Influence of Core-Veneer Thickness Ratio on the Fracture Load and Failure Mode of Zirconia Crowns.

PURPOSE: To evaluate the influence of different core-veneer thickness (CVT) ratios on fracture loads and failure modes of zirconia crowns with or without thermomechanical cycling (TC). MATERIALS AND METHODS: Forty yttrium-stabilized tetragonal zirconia polycrystalline (Y-TZP) cores were veneered with a pressable ceramic and divided into four groups (n = 10): group 1 - 1:3 CVT ratio (0.5-mm core, 1.5-mm veneer); group 2 - 1:1 CVT ratio (1.0-mm core, 1.0-mm veneer); groups 3 and 4 - identical to groups 1 and 2, respectively, except that they underwent TC (1,000,000 cycles with an 84-N load applied at a 4-Hz frequency, temperature cycling at 4 degrees C, 37 degrees C, and 55 degrees C - 30 seconds each). Loads were applied axially to the central fossa of the crowns with a 1 mm/min crosshead speed at 200 N until failure. RESULTS: Two-way ANOVA showed that the crowns with a 1:3 CVT ratio had a lower mean fracture load than crowns with a 1:1 CVT ratio (group 1 = 1666.4 +/- 466.4 N; group 2 = 2156.6 +/- 472.3 N). TC had no effect on the fracture load for crowns with the same CVT ratio. The failures observed for the crowns in groups 1, 3, and 4 were fractures within the veneering ceramic, which exposed the core. The failure severity of fractures for group 2 was less extensive, characterized by chipping within the ceramic veneer. CONCLUSION: Crowns with a 1:1 CVT ratio had higher fracture load values and more conservative failures. TC had no effect on the fracture load of the ceramic crowns.