## 4 Discussion  
Despite both flexural and torsional stress occurring simultaneously, very few studies have compared these aspects using both tests [25; 26]. It has been demonstrated that cyclic flexural loading significantly decreased the torsional resistance of heat-treated rotary files; previous studies also showed that adding torsional loads can move the location of the flexural fracture in the direction of the additionally applied torsional stress [27; 28]. The torsional resistance was assessed by adopting a technique that is derived from previous studies [28]. The main difference from the traditional ISO standardized test is that the speed in the present study was the clinical one (300 rpm), while ISO tests are conducted at 2 rpm. This protocol allowed for a more clinically relevant analysis of the time needed to reach the fracture limit of the NiTi rotary files. For the cyclic fatigue test, a static method was used because, in the dynamic fatigue tests, the amplitude of the motions could affect the results. When measuring the torsional resistance, files were tested under very stressful conditions, and the instrument was locked close to the tip. Thus, for the bending tests, the choice was made to subject the files to a similar stressful simulation, using both a severe curvature and a static test [29]. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Ttf (s) (SD)** & **FL (mm) (SD)** \\ \hline EdgeTaper & 19.50 (\(\pm\)2.45) & 3.15 (\(\pm\)0.71) \\ Protaper Universal & 10.905 (\(\pm\)2.87) & 3.05 (\(\pm\)0.93) \\ EdgeTaper Platinum & 88.43 (\(\pm\)2.53) & 2.95 (\(\pm\)0.37) \\ ProTaper Gold & 43.99 (\(\pm\)1.81) & 3 (\(\pm\)0.35) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mean and standard deviation of time to fracture (Ttf) and length of the fractured fragments (FL). \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Torque Limit (Ncm) (SD)** & **Time to Fracture (s) (SD)** \\ \hline EdgeTaper & 1.08 (\(\pm\)0.04) & 0.24 (\(\pm\)0.06) \\ Protaper Universal & 1.01 (\(\pm\)0.02) & 0.20 (\(\pm\)0.04) \\ EdgeTaper Platinum & 0.93 (\(\pm\)0.09) & 0.30 (\(\pm\)0.08) \\ ProTaper Gold & 0.97 (\(\pm\)0.01) & 0.22 (\(\pm\)0.06) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean and standard deviation of torque to fracture and time to fracture. As shown in the results, the heat treatment processes significantly increased the cyclic fatigue resistance of the NiTi rotary files, while slightly decreasing the torsional resistance. These findings are in accordance with many previously published articles. Such a big improvement in the cyclic fatigue resistance (more than 100%) may have a clinical relevance, even if endodontic rotary files are usually used in vivo for a shorter amount of time and a lower number of cycles. Another advantage of the heat treatment--the increase in the instruments' flexibility--surely adds more clinical benefits by making the instrumentation easier and reducing the risk of iatrogenic errors. On the other hand, the torsional resistance was decreased by the heat treatment. These findings are in accordance with other studies that have locked the instruments at 3 mm from the tip [30]. Nevertheless, for all the tested instruments, the mean values of the torque at the point of failure were significantly lower (ranging from 0.93 to 1.08 Ncm) than the torque values recommended by the manufacturers (usually 2-2.5 Ncm). As a consequence, if an instrument is locked inside the canal, the motor will, in any case, provide  
Figure 4: SEM analysis. Flexural fracture evidenced by the fatigue striations. Figure 3: SEM analysis. Torsional fracture evidenced by the round shaped circumferential lines. enough torque to break the file. Interestingly, this could happen in a few seconds, as recorded in this study, for all the instruments. No statistically significant difference in the time to fracture was found between the four tested instruments, showing that the influence of the heat treatment on torsional resistance is probably less relevant.