## 4 Discussion  
The results of the present study show that the NCF of PTN in the dynamic condition is the largest among the three experimental groups. The material, design, and the mode of the dynamic movement of the PTN might be the reasons for the significant difference in NCF among the three groups. Using the distribution analysis in the Minitab, the survival probabilities of PTU and PTN in the dynamic condition are about 80% at the NCF of 360 and 720, respectively. In the static condition, the survival probability of PTU is smaller than 60% at the NCF of only 90. This result supports the concept of dynamic manipulation of endodontic rotary NiTi instrument for enhancing the lifetime of the instrument. The data in each group have normal distribution when analyzed by the distribution function of the Minitab. However, the best distribution in all four distributions should be selected for further analysis using the Minitab. This is an important issue that facilitates and ensures the accuracy of further statistical analysis. The phase when the file is broken seems an important factor under the conditions of the present study. This phenomenon is not mentioned in any previous studies. For the present study, almost all fractures occurred when they were at full length inside the artificial canal. Two PTN files were broken in the out-phase of the dynamic situation. In contrast, the fractures of the PTU files occurred at the full length of the canal or the in-phase in the same dynamic condition. Although this difference is not significant in the present study, this result warns the clinician concerning the manipulation of the instruments in a real clinical setting. The results also reveal that there is only one fracture segment for PTU in both the static and dynamic conditions in the present study, although the double-curved artificial canal was used for the present study. This result differed from previous studies [22; 23; 24]. The oil environment, body temperature, material, and design of the PTU could be the factors that influence and change the results of the present study. This result reveals that  
Figure 3: The SEM surface of PTN in dynamic cycles. There is no striation on the entire image. Figure 2: The SEM surface of PTU in dynamic cycles. There is no striation on the entire image. the two-millimeter apical third of the PTU and PTN hardly broke in the condition of the present study. The regression with life data shows that the NCF of the PTN was significantly higher than that of PTU because of \(p<0.05\), and the coefficient was positive. The regression table also reveals that the dynamic movement of the file elongated the NCF of the instrument much longer than the static condition because of \(p<0.05\), and the coefficient was also positive. This result supports the concept of the dynamic movement of an endodontic instrument whenever it is inside the root canal. The reliability/survival function in the Minitab is an alternative approach for the statistical analysis method of the cyclic fatigue resistance experiment. The failure time in this kind of investigation is the exact recorded time when the instrument failed, not right censoring or arbitrary censoring, as identified in the Minitab guidelines. This statistical method offers the options of four distributions. It analyzes the data as identified by the user based on the lowest Anderson-Darling value from the analysis results. The regression with life data offers significant differences, if present, between the different brands of products and between the static or dynamic situations when the experiment is performed. \(p\)-values and positive or negative coefficients are important values for determining the differences between the experimental groups. The time recorded from the beginning of rotation to the fracture of the instrument seems more suitable and significant than the RPM to the clinician. However, because of the difference in the RPM for each unique instrument system, the manipulation of time-to-fracture is not fair for an instrument with a higher RPM.