## 3 Results  
The tissue destruction zone was significantly lower when saline was used to cool down the drill in all three drill rotation speeds and both drilling sites (apex: 800 rpm: \(p\) = 0.0001; 1200 rpm: \(p\) < 0.0001; 1500 rpm: \(p\) < 0.0001; side: 800 rpm: \(p\) = 0.0001; 1200 rpm: \(p\) = 0.0002; 1500 rpm: \(p\) < 0.0001). A post-hoc analysis showed no differences between 20 \({}^{\circ}\)C and 3 \({}^{\circ}\)C saline, regardless of drill rotation speed and drilling site (Figure 1). Spearman correlation coefficients between the actual temperature and tissue destruction were calculated for each drilling site, drilling speed, and cooling method (Table 2). The analysis of microanatomical structure showed the different morphological changes in samples referring to the drilling speed and cooling method used. During drilling, with a speed of 800 rpm and cooling liquid at temperature 3 \({}^{\circ}\)C, the zone of compressed bone tissue was visible (Figure 2). Simultaneously, the same speed of drilling without any cooling caused the moderate destruction of bone tissue and bone marrow (Figure 3). The microanatomical picture of changes generated by drilling with a speed of 1200 rpm (cooling liquid at 20 \({}^{\circ}\)C) showed the apical zone of rib with visible connective tissue without any areas of strong tissue compression (Figure 4). The same drilling speed, but with a cooling method at 3 \({}^{\circ}\)C, caused total destruction of the bone tissue and bone marrow. The empty bone marrow cavity was clearly distinguished from other parts of the rib's wall (Figure 5). Finally, the drilling speed of 1500 rpm resulted in the lack of strong compression of the bone tissue and the apical area of rib filled with the connective tissue (Figure 6), when any cooling system was not used. In contrast, the use of cooling liquid at 20 \({}^{\circ}\)C changed the result of drilling, causing total bone tissue and bone marrow destruction and the presence of damaged trabecular architecture of bone with empty bone marrow cavity (Figure 7). The history of the damage zone at the wall of the drilling canal (Figure 8) and the thickness of compressed bone tissue at the top of drill (Figure 9) proved that the use of an intermediate speed of drilling (1200 rpm) was the least traumatic for the model. \begin{table}
\begin{tabular}{c c c} \hline
**Measurement** & \(\mathbf{p}\)**-Value** & \(\mathbf{r}\)**-Value** \\ \hline apex/800/without cooling & 0.0004 & 0.95 \\ \hline apex/800/with cooling & 0.00006 & 0.8 \\ \hline Apex/1200/without cooling & 0.00005 & 0.983 \\ \hline apex/1200/with cooling & 0.0006 & 0.731 \\ \hline apex/1500/without cooling & 0.0007 & 0.929 \\ \hline apex 1500/with cooling & 0.0007 & 0.724 \\ \hline side/800/without cooling & 0.0002 & 0.967 \\ \hline side/800/with cooling & 0.0003 & 0.76 \\ \hline side/1200/without cooling & 0.0007 & 0.933 \\ \hline side/1200/with cooling & 0.077 & 0.428 \\ \hline side/1500/without cooling & 0.0004 & 0.946 \\ \hline side/1500/with cooling & 0.00003 & 0.901 \\ \hline \end{tabular}
\end{table} 
Table 2: Spearman correlation coefficients between the actual temperature and tissue destruction. Apex—at apical part of drill; Side—at lateral part of drill. Figure 2: Rotation of 800 rpm (3 \({}^{\circ}\)C). The apical area of rib filled with connective tissue. Visible area of tissue compression caused by the drill (arrow). Figure 4: Rotation of 1200 rpm (20 \({}^{\circ}\)C). The apical area of rib filled with connective tissue. The lack of strong tissue compression areas caused by the drill. Figure 5: Rotation of 1200 rpm (3 \({}^{\circ}\)C). Total destruction of bone tissue and bone marrow. Visible empty bone marrow cavity (arrow) clearly distinguished from other parts of unchanged rib tissue. Figure 3: Rotation of 800 rpm (without cooling). Moderate destruction of the bone tissue and bone marrow. Visible debris of bone trabecular architecture and bone marrow (arrow). Figure 8: The thickness of the damage zone in the tested drilling speeds [\(\mu\)m].