Factors Affecting the Fracture Strength and Fatigue Resistance of Molar Crowns Produced Using a Zirconia-based System

Zirconium-oxide ceramic was introduced as a potential restorative material for posterior teeth due to its superior mechanical properties. However, zirconia is opaque and needs to be masked with an esthetic veneer. Short-term clinical studies showed a high rate of veneer chipping/delamination for zirconia-based posterior crowns. This may be attributed to either inferior strength of the veneering porcelain or weakness in the zirconia-veneer bond. The aims of this in-vitro project were to analyze some of the factors that can influence the performance of zirconia-based molar crowns and to characterize the zirconia-veneer interface. The project consisted of four parts. The first part aimed to evaluate the effect of total thickness, core thickness and veneering technique on fracture mode and strength of zirconia-porcelain specimens of simple geometry. Results indicated that all three variables significantly affected mean fracture load, while only two (total thickness and core thickness) affected fracture mode. The second part aimed to assess the effect of core design and thickness on fatigue resistance, fracture strength and fracture mode/extent of zirconia-based molar crowns. Results indicated that core thickness, but not core design, had a significant effect on fracture strength. The anatomically-shaped core design reduced the size of the veneering porcelain fracture. In the third part, the effect of core design and two different CAD-CAM-produced veneering materials on fatigue resistance, fracture load and fracture mode of zirconia-based molar crowns was examined. Results indicated that the veneering material, but not core design, had a significant effect on fracture load and mode. The fourth part aimed to characterize the zirconia-porcelain interface using modern surface analysis techniques (Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS)). Results indicated chemical changes of zirconium along the interface. In conclusion, improving the strength of the veneering layer by using pressed veneering porcelain can enhance performance of posterior zirconia-based crowns in comparison to manual build-up technique. Furthermore, fusing a CAD-CAM-produced veneering layer to the underlying zirconia core is a promising technique that can augment veneering porcelain strength and enhance zirconia-porcelain bond. Modifying core design, crown thickness and core thickness can improve performance of zirconia-based molar crowns.Ph