## 4 Discussion  
The results obtained in the present study reject the null hypothesis (H\({}_{0}\)) which stated that the apical diameter and taper would not affect the resistance of NiTi endodontic rotary files to dynamic cyclic fatigue. Research reported a prevalence of fractures of NiTi endodontic rotary files between 0.9% [26] and 5% [27], and also highlighted the influence of this intraoperative complication on the outcome of root canal treatment. In addition, [PERSON] stated that the fracture of an endodontic instrument inside a root canal system demonstrated a further significant periodical pathology decrease in the success rate of the root canal treatment [28]. [PERSON] highlighted the importance of bacterial reduction during the cleaning and shaping procedures of a root canal treatment on the prognosis of the endodontic therapy, and reported that negative microbiological cultures obtained from the root canal system led to an endodontic success rate close to 94%, whereas positive cultures reduced the success rate to 68% [29]. [PERSON] pointed out that the persistent bacterial load is the main aetiology factor of endodontic failure and secondary endodontic infections [30]. This is the reason why the cyclic fatigue resistance of NiTi endodontic rotary files has been widely analyzed. Previous studies were conducted to analyze the resistance of both NiTi endodontic rotary and reciprocating instruments to different conditions related to cyclic fatigue [31]. However, the  
Figure 11: Box plots of the length of fractured files regarding the (**A**) apical diameter and (**B**) taper study groups. The horizontal line in each box represents the respective median value. Figure 12: The Weibull probability plots of the length of fractured files regarding the (**A**) apical diameter and (**B**) taper study groups. absence of a normative which regulates the cyclic fatigue tests of the NiTi endodontic rotary and reciprocating files led to the appearance of a heterogeneous multitude of cyclic fatigue test devices, which makes comparison of the results difficult [32]. In the present study, a dynamic cyclic fatigue device was used because it reproduces the operator's movements more accurately, and the results can be extrapolated to the clinical situation [33]. [PERSON] et al., showed that the 25.06 NiTi endodontic reciprocating files (ProDesigner R, Easy, Belo Horizonte, Brazil) presented higher (\(p>0.05\)) cyclic fatigue resistance and angular rotation before fracture compared to 25.08 Ni Ti endodontic reciprocating files (Reciproc, VDW, Munich, Germany) and 25.07 (WaveOne Gold, Dentsply Sirona, Ballaigues, Switzerland) [34]. However, the different cross-section design, crystalline structure of the NiTi alloy and counterclockwise direction could influence the results obtained in this study. The NiTi endodontic rotary system used in this study was selected because it provided NiTi endodontic rotary files with different apical diameters while maintaining the cross-sectional design, NiTi alloy crystal structure, and taper, as well as providing NiTi endodontic rotary files with different tapers while maintaining the same apical diameter, cross-sectional design, and crystal structure of the NiTi alloy. In addition, the selection of an NiTi endodontic rotary system instead of a reciprocating system was because the reciprocating movement associated with single-file systems has been shown to extend the lifetime of NiTi endodontic rotary files compared with continuous rotation, thus increasing the cyclic fatigue resistance of the reciprocating files [35]. Even the metallurgical characteristics and thermal treatments of the NiTi alloy influenced the flexibility and cyclic fatigue resistance of NiTi endodontic files [36, 37]. For this reason, a conventional NiTi wire alloy endodontic rotary system was also selected.