## 4 Discussion  
Cutting efficiency is an important mechanical property because it affects the ability of instrumentation to safely prepare the root canal system [14]. Several factors affect the cutting ability of NiTi instruments, such as rake angle and helical angle, depth and number of grooves, cross-sectional area and design (which appears to be a decisive feature of the tip), chip elimination ability, hardness, kinematics, and manufacturing processes, including heat treatments [1; 14]. An additional brushing motion could also modify the cutting behavior of NiTi files [14]. This study aimed to evaluate the effect of brushing motion on the cutting efficiency of 2 Shape and Hyflex CM rotary systems. 2 Shape and Hyflex CM are two heat-treated files with a similar design but a different heat treatment [15; 16]. To evaluate the cutting efficiency, the use of a suitable substrate is necessary. Acrylic and plexiglass blocks with simulated canals were employed to determine cutting efficiency and other mechanical properties of endodontic files. In the current investigation, the human dentin (i.e., extracted mandibular \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Root Canal Volume (mm\({}^{3}\))**} \\ \hline  & **Groups** & **Mean** & **SD** \\ \hline \multirow{2}{*}{**Volume pre**} & **2 Shape** & 5.1348 \({}^{\text{a}}\) & 1.89759 \\  & **Hyflex CM** & 6.6940 \({}^{\text{a}}\) & 4.09832 \\ \hline \multirow{2}{*}{**Volume post**} & **2 Shape** & 5.7057 \({}^{\text{a}}\) & 1.84434 \\  & **Hyflex CM** & 7.2657 \({}^{\text{a}}\) & 3.90572 \\ \hline \end{tabular} Different superscript letters indicate significant difference between different files (\(p\) < 0.05).  
\end{table} 
Table 4: Root canal volume pre- and post- brushing with 2 Shape and Hyflex CM rotary instruments. Figure 3: Illustrative 3D images of the root canals of distal root of mandibular molars before (in green) and after (in red) canal shaping associated with brushing motion. The non-touched canal wall areas were reported in green in the superposition view ((**A**–**F**) = 2 Shape; (**G**–**N**) = HyFlex CM). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Percentage of Untouched Areas (\%)**} \\ \hline
**Instrument** & **Mean** & **SD** & **Minimum** & **Maximum** \\ \hline
**2 Shape** & 4.6023 \({}^{\text{a}}\) & 3.18867 & 1.77 & 8.59 \\
**Hyflex CM** & 3.7965 \({}^{\text{a}}\) & 2.79915 & 0.76 & 7.13 \\ \hline \end{tabular} Different superscript letters indicate significant difference between different files (\(p\) < 0.05).  
\end{table} 
Table 3: Percentage of untouched areas by 2 Shape and Hyflex CM rotary instrumentation. molars) were essentially chosen for two reasons. First, natural dentin appears more appropriate than the artificial canals to simulate clinical conditions because of the different surface texture and hardness [9]. Moreover, the heat generated by rotary instruments may melt the resin and influence the results obtained [14]. Second, distal roots of mandibular molars often consist of oval canals especially in the apical third [6; 7; 17; 18]. Some authors suggested that cutting behavior of endodontic NiTi files should be investigated using plexiglass models because of their standardized production [19]. Yet, it has been previously reported that the hardness of plexiglass is significantly lower than the dentine one and thus not guaranteeing a reliable and adequate simulation of clinical conditions [1]. Since no standardized method to measure cutting efficiency is available, several methodologies have been described to investigate the cutting efficiency of instruments in oval-shaped root canals, such as reassembly [20], histological sections [21], and weight loss from acrylic blocks or extracted teeth [22]. Anyway, the above-mentioned methods have been proven to have limitations because they are two-dimensional, imprecise, invasive, and therefore less accurate [14].