# Accuracy of Guided Endodontics in Posterior Teeth
## 4 Discussion  
This ex vivo proof-of-principle study demonstrates that GE allows for precise and predictable access cavity preparation in posterior teeth. Other ex vivo studies showing that GE is a viable, fast and accurate method for the preparation of endodontic access cavities in anterior teeth already exist [7; 8; 16], and there are clinical case reports showing the feasibility of guided endodontic treatment of the upper second and third molars [14] as well as the first lower molar [17]. The evidence indicates that GE is operator-independent and allows less experienced operators to reliably access calcified root canals. The overall accuracy (mean angular deviation for all access cavities) measured in this study (1.39 degrees) is comparable to that reported in previous studies [7; 18]. One interesting new finding of this study is that the number of main root canals per tooth had a significant effect on angular deviation and spatial deviation at the tip of the bur in the bucco-lingual dimension. Access cavity preparation was less accurate in teeth with one main root canal than in multi-canal teeth. This could be explained by the fact that the teeth used in this study did not have severe calcification, and that the teeth with one root canal had a larger, possibly oval root canal lumen. Since there was a certain looseness of fit between the utilized burs and sleeves, the bur might have been able to center itself in smaller and rounder root canals once the root canal orifice was reached. This would explain the smaller deviation in teeth with two or three main root canals per tooth. However, operator-related sources of error (e.g., manual positioning during certain steps) may have led to inaccuracies in access cavity preparation as well. Semi-automatic registration of CBCT and surface scan data by the planning software could be another source of inaccuracy, as there is some evidence suggesting that full arch surface scans may be subject to local deviation [16]. Other sources of inaccuracy described in previous studies also apply to the present study (e.g., CAD/CAM processing of templates or loose fit between the bur and sleeve) [19]. There are limiting factors to consider before performing GE in the posterior region [7]. A limited mouth-opening ability and, therefore, limited space for the GE template may complicate the application of GE in posterior regions. However, it should be noted that the templates used in clinical practice can be designed with a lower height than the ones used in this study. For GE, the root canal orifice must be located in a position that can be reached by a straight and rigid bur [7]. Therefore, root curvature is considered a contraindication if the root canal orifice is located far apically. In contrast to anterior teeth, molars often have roots with greater curvature. Fortunately, curvatures most commonly occur in the apical third of molar roots, resulting in a generally promising prognosis for guided endodontics treatment in molars, since root canal calcifications rarely occur up to the apical third of the root canal of these teeth [15]. Even though complex canal morphologies can be a limiting factor for the application of GE, on the other hand, GE may be advantageous in teeth with root variations. GE could be aid in the identification of a calcified radix entomolaris or a middle mesial canal. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Bucco-Oral Deviation at Tip of the Bur [mm]** & \multirow{2}{*}{**Mean Difference: Group 1 vs. Group 2 [mm]**} & \multirow{2}{*}{_p-Value_} \\ \hline
**Group 1** & **Group 2** & & \\ \hline
1 CPT & 2 CPT & 0.16 & 0.04 \\  & 3 CPT & 0.12 & 0.21 \\
2 CPT & 1 CPT & \(-\)0.16 & 0.04 \\  & 3 CPT & \(-\)0.04 & 1.00 \\
3 CPT & 1 CPT & \(-\)0.12 & 0.21 \\  & 2 CPT & 0.04 & 1.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mean difference in bucco-oral deviation at the tip of the bur between planned and prepared access cavities according to the number of main root canals per tooth (CPT).