## 4 Discussion  
When considering cyclic fatigue test results, the experimental conditions must be carefully taken into account as they predetermine their practical significance. In this study, the fatigue tests were performed at the typical intracanal temperature of 35 \({}^{\circ}\)C, which approaches the temperature of the body. Furthermore, the reciprocating motion in the mode RECIPROC ALL (150\({}^{\circ}\) counterclockwise and then 30\({}^{\circ}\) clockwise rotation with a speed of 300 rpm) was used. In general, other operating modes can also be used such as WAVEONE ALL (170\({}^{\circ}\) counter clockwise and then 50\({}^{\circ}\) clockwise rotation with a  
Figure 6: Results of the nanoindentation test performed with a Berkovich indenter at two distances from the apex of the instruments. Values of (**a**) hardness \(H_{r}\) (**b**) reduced elastic modulus \(E\) and ratios of (**c**) \(H/E\) and (**d**) \(H^{3}/E^{2}\) for tested instruments. Dark bars represent test done at the distance of 2 mm and light bars represent test at the distance of 6.4 mm from the apex of the instrument. Figure 7: Normalized indentation creep curves. speed of 350 rpm) [24]. Nevertheless, the literature data comparing cyclic fatigue using both reciprocating motions and converting time to fracture into number of cycles leading to fracture of the instrument [21; 24; 40] does not show any differences in cyclic fatigue resistance [41]. It is well known that high importance should also be given to the trajectory of the instrument when introduced into the tooth/artificial canal, as this can significantly influence results [42]. Within this study, all instruments were tested in the same artificial canal and no statistically significant differences in the length of fractured fragments were observed. This can be considered as a proof of the correct positioning of instruments inside the artificial canal [14; 43], and in turn also that all instruments were confronted by similar stresses, thereby ensuring sufficient reproducibility [44]. Comparison of the fatigue resistance (see Figure 2) clearly shows how peculiarities of different manufacturing processes significantly affect cyclic fatigue resistance [18; 19] and in general also mechanical and tribological properties. It is clear that the Unicone made of M-wire was significantly less resistant to cyclic fatigue in comparison to Reciproc Blue, WaveOne Gold and Unicon Plus, all made of CM-wire; this agrees with other studies comparing effect of phase composition on NiTi-based alloys [14; 20; 21; 22]. Our study also shows that Reciproc Blue is more resistant to cyclic fatigue than WaveOne Gold, similar to previous studies [16; 20]. The higher fatigue resistance of Reciproc Blue compared to WaveOne Gold is mainly attributed to different thermomechanical treatment. Reciproc Blue is ground before appropriate heat-cooling process, leading to the creation of a blue surface layer, while WaveOne Gold is ground after an additional slow heat-cooling process, leading to a gold surface layer made from Ni\({}_{3}\)Ti\({}_{4}\) precipitate [16; 45]. Smaller differences in the cyclic fatigue resistance of Reciproc Blue and Unicone Plus compared to Reciproc Blue to WaveOne Gold may be due to the fact that both Reciproc Blue and Unicone Plus are ground prior to their thermal treatments, unlike WaveOne Gold. In addition, the transformation temperatures and phase composition of the used NiTi alloys are important parameters. It is known that the durability of Reciproc Blue compared to WaveOne Gold is mainly attributed to different thermomechanical treatment, despite having a lower transformation temperature [46]. This is mainly explained by the presence of more stable martensite [13]. Being composed mainly of martensite, these instruments are able to resist cyclic fatigue due to its reorientation properties more than instruments based on austenite, in our case Unicone [13]. Another factor influencing cyclic fatigue resistance is cross-section area.