## 3 Results  
### Mechanical Test Results  
The means and standard deviations of the mechanical test results are presented in Table 2 and Figure 1. Spring H had the greatest bending resistance, followed by Spring Endo (\(p<0.05\)). The buckling resistance of Spring Endo and Spring H files was the highest (\(p<0.05\)). XP-endo Shaper showed the highest NCF, followed by TruNatomy Prime (\(p<0.05\)). The UTS of the Spring Endo and Spring H files was more than that of the TruNatomy Prime and XP-endo Shaper files. The ARF of the XP-endo Shaper was the highest, followed by TruNatomy Prime. Representative graphs of torsional strength and angle of rotation during the torsional resistance test are shown in Figure 2. EM was obtained from the slope of the initial linear portion of the graph. Spring Endo had the highest EM, followed by Spring H. XP-endo Shaper demonstrated the lowest bending and buckling resistances, UTS, and EM, along with the highest NCF and ARF. \begin{table}
\begin{tabular}{c c c c c} \hline  & **TruNatomy Prime** & **XP-Endo Shaper** & **Spring Endo** & **Spring H** \\ \hline Bending resistance (Ncm) & 0.125 \(\pm\) 0.026 \({}^{\rm a}\) & 0.106 \(\pm\) 0.016 \({}^{\rm a}\) & 0.544 \(\pm\) 0.063 \({}^{\rm b}\) & 0.643 \(\pm\) 0.074 \({}^{\rm c}\) \\ \hline Buckling resistance (gf) & 226.34 \(\pm\) 23.69 \({}^{\rm b}\) & 103.99 \(\pm\) 17.64 \({}^{\rm a}\) & 599.28 \(\pm\) 86.16 \({}^{\rm c}\) & 614.35 \(\pm\) 97.51 \({}^{\rm c}\) \\ \hline NCF & 712.81 \(\pm\) 54.49 \({}^{\rm b}\) & 5344.46 \(\pm\) 665.86 \({}^{\rm c}\) & 376.04 \(\pm\) 60.11 \({}^{\rm a}\) & 433.20 \(\pm\) 52.34 \({}^{\rm a}\) \\ \hline UTS (Ncm) & 0.540 \(\pm\) 0.090 \({}^{\rm a}\) & 0.471 \(\pm\) 0.037 \({}^{\rm a}\) & 1.019 \(\pm\) 0.135 \({}^{\rm b}\) & 1.217 \(\pm\) 0.192 \({}^{\rm b}\) \\ \hline ARF (degrees) & 654.71 \(\pm\) 83.14 \({}^{\rm c}\) & 1834.09 \(\pm\) 176.82 \({}^{\rm d}\) & 417.41 \(\pm\) 42.94 \({}^{\rm a}\) & 515.33 \(\pm\) 62.85 \({}^{\rm b}\) \\ \hline EM (Ncm/degrees) & 0.00149 \(\pm\) 0.00022 \({}^{\rm b}\) & 0.00056 \(\pm\) 0.00006 \({}^{\rm a}\) & 0.00961 \(\pm\) 0.00089 \({}^{\rm d}\) & 0.00597 \(\pm\) 0.00048 \({}^{\rm c}\) \\ \hline \end{tabular}
\begin{tabular}{c c c c} \hline  & Different superscript letters in the same row indicate significant differences between NiTi files (\(p<0.05\)). NCF, number of cycles to fracture; UTS, ultimate torsional strength; ARF, angle of rotation to fracture; EM, elastic modulus. \\ \end{tabular}
\end{table} 
Table 2: Mechanical test results (mean \(\pm\) standard deviation). Figure 1: Mechanical test results of TruNatomy Prime, XP-endo Shaper, Spring Endo, and Spring H. files. Different lowercase letter indicates statistically significant differences between files. Bending resistance (**A**), buckling resistance (**B**), NCF from cyclic fatigue resistance test (**C**), UTS (**D**), ARF (**E**), and EM (F) from torsional resistance test. The FE-SEM images of fractured instruments from the cyclic fatigue resistance test presented typical patterns of fatigue fracture, such as multiple fatigue striations located near the cracks (Figure 3). The FE-SEM images of the fractured instruments from the torsional resistance test showed a torn-off appearance at the center of the fractured surfaces (Figure 4). The cyclic fatigue and torsional resistance tests revealed brittle and ductile fracture patterns on lateral view, respectively (Figures 3 and 4). Figure 2: Representative torque–angle of rotation curves of the tested files based on the torsional resistance test results. The results of the Pearson correlation analysis are shown in Table 3. The bending and buckling resistances, UTS, and EM had strong positive correlations with each other (\(p\) < 0.001, Pearson's r = 0.848-0.932). A strong positive correlation was observed between the NCF and ARF (\(p\) < 0.001, Pearson's r = 0.961).