Aim: The present study aims to evaluate the effect of different preparation designs on fracture resistance and failure mode of hybrid nanoceramic endocrown on premolars.

Methodology: Fourteen endodontically treated maxillary premolars were prepared to receive a hybrid nanoceramic endocrown restorations and divided into 2 groups according to the preparation designs (butt joint and 2 mm ferrule). The endocrown restorations were cemented to the prepared teeth using dual cure adhesive resin cement. All specimens were subjected to a compressive force 5KN until fracture occurs by using universal testing machine. The maximum compressive force was recorded for all specimens, then failure mode of all samples were examined visually and photographically. All repairable samples were examined for extension of the cracks using scanning electron microscope.

Results: Endocrowns with ferrule preparation recorded a highly statistically significant difference with the higher mean value of fracture resistance (1911.57 ± 341.29 N), compared to the endocrowns with butt joint which recorded the lower mean value of fracture resistance (1192.70±196.04 N). Regarding the failure mode; endocrowns with ferrule design showed a high percent of catastrophic failure while butt joint showed more repairable failure.

Conclusion: Endocrowns with ferrule design showed a higher fracture resistance value but with a high percent of catastrophic failure mode compared to butt joint design that showed lower fracture resistance value with mostly repairable mode of failure.