## 4 Discussion
ber preparation designs were found to enhance restorative retention due to the increased bonding surface and high fracture resistance values, but transmitted the loads to the root dentin [28; 50]. However, the failure mode in the current study, fractographic inspection, showed different results between the conventional and proximal boxes endocrown designs (Table 3). This difference is seen with other changes in endocrown design. [PERSON] et al. found that increasing the thickness of lithium disilicate endocrowns enhances the restoration's fracture resistance, but increases the chance of tooth fracture [51]. Previous studies in the literature mostly investigated the effect of the ferrule preparation design on the fracture resistance of endocrowns. [PERSON] et al. [25] found that a 1 mm ferrule increased the fracture resistance and retention of endocrowns. Minimal proximal adjustments, where the amount of remaining coronal tooth structure is contraindicative of a conventional endocrown preparation, were proposed in a case report by [PERSON] et al. [52]. Another study investigated the effect of the type of preparation finish line on endocrown fracture resistance and found that shoulder finish lines enhanced fracture resistance of the restorations [53]. Comparing our results with other studies in the literature was complicated due to variations in the teeth selected (anterior versus posterior), the depth of the pulp chamber, study design (finite element analysis, in vitro study), presence of ferrule, design of the study (in vitro versus finite element analysis), restorative materials (glass-based, resin-based, or zirconia-based), presence of periodontal ligaments replica, type of fracture load (static loading versus cyclic loading), and loading direction (axial versus oblique). The periodontal ligament (PDL) allows some degree of tooth displacement under the occlusal load [54]. Without simulating such an effect, load test results would be misleadingly high [55]. In the current study, polyvinyl siloxane material was used to simulate the PDL, which was demonstrated to be a good option by [PERSON] et al. [42]. In the current study, acrylic resin with an elastic modulus of around 2000 MPa [56] was used to embed the teeth, resembling the alveolar bone [43; 57]. Various resins have been used in the literature, with elastic modulus values lower than the elastic modulus of alveolar bone (about 10,000-14,000 MPa [58; 59]). However, their elastic moduli are still considered much closer to the alveolar bone than those of metal alloys (around 100,000 MPa [60]). The use of acrylic and polyurethane resins to simulate alveolar bone during restorative mechanical testing was deemed adequate by previous studies, and PDL simulation significantly influenced fracture test results more than embedding resin [61; 62; 63]. In the current study, extracted human molars were used to evaluate the fracture resistance of endocrowns with two preparation designs, since the pulp chamber of molars is larger than premolars, providing more surface for bonding to the restoration [44]. To reduce variability between specimens, lithium disilicate endocrowns were milled from prefabricated CAD/CAM blocks (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) using the same software and milling machine for Cerec CAD/CAM system, and all endocrowns had the same occlusal anatomy. Lithium disilicate ceramic material is considered one of the best esthetic materials for fabricating indirect single-tooth restorations [44; 47]. Using lithium disilicates for endocrowns offers the ability to bond to the remaining tooth structure, besides their excellent mechanical properties [26; 27]. For conventional lithium disilicate endocrown design restorations in molar teeth with static loading, the fracture resistance in the current study showed higher mean values than [PERSON] et al. [47] (915.91 \(\pm\) 182.06 N), [PERSON] et al. [25] (638.5 \(\pm\) 238.5 N), and [PERSON] et al. [64] (584.48 \(\pm\) 5.80 N). However, [PERSON] et al.