## 3 Results  
The results of the cyclic fatigue test, represented by the time of operation until fracture, are summarized in Table 1. Reciproc Blue, Unicone Plus and WaveOne Gold instruments exhibited statistically significantly higher cyclic fatigue resistance than Unicone (\(p<0.05\)). Reciproc Blue and Unicone Plus showed statistically significant higher cyclic fatigue resistance than WaveOne Gold (\(p<0.05\)). Although Unicone Plus showed lower cyclic fatigue resistance than Reciproc Blue, the difference was not statistically significant (\(p>0.05\)). For clarity a cumulative frequency plot is shown in Figure 2, representing the percentage of fractures of each instrument at certain times. One can see that all Unicone instruments had failed already before the first instrument in the other groups. All fractures took place over a range of distances from 5.0 mm to 6.3 mm from the apex, with no statistical differences among the instruments. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **WaveOne Gold** & **Reciproc Blue** & **Unicone Plus** & **Unicone** \\ \hline Time to fracture (s) & 214.6 \(\pm\) 20.7 & 262.9 \(\pm\) 35.5 & 245.1 \(\pm\) 32.1 & 78.6 \(\pm\) 11.9 \\ Fracture position (mm) & 5.5 \(\pm\) 0.4 & 5.7 \(\pm\) 0.5 & 5.8 \(\pm\) 0.5 & 5.4 \(\pm\) 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mean of time to fracture of tested samples. The comparison of the transversal cross-section of the instruments reveals their completely different design, as can be seen in Figure 3, showing polished cross-sections at a distance of 6.4 mm from the apex. Both WaveOne Gold and Unicone exhibit the smallest cross-sectional areas, as summarized in Figure 4, for 2.0 mm and 6.4 mm distances from the apex. The ratio of the cross-sectional areas between the two distances of the different tools differs due to their variable conicity. Line roughness was measured using a non-contact procedure on the side of the file. The defined areas are shown in real colours in Figure 5, including the representative surface profiles (out of ten) which were used for roughness calculation. Three samples were measured for each type of endodontic file; the calculated data are summarized in Table 2. It can be seen that Unicone Plus exhibits the lowest values by far in all observed roughness parameters. Unicone and Reciproc Blue files exhibit intermediate roughness values, while the WaveOne Gold shows the highest values for the most used parameters in practice, Ra and Rq (Rms). Besides Ra and Rq (also called RMS), other topographical measures are shown in Table 2, where Rz is the maximum height of profile, Rc is the mean height of profile elements and Rt stands for the total height of profile [39]. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Ra [\(\mu\)m]** & **Rq [\(\mu\)m]** & **Rz [\(\mu\)m]** & **Rc [\(\mu\)m]** & **Rt [\(\mu\)m]** \\ \hline Unicone & 0.29 \(\pm\) 0.08 & 0.39 \(\pm\) 0.12 & 2.22 \(\pm\) 0.64 & 1.28 \(\pm\) 0.35 & 3.47 \(\pm\) 1.54 \\ WaveOne gold & 0.54 \(\pm\) 0.05 & 0.67 \(\pm\) 0.06 & 3.34 \(\pm\) 0.30 & 2.01 \(\pm\) 0.27 & 4.16 \(\pm\) 0.55 \\ Unicone Plus & 0.19 \(\pm\) 0.03 & 0.26 \(\pm\) 0.05 & 1.63 \(\pm\) 0.36 & 0.78 \(\pm\) 0.20 & 2.46 \(\pm\) 0.62 \\ Reciproc Blue & 0.36 \(\pm\) 0.10 & 0.45 \(\pm\) 0.13 & 2.62 \(\pm\) 0.80 & 1.56 \(\pm\) 0.49 & 3.41 \(\pm\) 0.95 \\ \hline \end{tabular}
\end{table} 
Table 2: Roughness parameters on the side wall of the endodontic files. Figure 2: Probability of fracture vs. time to failure sets of ten samples for each evaluated instrument. Figure 4: Surface area measured on two transversal-cross section positions at a distance of (**a**) 2 mm and (**b**) 6.4 mm from the apex of the instruments. Figure 3: Microscopic image of the cross-sections at a distance of 6.4 mm from the apex of the instruments (**a**) Unicone, (**b**) WaveOne Gold, (**c**) Unicone Plus, (**d**) Reciproc Blue. Red squares are used for better comparison of the shape of the instruments.