## 4 Discussion  
Simulated root canals in training endo-blocks are characterized by similar parameters. Therefore, it can be assumed that the experiment environment in which they are used provides uniform, reproducible features for a single operator. The only variable seems to be a type of the instrument used for canals' preparation. However, the fact that endo-blocks are made of acrylic resin, this might be a limitation because material's mechanical properties are different from those of dentin. As a result, it is speculated whether the obtained results could be also validated for clinical outcomes prognosis. [PERSON] following [PERSON], described that while the canals are being prepared with the rotary instruments, the generated heat can result in resin softening and blocking of instruments' cutting surfaces, subsequently leading to ineffective instrumentation [8]. However, [PERSON] et al. obtained matched results in simulated endo-blocks also assessing the instruments interaction in human teeth [5]. Analyzing the advantages and disadvantages of utilizing training endo-blocks and natural hard tooth tissue, it is not recommended to classify the endo-blocks as \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Endodontic Instrument Type**} & \multicolumn{2}{c}{**Angles’ Curvature Length**} & \multirow{2}{*}{**Test _t (p)_**} & \multirow{2}{*}{**Difference**} & \multirow{2}{*}{**ANOVA (p)**} & \multicolumn{4}{c}{**Test NIR (_p_)**} \\ \cline{2-2} \cline{6-9}  & **before** & & & & & **MT** & **WO** & **K** & **PN** \\ \hline MT & 1.56 \(\pm\) 0.01 & 1.75 \(\pm\) 0.03 & \textless{}0.0001 & 0.19 \(\pm\) 0.03 & \multicolumn{2}{c}{——} & 0.0460 & \textless{}0.0001 & 0.2812 \\ WO & 1.61 \(\pm\) 0.12 & 1.85 \(\pm\) 0.17 & \textless{}0.0001 & 0.24 \(\pm\) 0.10 & \textless{}0.0001 & 0.0460 & \textasci{}0.0001 & 0.3373 \\ K & 1.58 \(\pm\) 0.06 & 1.63 \(\pm\) 0.10 & 0.0224 & 0.05 \(\pm\) 0.05 & \textless{}0.0001 & \textless{}0.0001 & \textasci{}0.0001 & \textasci{}0.0001 \\ PN & 1.57 \(\pm\) 0.02 & 1.79 \(\pm\) 0.04 & \textless{}0.0001 & 0.22 \(\pm\) 0.05 & & 0.2812 & 0.3373 & \textless{}0.0001 & — \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The mean (X) and standard deviation (SD) of angles’ curvature length before and after their mechanical preparation, and the differences (after-before). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Endodontic Instrument Type**} & \multicolumn{2}{c}{**Angle’ Curvature**} & \multirow{2}{*}{**Test _t (p)_**} & \multirow{2}{*}{**Difference**} & \multirow{2}{*}{**ANOVA (p)**} & \multicolumn{4}{c}{**Test NIR (_p_)**} \\ \cline{2-2} \cline{6-9}  & **before** & & & & & **MT** & **WO** & **K** & **PN** \\ \hline MT & 44.80 \(\pm\) 0.42 & 50.10 \(\pm\) 0.86 & \textless{}0.0001 & 5.30 \(\pm\) 0.83 & \multicolumn{2}{c}{——} & 0.0460 & \textless{}0.0001 & 0.2812 \\ WO & 46.00 \(\pm\) 33.00 & 53.00 \(\pm\) 4.83 & \textless{}0.0001 & 7.00 \(\pm\) 2.87 & \textless{}0.0001 & 0.0460 & \textasci{}0.0001 & 0.3373 \\ K & 45.30 \(\pm\) 1.77 & 46.60 \(\pm\) 2.84 & 0.0224 & 1.30 \(\pm\) 1.49 & \textless{}0.0001 & \textless{}0.0001 & \textasci{}0.0001 & — & \textless{}0.0001 \\ PN & 45.10 \(\pm\) 0.57 & 51.30 \(\pm\) 1.42 & \textless{}0.0001 & 6.20 \(\pm\) 1.55 & & 0.2812 & 0.3373 & \textless{}0.0001 & — \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The mean (X) and standard deviation (SD) of angles’ curvature before and after their mechanical preparation, and the differences (after vs before). an optimal material for pre-clinical research regarding the comparative analysis of canals shaping with of endodontics instruments in vitro. While for canal preparation comparison it is important to reach the same size at the apex [6]. In this research simulated canal was enlarged in the apical diameter to size ISO 25. The same value was set up in the experiments conducted by other authors [7; 17]. None of the canals was blocked with the resin debris.