Comparison of fracture resistance for CAD/CAM overlay restorations made of different ceramic materials and designs

Aim: To evaluate the fracture resistance and failure mode of complete crown and different designs of overlay restorations fabricated from different CAD/CAM ceramic materials. Methods: Ninety upper premolars were randomly divided into nine groups (n=10/group) according to ceramic material and restoration design into: LD4: Lithium disilicate overlay with margin located 4millimeter (mm) above the Cementoenamel junction (CEJ). LD2: Lithium disilicate overlay with margin located 2mm above the CEJ. LD-F: Lithium disilicate crown. ZLS4: Zirconia-reinforced lithium silicate overlay with margin located 4mm above the CEJ. ZLS2: Zirconia-reinforced lithium silicate overlay with margin located 4mm above the CEJ. ZLS-F: Zirconia-reinforced lithium silicate crown. HC4: Hybrid ceramic overlay with margin located 4mm above the CEJ. HC2: Hybrid ceramic overlay with margin located 2mm above the CEJ. HC-F: Hybrid ceramic crown. The ceramic restorations were fabricated according to the manufacturer instruction and cemented to the prepared tooth surfaces. Universal testing machine was used to evaluate the fracture resistance at cross head speed of1mm/min until failure. The obtained results were statistically analyzed using One-way ANOVA test at 0.05 level of significance and failure mode were evaluated. Results: There was a significant difference (p=0.00) between the restoration designs of the same ceramic material, with a higher fracture resistance for the overlay restoration design with 4mm above the CEJ. Also, there was a significant difference (p=0.00) between the restoration materials with the same restoration design. The failure mode was mainly of type III. Conclusion: Minimal invasive restorations like overlay restorations can be used as an alternative for the full crown restorations. Keeping the finish line coronary as possible during indirect restoration preparation increases the fracture resistance of the tooth. The Type III fracture of the tested samples can probably be attributed to the high modulus of elasticity of the used materials.