Influence of Dental Tissue Substructure and Dimension on the Fracture Strength of Lithium Disilicate and Zirconia Ceramics

Purpose: To assess the influence of substructure (enamel vs dentin) and thickness on the fracture strength of ceramic discs made from lithium disilicate (LDS) vs zirconia (Zi). Materials and Methods: A total of 128 intact maxillary third molars were collected, and standardized enamel and dentin discs were fabricated. LDS (n = 64) and Zi (n = 64) discs with 0.5-mm (n = 32 each) and 1-mm (n = 32 each) thicknesses were produced, and each group was divided into two subgroups (n = 16 each), which were luted to the enamel/dentin discs. Half of the specimens in each subgroup were aged via chewing simulation and thermocycling, and all specimens were loaded until fracture in a universal testing machine. Differences between LDS and Zi with respect to enamel vs dentin as substructure and ceramic thickness were analyzed by use of nonparametric testing (Mann-Whitney U test). Results: In unaged specimens, fracture loads were not significantly (P > .05) different between Zi and LDS specimens for the 1-and 0.5-mm thicknesses. However, fracture loads were significantly higher (P < .001) in specimens supported by enamel independent of the ceramic material. In aged specimens, the fracture loads were significantly (P < .01) higher when supported by enamel; however, in the 0.5-mm groups, Zi achieved significantly higher breaking loads than LDS when luted to dentin. Conclusion: When 0.5-mm ceramic discs were luted to dentin, Zi outperformed LDS with respect to fracture loads.