Effect of inlay, onlay, and endocrown restorations on the mode of failure and fracture resistance of endodontically treated maxillary premolar teeth. An in vitro study

Aim: To assess the impact of various restoration designs on the fracture resistance and failure modes of maxillary premolars that had endodontic treatment. Methods: Eighteen extracted upper premolars were used. All teeth were endodontically treated, root canals were shaped with Protaper Next files up to #X3 and obturated with matching gutta percha cones. Afterwards, teeth were randomly assigned to 3 groups according to the restoration design. Group1, EC: Endocrowns, Group2, O: onlays and Group3, I: inlays. All restorations were fabricated from IPS e-max press. Samples underwent thermocycling before compressive stress was applied at a 30-degree angle to their long axes. Data were collected and analyzed statistically by ANOVA and Tukey post-hoc tests. Results: the inlay group failed at a significantly lower load compared to both onlay group and Endocrown group, (p=0.032). Regarding the failure mode, statistical analysis revealed a significant difference amongst groups. All samples in "EC" group (100%) had Fractured tooth & restoration above CEJ, all samples in "I" group (100%) had Fractured tooth & restoration below CEJ, while in "O" group, 66.666% of samples had Fractured tooth & restoration above CEJ while 33.333% showed only a fractured restoration.Conclusion: Compared to inlays and onlays, endocrowns had the highest mean fracture resistance; however, the difference was not significant when compared to onlays. When compared to both inlays and endocrowns, onlays had the failure mode that was most favorable.Recommendation: Based on those findings, onlays could be suggested as a final restoration for maxillary premolars following endodontic therapy.