**3. Results**

SEM analysis of the T Pro E2 NiTi endodontic rotary files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China) showed accumulation of organic matter, but none of the NiTi endodontic rotary files showed relevant structural alterations. Moreover, manufacturing lines were distributed perpendicularly to the longitudinal axis in all of the endodontic rotary files and also parallel to each other due to the manufacturing process by laser machining. The width and spacing of the manufacturing lines and tubular porosity correspond to the precision and intensity of the laser machining process. In addition, the macroscopically geometrical design of the double S-shaped cross-section of Mtwo NiTi alloy endodontic rotary files (Ref.: 0236025025, Mtwo, VDW, Munich, Germany) showed a higher pitch than the rectangular cross-section of T Pro E1 NiTi endodontic files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China), the convex triangular cross-section of T Pro E2 NiTi endodontic files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China), and the triangular cross-section of T Pro E4 NiTi endodontic files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China).

EDX micro-analysis of the double S-shaped cross-section of Mtwo NiTi alloy endodontic rotary files (Ref.: 0236025025, Mtwo, VDW, Munich, Germany), the rectangular cross-section of T Pro E1 NiTi endodontic files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China), the convex triangular crosssection of T Pro E2 NiTi endodontic files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China), and the triangular cross-section of T Pro E4 NiTi endodontic files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China) was performed at 20 kV as this allowed a deeper analysis of the NiTi endodontic rotary files surface. In summary, the double S-shaped cross-section of Mtwo NiTi alloy endodontic rotary files (Ref.: 0236025025, Mtwo, VDW, Munich, Germany) differs in the chemical elements present in the metal alloy, in accordance with the rectangular cross-section of T Pro E1 NiTi endodontic files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China), the convex triangular cross-section of T Pro E2 NiTi endodontic files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China), and the triangular cross-section of T Pro E4 NiTi endodontic files (Ref.: 20010103, T Pro E1, Perfect Endo, Shenzhen Perfect Medical Instruments, Shanwei City, China), which include aluminum in the chemical composition of the metal alloy (Table 1).

**Table 1.** Mean atomic weight percent (%) of Mtwo NiTi alloy endodontic rotary files, T Pro E1 austenite phase NiTi alloy endodontic rotary files, T Pro E2 austenite phase NiTi alloy endodontic rotary files, and T Pro E4 austenite phase NiTi alloy endodontic rotary files.


The mean and SD values for the time to failure (in seconds) for each of the study groups are displayed in Table 2 and Figure 4.


**Table 2.** Descriptive statistics of the time to failure of Mtwo NiTi alloy endodontic rotary files, T Pro E1 austenite phase NiTi alloy endodontic rotary files, T Pro E2 austenite phase NiTi alloy endodontic rotary files, and T Pro E4 austenite phase NiTi alloy endodontic rotary files.

a,b,c,d Statistically significant differences between groups (*p* < 0.05).

**Figure 4.** Box plot of the time to failure of Mtwo NiTi alloy endodontic rotary files, T Pro E1 austenite phase NiTi alloy endodontic rotary files, T Pro E2 austenite phase NiTi alloy endodontic rotary files, and T Pro E4 austenite phase NiTi alloy endodontic rotary files.

The ANOVA test showed statistically significant differences between the time to failure of all NiTi endodontic rotary files (*p* < 0.001) (Figure 4). The results related to the number of cycles to failure are similar as the dynamic cyclic fatigue device had a frequency of 60 pecking movements/min.

The scale distribution parameter (η) of the Weibull statistical analysis found statistically significant differences between the time to failure of all NiTi endodontic rotary files (*p* < 0.001) (Table 3, Figure 5). However, the shape distribution parameter (β) of the Weibull analysis found no statistically significant differences between the time to failure of any of the NiTi endodontic rotary files (*p* > 0.05). The results related to the number of cycles to failure are similar as the dynamic cyclic fatigue device had a frequency of 60 pecking movements/min (Table 3, Figure 5).


**Table 3.** Weibull statistics of time to failure of replica-like and original brand NiTi endodontic rotary files study groups.

**Figure 5.** Weibull probability plot of time to failure of Mtwo NiTi alloy endodontic rotary files, T Pro E1 austenite phase NiTi alloy endodontic rotary files, T Pro E2 austenite phase NiTi alloy endodontic rotary files, and T Pro E4 austenite phase NiTi alloy endodontic rotary files.
