## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**Cooling (Exothermic) Curve**} \\ \hline \(\mathbf{R_{s}}\) (\({}^{\circ}\)C) & \(\mathbf{R_{f}}\) (\({}^{\circ}\)C) & \(\Delta\)H (J/g) & \(\mathbf{M_{s}}\) (\({}^{\circ}\)C) & \(\mathbf{M_{f}}\) (\({}^{\circ}\)C) & \(\Delta\)H (J/g) \\ \hline ProFile & & & 10.13 & –13.54 & 1.98 \\ \hline ProTaper Gold & & & 40.55 & 19.79 & 4.68 \\ \hline ProfaTaper Gold & & & 43.52 & 29.17 & 3.36 \\ \hline HyFlex CM & 36.40 & 21.35 & 4.05 & –18.84 & –46.35 & 7.63 \\ \hline V Taper 2H & 27.89 & 14.58 & 4.22 & –25.95 & –53.44 & 9.36 \\ \hline ZShape & & & 16.00 & –11.46 & 2.07 \\ \hline K3 XF & & & 18.11 & 6.77 & 3.07 \\ \hline \hline \end{tabular}  
* \(\mathbf{R_{s}}\) and \(\mathbf{R_{f}}\) are the start and finish temperatures, respectively, of the R-phase transformation. \(\mathbf{A_{s}}\) and \(\mathbf{A_{f}}\) are the start and finish temperatures, respectively, of the austenitic transformation. \(\mathbf{M_{s}}\) and \(\mathbf{M_{f}}\) are the start and finish temperatures, respectively, of the martensitic transformation.
* represents the total energy of the martensite to R-phase and R-phase to austenitic transformations.  
\end{table} 
Table 3: _Cont._K3 XF exhibited the highest bending resistance, but its buckling resistance was low (Figure 2A,B). K3 XF has a larger number of threads and a constant 6% taper. The volume of the K3 XF file in the 3 mm tip is lower than those of ProTaper Gold and ProfaTaper Gold. The small volume of K3 XF possibly resulted in its lower buckling resistance. NiTi file fracture occurs because of cyclic and torsional fatigue. Cyclic fatigue fractures result from the repetitive compressive and tensile stresses that occur when the NiTi file is rotated inside a curved canal [41]. Torsional fractures occur when the file tip is locked in a canal while the shaft continues to rotate [41]. Cyclic fatigue resistance is related to the flexibility of the NiTi file [18]. According to the results of the Pearson correlation analysis, the bending resistance and NCF had a strong negative correlation, indicating that the more flexible the NiTi file, the more resistant it was to cyclic fatigue fracture. HyFlex CM had the highest NCF, followed by ProfaTaper Gold and V Taper 2H (Figure 2C), which is consistent with previous research findings that martensitic NiTi files, such as CM-wire NiTi files, exhibit superior cyclic fatigue resistance [24; 25; 26; 27; 28; 29]. In this study, K3 XF exhibited the lowest NCF, even lower than that of ProFile, the non-heat-treated NiTi file (Figure 2C). K3 XF was mostly in its austenite phase during the cyclic fatigue resistance test, which was performed at 22 \({}^{\circ}\)C (Figure 6G). The cross-sectional area of K3 XF was larger than that of ProFile (Figure 4A,G), which contributed to its lower cyclic fatigue resistance. A previous study using NiTi files with 4% taper and #25 tip size reported that the cyclic fatigue resistance of K3 XF is comparable to that of ProFile and lower than that of HyFlex CM at 22 \({}^{\circ}\)C and 37 \({}^{\circ}\)C, respectively [42]. The disparity between the results of the previous study and our study was caused by the different sizes of the tested NiTi files. Torsional fracture resistance is defined by both the ARF and UTS of a material. V Taper 2H had the largest ARF (Figure 2D) despite having the lowest bending and buckling resistances (Figure 2A,B). According to its torque versus rotation angle plot, V Taper 2H underwent significant plastic deformation after elastic deformation (Figure 3). Overall, V Taper 2H was the least resistant to lateral and vertical compressive loads, but more resistant to continuous rotation stresses. Furthermore, V Taper 2H had the second highest UTS, while HyFlex CM, another brand of the CM-wire NiTi file, had the lowest. The superior torsional resistance of the V Taper 2H file is presumably due to design features such as a larger number of threads and a modified triangular cross-section [43].