## 2 Materials and Methods
### Root Canal Shaping and Postinstrumentation Scanning of Tenth  
Access cavities were performed by using an 856-diamond bur (Komet Italia SRL, Milan, Italy) with a high-speed handpiece under running water. The root canal apical patency was verified applying a size 10 K-type file (Flexofile; Dentsply Sirona, Ballagues, Switzerland) until it was detectable at the apical foramen. The working length (WL) was established 0.5 mm short of this length. Then a glide path was obtained using One G 14.03 (Micro Mega, Besancon, France).  
Once the comparability of samples was checked, the ten molars were randomly assigned to the 2 groups (\(n=10\)) on the basis of the shaping system used. Distal canals in Group 1 were prepared with 2 Shape and HyFlex CM in Group 2. The technical features of the analyzed instruments are listed in Table 1 and cross-sections reported in Figure 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Instrument** & **Manufacturer** & **Size (mm)** & **Conicity (\%)** & **Cross Section** & **Cutting Edge** & **NiTi Alloy** \\ \hline \multirow{3}{*}{**2 Shape**} & Micro-Mega, & & & & & \\  & Besancon, France & 0.25 & 4 & Triangular asymmetrical & 2 main and 1 secondary & T-wire \\ \cline{2-6}  & & 0.25 & 6 & Triangular asymmetrical & 2 main and 1 secondary & T-wire \\ \hline \multirow{3}{*}{**HyFlex CM**} & Coltene-Whaledent, & & & & & \\  & Alstatten, Switzerland & & & & & \\ \cline{1-1} \cline{2-6}  & & 0.25 & 6 & Triangular & 3 & CM-wire \\ \hline \hline \end{tabular}
\end{table}
Table 1: Technical features (manufacturer, size (mm), conicity (%), cross section, cutting edge and NiTi alloy) of analyzed instruments.  
Figure 1: Periapical radiographs of two mandibular molars selected for the study. D: distal, M: mesial.  
Group 1: The TS1 (25.04) was introduced until the WL, followed by TS2 (25.06) with the same dynamic movement in a step-down motion. For both files, the torque was set to 2.5 Ncm and the speed to 300 rpm using an electronic motor MM Control (Micro Mega, Besancon, France). The instruments were used in a pecking motion with an amplitude of 3 mm. With a sterile gauze, the instruments were cleaned after each movement and inspected. This procedure was carried out until the WL was achieved [14]. Each set of instruments was employed for the preparation of 1 canal and then discarded.  
Group 2: Hyflex CM 20.04 was used in a gentle in-and-out motion until WL, followed by the Hyflex CM 25.06. For both files, the torque was set to 2.5 Ncm and speed to 500 rpm on the MM Control motor according to manufacturer recommendations.  
A second micro-CT acquisition was done after initial shaping.  
Afterward, in Group 1, the TS2 file was used in five lateral brushing motions on the buccal, distal and lingual walls respectively and in Group 2, Hyflex CM 25.06 was then used in five lateral brushing motions on the buccal, distal and lingual walls respectively. A third micro-CT scan was performed in the same way to determine the effect of the brushing motion applied in both groups.  
During shaping procedures, the canals were irrigated with 3 mL of 5.25% sodium hypochlorite (Vista Apex Dental Products, Racine, WI, USA) using a side-vented 27-G needle (Endo-Eze Irrigator, Ultradent Products, South Jordan, UT, USA). After instrumentation, the root canals were irrigated with 17% EDTA solution delivered at a 1 mL/min rate for 5 min. Then, a 5-min 5-mL rinse with distilled water was applied and the canals were dried using paper points (Meta Biomed Inc., Colmar, PA, USA). All instrumentation procedures were performed by a single expert operator to avoid any operator bias.