## 3 Results  
Descriptive statistics of endocrown designs' fracture resistance (load to fracture) of conventional and proximal boxes in control and test groups are demonstrated in Figure 4.  
Figure 3: Specimens were placed into fixtures mounted of the universal testing machine. Results of the Mann-Whitney U-test of fracture resistance test in Newtons (N) are detailed in Table 1. The test showed an insignificant difference between the two endocrown preparation design groups (\(p\) = 0.142). Cohen's Kappa agreement between the two evaluators on the assessment criteria after the fracture resistance test criteria is presented in Table 2.  
* The estimation of the weighted kappa uses linear weights (0-0.2 = non-agreement, 0.21-0.39 = minimal agreement, 0.4-0.59 = weak agreement, 0.6-0.79 = moderate agreement, 0.8-0.9 = strong agreement, >0.9 almost perfect agreement). * Value does not depend on either null or alternative hypotheses. * Estimates the asymptotic standard error assuming the null hypothesis that weighted kappa is zero. In comparing the test group (endocrown with proximal boxes) with the control group (no proximal boxes), we can observe the following: Endocrown with proximal extension boxes had the apical location of the fracture line, mainly occurring below the simulated bone level. Almost one-third of the specimens were at the finish line. None were detected at or below, or above CEJ. That may reflect on the fracture classification as catastrophic or non-restorable, as shown in Table 3, as 80% of fractures were classified as catastrophic. Agreement represented by Kappa of catastrophic failure mode was 0.255 between the two evaluators. However, when comparing each previous rater's rating to the reached consensus rating, evaluator one kappa was 0.315 in agreement with the consensus rating and evaluator two was 0.787. Fracture lines were primarily involved in both materials (tooth and ceramic) at 80%. None was in tooth structure only. The separation of pieces into more than two pieces non-attached (separated) occurred in half of the specimens. The remaining other half were distributed among the other classifications. The fracture direction was either mesiodistal or multidirectional, with an abundance of multidirectional at 70%. \begin{table}
\begin{tabular}{c c c c c} \hline
**Endocrown Design** & **N** & **Mean (N)** & **Standard Deviation** & \(p\)***** \\ \hline Conventional & 9 & 1196.971 & 318.364 & \\ proximal boxes & 10 & 1019.766 & 266.348 & 0.142 \\ \hline \end{tabular}
* Significant at \(p\) \(<\) 0.05.  
\end{table} 
Table 1: Mann-Whitney U-test results of fracture resistance test in Newtons (N). Figure 4: Analysis of endocrown designs’ fracture resistance of conventional and proximal boxes. Endocrown with conventional design had the apical location of the fracture at or below the simulated bone level in more than half of the specimens. A third of the specimens were at the finish line, and only one specimen showed fracture below CEJ and above the simulated bone level, which may reflect on the classification into catastrophic or non-restorable teeth being almost equally distributed. Fracture lines were mostly involved in both materials (tooth and ceramic) at 77.8%, 22.2% of the fracture lines was in the endocrown material itself, and none in the tooth structure only. The separation of pieces into more than two pieces non-attached (separated) was 55% of the specimens, while 33.3% was fractured into two pieces, and only one specimen was fractured into two pieces attached. The fracture direction was predominantly multidirectional at 44.4%. Except for the distolingual ones, the remaining directions were distributed almost equally among the other five directions. Table 3 shows the mean fracture resistance values of specimens for each mode of failure. Figure 5 shows the failure mode of two specimens.