## 3 Results
The mean fracture load observed in our study LD ceramic was well above the maximum bite force reported by earlier studies of 738 N [33]. Although the fracture resistance observed in HTZ and PIC endocrowns were lower, but at par with expected masticatory load at premolar region. [PERSON] et al. [34] from Finite element analysis and Weibull analysis reported the similar performance of endocrowns and conventional crowns at premolar regions under fatigue load. LD ceramic endocytosis had 62.55 MPa at 2 mm occlusal thickness, while at 4.5 mm occlusal thickness fracture resistance reduced to 45.80. The higher fracture resistance of LD ceramic could be attributed to its superior bonding strength to natural structure and biomechanical properties. LD glass-ceramics have a high flexural strength of between 300 and 400 MPa, high fracture toughness of between 2.8-3.5 MPa and elastic modulus of 95 GPa [35]. The superior mechanical strength of LD ceramics is credited to the interlocked microstructure and shape of crystals [36]. They are characterized by high crystallinity and high aspect ratio grains; which promote bridging and hinder crack propagation [37]. Hydrofluoric acid etching dissolves the glass phase within the matric, consequently creates micro-undercuts and enlarges the bonding surface [38]. The chemical bonding between the resin and ceramic is facilitated by the silane coupling agent application. The trialkoxy silane in methacrylic monomers initially hydrolyzed; interact with one another (hydrogen bonding), partner with silicate surface through a condensation polymerization and develops a covalent bond with hydroxyl (OH) groups on the ceramic surface [39]. [PERSON] et al. [40] concludes the better adhesion of LD ceramics to tooth structure facilitates the transmission of occlusal forces to pulp chamber walls. LD ceramics possess an excellent ability to resist damage induced by cyclic expansion and contraction from thermocycling due to combined outcomes of micromechanical and chemical bonding by the formation of Si-O-Si and -C-C- at the interface [41]. Higher fracture resistance of LD ceramic endocytosis observed in our study is also corroborated by [PERSON] et al. [42]. The 2 mm radicular extension demonstrated the improved fractured resistance even with 4.5 mm occlusal reduction. The result corroborates with the findings from [PERSON] et al. [26]; they reported the higher loads-to-failure with 2.5 mm deep endocrowns irrespective of the material utilized for the fabrication. [PERSON] et al. [43] reported the Endocrowns with both 2-mm and 4-mm long endo-cores displayed results after fatigue load equivalent to classical crowns. Hence, they recommended the adhesive strategies should be complemented with the macro-mechanical retention in the root. Finite element analysis by [PERSON] et al. [44] showed lower intensity stress and better stress distribution in endocrowns with longer intra-coronal depth. The 2 mm radicular extension proposes providing the larger bonding surface area within the root and enhances micro-mechanical retention; hindering their displacement from the root cavity under lateral stresses. The improved fracture resistance could also be influenced by the modulus of elasticity of the material. The results of our study suggested a substantial reduction in fracture resistance with higher occlusal thickens in both LD ceramic and PIC ceramics. The Fracture resistance of LD ceramic at 4.5 mm thickness was reduced to 45.80 MPa from 62.55 MPa. These results were in agreement with the Finite element analysis reported by [PERSON] et al. [45]. They found that the Von Mises stress with the remaining tooth structure was 54-57 MPa with the load applied 5 mm above the restoration-toothjunction. The Von Mises stress was substantially increased to 87-90 MPa when the load was placed at a height of 8 mm. The total equivalent strain recorded was at 2000-2300 \(\upmu\)\(\varepsilon\) for 5 mm height load application, 3800-4200 \(\upmu\varepsilon\) for the loading height of 8 mm.