## 4 Discussion  
Endocrowns are considered good conservative options for restoring endodontically treated posterior teeth, especially molars, with inadequate remaining tooth structure [39]. [PERSON] et al. evaluated both ceramic and composite endocrown fracture resistance, and they were comparable to intact natural teeth used in the study [44]. Several studies reported superior fracture resistance of endocrowns compared to post-and-core supported restorations [45; 46; 47]. Lithium disilicate endocrowns showed higher fracture resistance values than the average masticatory force in several previous studies [6; 48]. However, endocrowns are contraindicated in cases with less than 3 mm deep pulp chambers, less than 2 mm wide cervical margins, and cases where adhesion is compromised [49]. Unfortunately, there is limited data regarding the mechanical properties of endocrowns with different designs and materials. The purpose of this in vitro study was to compare the conventional endocrown preparation design with endocrowns prepared with mesial and distal extension boxes on the fracture resistance of molar teeth. One of the control specimens was not stable during testing and therefore was eliminated from the group. The null hypothesis was accepted. There were no differences in the fracture resistance of the two endocrown designs: the conventional and proximal extension. Furthermore, there was no statistically significant difference between the two endocrown designs in the fracture resistance test (\(p=0.142\)). This result is similar to a previous study by [PERSON] et al. [30], investigating endocrown preparation modification effects on restoration fracture resistance and failure. They explored various pulp chamber extension designs and found no significant differences in fracture resistance between the different designs. In the current study, a 3 mm standardized preparation depth into the pulp chamber was ensured for all samples. Deeper pulp cham \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Assessment**} & \multicolumn{2}{c}{**Criteria**} & \multicolumn{2}{c}{**Proximal Boxes**} & \multicolumn{2}{c}{**Conventional**} \\ \cline{3-8}  & & & **Mean (N)** & **Count** & **Mean (N)** & **Count** \\ \hline \multirow{8}{*}{Fracture Location/End Level Apically} & a. & Above Cementoenamel Junction & - & - & - & - \\  & b. & Below Cementoenamel Junction and & & & & \\  & & above the Simulated bone level (not at & & - & - & 1240.13 & 1 \\  & & the finish line) & & & & \\  & c. & At Cementoenamel Junction & & & & \\  & d. & Below Simulated Bone Level & - & - & - & - \\  & e. & At the finish line & 1104.05 & 7 & 1161.88 & 5 \\  & e. & At the finish line & 823.10 & 3 & 1241.07 & 3 \\ \hline \multirow{8}{*}{Fracture line involvement Endocrown/tooth structure} & a. & Tooth Structure only & & - & - & - \\  & b. & Endocrown Ceramic only & 746.85 & 2 & 1435.62 & 2 \\  & c. & In Both Materials & 1087.99 & 8 & 1128.79 & 7 \\ \hline \multirow{8}{*}{Catastrophic/Non-reparability} & a. & Yes & 1053.88 & 8 & 1169.36 & 4 \\  & b. & No & 883.33 & 2 & 1219.06 & 5 \\ \hline \multirow{8}{*}{Number of pieces and separation} & a. & Two pieces separated & 791.07 & 1 & 1071.44 & 3 \\  & b. & \(>\)two pieces separated & & & & \\  & c. & Two pieces attached & 1238.86 & 5 & 1215.48 & 5 \\  & d. & \(>\)two pieces attached & 739.67 & 2 & 1481.01 & 1 \\  & & & 866.48 & 2 & - & - \\ \hline \multirow{8}{*}{Fracture direction} & a. & Mesiodistal & 756.80 & 3 & 991.98 & 2 \\  & b. & Buccolingual & & & & \\  & c. & Buccomesial & - & - & 1481.01 & 1 \\  & d. & Distobuccal & - & - & 1420.84 & 1 \\  & e. & Distolingual & - & - & 1450.40 & 1 \\  & f. & Multidirectional & 1132.47 & 7 & 1109.14 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison between the conventional and the proximal boxes extension groups of each examined (observed) item and the number of specimens after consensus.