Comparative evaluation of fracture resistance and failure modes in endodontically treated molars restored with zirconia endocrown and onlays

Introduction: Developments in dental materials, CAD/CAM technologies and adhesive dentistry have improved the application of conservative restorations such as endocrowns and onlays. Among ceramics, zirconia has properties such as high strength, transformation toughening, chemical and structural durability, and biocompatibility, which enable zirconia to be used in the posterior area.

Aim: This study is a comparative evaluation of fracture resistance and failure modes in endodontically treated molars restored with zirconia endocrown and onlays.

Materials and methods: This study was performed on 20 human mandibular first molars with similar dimensions. After root canal treatment, the samples were divided into two groups: endocrowns and onlays (n=10). Restorations were made using a CAD-CAM milling machine with zirconia CAD blocks and, after cementation, subjected to 10,000 thermocycling and 500,000 fatigue cycle procedures, respectively. Each specimen was placed on a Universal Testing Machine and subjected to axial compressive force applied at a crosshead speed of 0.5 mm/min. The mean loads of failure of each group were statistically compared using the Student t-test. Chi-square tests were used to compare frequencies of failure modes among groups.

Results: Fracture resistance showed a statistically significant difference between endocrown (5374.6810±670.03445 N) and onlay (3312.5000±804.01428 N) (p<0.001). No statistically significant difference was detected in the distribution of failure types among the groups (p>0.05).

Conclusions: The fracture resistance of endocrown is substantially higher than that of onlay, and failure type does not differ in both restorations. Zirconia is a reliable material to use in conservative restorations.