## 3 Results
The higher the crown height from the restoration-tooth junction, the stress and deformation are expected to rise because of an increase in lever arm length. The PIC endocrowns showed lesser fracture resistance of 26.30 MPa at 2.5 mm occlusal thickness. The radicular extension indicated marginal improvement in the fracture resistance. The results were in agreement with the findings from [PERSON] et al. [46]; they reported no significant difference in fracture resistance between the preparation depths of 2 mm and 4 mm within the similar marginal design. [PERSON] et al. [47] reported as the thickness of restorative material increased, the von Mises stress in dentin increased. An increase in the elastic modulus of the material may benefit the durability of bonding between the endocrowns and the abutment tooth; however, it may cause a fracture of the residual tooth structure. [PERSON] et al. [48] described the modulus of elasticity of PIC at 27.26 GPa, the lower modulus of elasticity could be the reason for the lower fracture resistance of PIC endocrowns. Hence the conservative teeth preparation for the endocrowns is likely to improve the fracture resistance, as confirmed from the results of our study. The results of the study indicated the high HTZ endocrowns with the least fracture resistance of 23.47 MPa at 2 mm thickness. In contrast to other tested material, the fracture resistance was moderately enhanced to 27.30 MPa with an increased thickness of 4.5 mm. The radicular extension was a significant aid in increasing fracture resistance up to 37.29 MPa. The flexural strength and modulus of elasticity of HTZ zirconia is reported to be at 800-1100 MPa and 200 GPa. The HTZ has more natural esthetic properties with increased optical properties attributed to the inclusion of cubic zirconia [49]. Several studies concluded the lower mechanical properties of HTZ ceramic is due to the absence of transformation toughening mechanism [50]. High flexural strength and fracture resistance in zirconia is credited to volume expansion associated with transformation from the tetragonal to the monoclinic phase. Consequently, it creates the compressive stress behind the crack tip [51]. Reduced mechanical properties of HTZ restorations are due to the absence of monoclinic peaks and grains below 1 \(\upmu\)m HTZ restorations [52]. Augmented fracture resistance of HTZ ceramic at increased thickness could be attributed to its correlation of fracture strength with thickness [53]. [PERSON] et al. [54] suggested the air-borne particle abrasion enhances the flexural strength. Previous studies have also reported a higher load-bearing capacity of LD ceramics compared to HTZ ceramics when bonded on to dentin [55]. [PERSON] et al. [56] suggested that the more stress is transferred to restorative material with the high elastic modulus, predisposing it for the early fracture. Most unfavorable failures type was observed in LD ceramic with an increased incidence of root fracture. [PERSON] et al. [20] from their 3D FE analysis described the concentration of an equivalent von Mises stress at the interface of materials with different elastic modulus. The stiffer material with higher modulus elasticity affects the distribution of an equivalent von Mises stress. The difference in modulus of elasticity between resin luting cement, indirect crown, and dentin predisposes the ETT for root fracture. The PIC ceramics with mechanical properties closely matching the natural tooth hard tissues lower the frequency of unfavorable root fracture [57]. The clinical implications of Study results include the LD ceramics is material of choice for fabrication of endocrowns at premolar region. The larger occlusal thickness of the endocrowns needs to be compensated with comparable radicular extension to enhance fracture resistance. Limitations of the study include, Even though due care is observed in the standardization of teeth samples some degree of variability is difficult to eliminate.