## 4 Discussion  
Guided endodontic microsurgery is becoming recognized as more accurate than freehand surgery [5; 16]. Guides evolve to best adapt to the clinical situation and are being improved with technical developments. The present study has proposed a new design for a printed SMG, and its accuracy was evaluated using a model ex vivo. Two parameters were assessed: the accuracy of the most apical point of the drill path and the angular deviation. These two parameters are often chosen in studies because they are easily identifiable and present reliable information concerning the assessment of the difference between the actual results and virtual planning. The angular deviation is an important parameter because it determines the root-end resection and whether all of the accessory canals can be resected [23]. Using SMGs, the angular deviation was 3.1 \(\pm\) 1.3 degrees. Data related to the angular deviation are very scarce. Only one study has reported an overall angulation of 3.95 degrees (95% CI: 2.1-5.9 degrees) using a trephination guide for endodontic surgery [14]. This result is in accordance with our results. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & \multicolumn{3}{c}{**n**} & \multicolumn{6}{c}{**Apical Deviation**} \\ \cline{2-9}  & & **Mean** & **Min** & **Max** & **Median** & **SD** & **95\% CI LL** & **95\% CI UL** & \(p\) \\ \hline SMG 1 & 4 & 2.38 & 1.55 & 3.11 & 2.43 & 0.77 & 1.55 & 3.11 & 0.0087 \\ SMG 2 & 4 & 4.90 & 2.67 & 6.38 & 5.28 & 1.66 & 2.67 & 6.38 & 0.0097 \\ SMG 3 & 4 & 4.24 & 2.46 & 6.58 & 3.95 & 1.72 & 2.46 & 6.58 & 0.016 \\ SMG 4 & 4 & 4.85 & 3.8 & 5.67 & 4.97 & 0.92 & 3.08 & 5.67 & 0.0019 \\ SMG 5 & 4 & 3.75 & 2.65 & 5.87 & 3.24 & 1.51 & 2.65 & 5.87 & 0.0156 \\ SMG 6 & 4 & 1.25 & 0.77 & 2.56 & 0.83 & 0.87 & 0.77 & 2.56 & 0.0646 \\ SMG 7 & 3 & 1.99 & 1.2 & 3.14 & 1.6 & 1.01 & 1.23 & 3.14 & 0.076 \\ SMG 8 & 2 & 1.86 & 1.24 & 2.78 & 1.86 & 0.86 & 1.24 & 2.47 & 0.2205 \\ Total & 29 & 3.28 & 0.77 & 6.58 & 2.97 & 1.76 & 2.61 & 3.95 & 0.001 \\ \hline \end{tabular} The apical deviations between the preoperatively planned and actual drill paths are expressed with mean, minimum, maximum, median, and standard deviation for each 3D metal guide (in millimeters). SMG: 3D-printed surgical metal guides; n, number of virtual implant planned; min, minimum; max, maximum; med, median; SD, standard deviation; LL: lower level, UL: upper level.  
\end{table} 
Table 1: Statistic summary of the deviation between the planned and the actual paths for each of the 3D-printed surgical metal guides. Of note, the angular deviation has been evaluated in multiple implantology studies. [PERSON] et al. compared the deviation between the position of virtually planned implants and the position of implants placed with a printed surgical guide in a human cadaver [24]. The mean angular deviation of the mandible was 2.26 degrees (range: 0.24-11.74 degrees; 95% CI: 2.01). [PERSON] et al. reported a mean angular deviation of 2.30 \(\pm\) 0.92 degrees [7]. [PERSON] et al. analysed the factors influencing the accuracy of a printed surgical guide for implant placement and reported a mean angular deviation of 2.25 degrees (0.07-5.82 degrees) for the mandible [25]. The overall mean deviation between the most apical points planned and those performed with the surgical guide were superior to previous studies performed using human mandibles and resin guides. [PERSON] et al. found a mean deviation of 0.79 \(\pm\) 0.33 mm [5] and [PERSON] et al. obtained a mean deviation of 1.47 \(\pm\) 0.75 mm for the same parameters [16]. These results may be explained because of the variation between the different SMGs. Three out eight of the different SMGs did not express any statistical difference between the most apical points planned and those performed with the surgical guide, whereas the others expressed quite high values. One explanation for the variation could be that several drill paths (up to four) were achieved with the same guide to maximize the number of roots in the study to be compared. [PERSON] et al.