Purpose: The aim of the study was to assess the effect of different endocrown extensions inside the pulp chamber on the fracture resistance and failure mode of maxillary premolars.
Materials and methods: Twenty four single rooted upper premolars were root canal treated, randomly divided into three main groups (n=8) according to extensions of endocrowns inside the pulp chamber (GI= 2mm , GII= 3mm and GIII= 4 mm) . Endocrowns were made-up from Brillient Crios (BC) CAD/CAM blocks and cemented with adhesive resin cement. Teeth were subjected to thermo-cycling and then to 45º oblique compressive load until fracture. The fracture resistance and failure modes were determined. Data were analyzed with One Way ANOVA followed by Tukey`s post hoc test for multiple comparisons. Results: The fracture resistances were significantly different between all groups; GII scored the highest mean fracture resistance value (647±72.37 N) followed by GI (475.6±69.92 N), and the lowest was scored by GIII (353.9±61.02 N). Regarding fracture analysis of different preparation depths; Gr I showed highest repairability percentage followed be Gr II and Gr III; there was significant difference between Gr (I) and Gr (II) and between Gr (I) and Gr (III) as P-value < 0.05. While, there was insignificant difference between Gr (II) and Gr (III) as P-value > 0.05 as revealed by Chi Square test. Conclusion: Different extensions of endocrowns inside the pulp chamber affect the fracture resistance and failure mode of maxillary premolars.