Marginal and Internal Misfit, Fracture Load, and Stress Distribution in Monolithic Zirconia-reinforced Lithium Silicate and Lithium Disilicate Crowns

Objectives: This study evaluated marginal and internal misfit, stress distribution, and fracture load with and without thermomechanical fatigue of monolithic zirconia-reinforced lithium silicate and lithium disilicate crowns. Correlations between misfit and fracture load were also examined.
Methods: Monolithic lithium disilicate and zirconia-reinforced lithium silicate crowns (n=20/material) were milled using a CAD/CAM system. Marginal gaps, absolute marginal discrepancy, axial gaps, and occlusal gaps were measured by X-ray microtomography on eight randomly selected crowns of each material. Crowns (n=20) were cemented on abutments and half of them were subjected to thermomechanical fatigue in a universal testing machine; fracture load was then recorded. The other half of samples served as controls and was not subjected to thermomechanical fatigue, but loads were applied until failure. Fracture load and misfit results were examined using Pearson correlation, and stress distribution during mechanical testing was evaluated using the finite element method.
Results: Average marginal gap and absolute marginal discrepancy were greater in zirconia-reinforced lithium silicate than in lithium disilicate crowns (both P<.05), but no difference in axial gap or occlusal gap was observed. Thermomechanical fatigue did not affect the fracture load of either material. However, fracture loads of zirconia-reinforced lithium silicate crowns with and without thermomechanical fatigue were lower than those of lithium disilicate crowns (P<.05). Fracture load was correlated moderately with marginal gap (r=–0.553) and absolute marginal discrepancy (r=–0.497). Finite element analysis showed similar stress concentration in the cervical area for both materials.
Conclusions: Zirconia-reinforced lithium silicate crowns showed satisfactory fracture load and adequate marginal and internal fit for safety clinical use.