper 2H and HyFlex CM underwent a two-phase transformation during cooling, as indicated by their DSC curves (Figure 6D,E). The CM-wire NiTi files, V Taper 2H and HyFlex CM, presented low stiffness and high flexibility at 22 \({}^{\circ}\)C owing to their martensite and R-phase composition. At 22 \({}^{\circ}\)C, 2 Shape was primarily composed of austenite. In this study, the flexibility of 2 Shape was found to be higher than that of ProFile and lower than that of the CM-wire NiTi files. The NCF of 2 Shape was comparable to that of ProFile and lower than those of CM-wire NiTi files, which is consistent with the results of a previous study [28]. The flexibility and cyclic fatigue resistance were mostly affected by heat treatment applied during manufacturing. The ARF of 2 Shape was lower than those of ProFile and V Taper 2H. As a superelastic NiTi file, 2 Shape exhibited lower ductility than V Taper 2H. By contrast, the ARF of 2 Shape was larger than those of HyFlex CM, ProfaTaper Gold, and ProTaper Gold. The ARF of 2 Shape is considered to be larger due to its small cross-sectional area, which results from its asymmetrical modified triangular shape.