## 4 Discussion  
On the basis of the results obtained, increased temperature negatively affected the cyclic fatigue resistance of the tested instruments, as previously reported [6; 8; 9; 14]. Consequently, the null hypothesis was rejected. The phase constitution of NiTi files at a specific temperature assumes a key role in defining their mechanical properties [24]. At higher temperatures than the transformation level, the NiTi alloy exhibits an austenite structure which is more rigid, whereas it possesses a martensite structure, which is more flexible, at lower temperatures [5]. Therefore, the progressive reduction in lifespan of the NiTi instruments tested at higher temperatures could be caused by the progressive transition to an austenite phase [6]. At 20 \({}^{\circ}\)C, NaOCl increased cyclic fatigue resistance of F6 ST and OC files, with liquid formulation performing significantly better than gel solution for F6 ST. It is possible to hypothesize that this result is due to wetting properties of the irrigant solution, which could reduce flexural stress of NiTi files; further research is needed to confirm these hypotheses. The outcomes obtained are in contrast with those of [PERSON] et al. who reported that NaOCl negatively affected the fatigue behavior of NiTi instruments [7]. These differences are probably due to the different experimental procedures. Indeed, in that study, the block containing the artificial canal was fixed inside a recipient that was filled with 2.5% NaOCl or 5.25% NaOCl. This condition is different from our methodological set-up, in which NaOCl flow occurs in the artificial canal simultaneously to the fatigue test. Moreover, an artificial metallic canal was used, while in the current study a ceramic canal, which reduces the risk of corrosive phenomena, was employed. In addition, a different type of  
Figure 3: (**A–L**) Scanning electron micrographs of the fracture surface of F6 SkyTaper and OneCurve fragments in the axial view (450 \(\times\) -600 \(\times\) magnification) after the cyclic fatigue tests in the different experimental conditions. The white arrows indicate the origins of the crack, and the dotted area indicates the final abrupt breakage. The typical surface pattern of the fibrous fatigue zone showed dimples and cones in all of the fracture surfaces. instrument (i.e., ProTaper Gold) with metallurgical properties different from those of F6 ST and OC was chosen. On the other hand, [PERSON] et al. reported 5.25% NaOCl did not significantly influence the cyclic fatigue resistance of NiTi files. In a similar way to the present study, [PERSON] et al. used an artificial ceramic canal, avoiding the potential corrosion of the test models made by metallic parts [14]. In addition, it is reasonable to think that liquid solutions are more effective than gel probably due to the wetting properties and penetration ability of liquid formulation [25]. Indeed, the modification of the consistency solution modifies its resistance to deformation (the so-called viscosity), wettability and surface tension. Specifically, gel components provide high viscosity to the formulation, reducing its capacity to flow in comparison with liquid [25; 26]. However, no significant difference was observed between liquid and gel formulation for OC at either temperature. These results are probably due to the differences in time to fracture between the two files. Indeed, OC instruments showed higher TtF in comparison with F6 ST ones, at both temperatures, probably due to the advantages provided by its heat treatment [6; 12; 27]. Consequently, OC files remain active for more time; it is plausible to suppose that the potential differences between gel and liquid NaOCl formulations are not such as to generate significant variations for prolonged periods. At 37 \({}^{\circ}\)C, no significant difference was observed among the control and experimental groups for F6 ST. However, hypochlorite significantly improved TtF of OC files.