## 4 Discussion
It is possible to speculate that the different behavior of the two files depends on the new proprietary thermal treatment employed to manufacture the OC files (C-Wire). Indeed, F6 ST files are made from a conventional austenite 55-nickel-titanium alloy which shows an austenite finish temperature (A\({}_{\text{f}}\)) below the body temperature [28; 29], while OC files are constituted of a supposed control memory NiTi wire exhibiting an A\({}_{\text{f}}\) between 40 \({}^{\circ}\)C and 50 \({}^{\circ}\)C [6]. Therefore, the gradual transition to the austenitic phase is more pronounced at 37 \({}^{\circ}\)C for F6 ST files in comparison with OC ones. The negative impact of increased temperature could overcome the irrigant benefits for F6 ST instruments [30]. Differently from our outcomes, a previous study reported that at body temperature, NaOCl did not significantly improve the cyclic fatigue resistance of OC [31]. This difference is likely a consequence of dissimilar methodological conditions, including the NaOCl concentration tested (2.5% vs. 5.25%) and experimental apparatus (stainless steel block immersed in 2.5% NaOCl solution vs. continuous NaOCl flow). It is difficult to compare NiTi instruments with different features (geometrical and metallurgical). Nevertheless, in agreement with previous findings, the fatigue life of heat-treated NiTi files (OC) was longer than non-heat-treated files (F6 ST) in all testing conditions. This behavior is probably due to the greater flexibility of OC files caused by the heat treatment [6; 32; 33]. The principal novelty and strength of the apparatus described is the possibility to activate instruments simultaneously to the irrigant flow. Even if the previous studies investigated the corrosive behavior of sodium hypochlorite on NiTi files, methodological conditions did not allow a good simulation of the real clinical situations. Indeed, fatigue tests were performed after a previous exposure of NiTi files to NaOCl for non-standardized time periods [17; 34; 35] or while the instruments were immersed in low NaOCl concentrations [36; 37]. Both scenarios could induce a prolonged or reduced contact between instrument and NaOCl solution. Moreover, the new customized apparatus ensured standardized methodological conditions, thus allowing the reproducibility of tests performed. In addition, a zirconium artificial canal was adopted for cyclic fatigue tests. Zirconia appears to be a suitable material because it possesses mechanical properties similar to stainless steel but avoids the potential corrosion phenomena of the metallic parts used [14; 38; 39]. Furthermore, the present study provides new relevant information about influence of irrigants on the cyclic fatigue resistance of F6 ST and OC instruments, which are both single-file systems widely used in clinical practice. Despite these advantages, the clinician should apply these results in clinical condition with attention. First of all, in vitro conditions vary from intracranial instrumentation, in which the fracture is the result of numerous factors such as torsional stress [40], anatomic canal configuration and the kind of instrument used [20]. Although methodology ensured a standard approach, instruments could be subjected to different friction levels due to the configuration of their blades during clinical use [41]. In addition, a possible comparison between the two files requires caution because cyclic fatigue resistance is influenced by several factors [6; 12; 20], including geometrical variables such as cross section (a constant S-shaped for F6 ST and triangular for OC at 5 mm from the tip) [12; 28], pitch width and rake angle as well as metallurgical features. Lastly, the type of irrigant used as well as its concentration and usage conditions (such as temperature and flow rate) may influence results obtained.