Objective: The goal of this study was to test the hypothesis that fracture toughness of the veneers in clinically failed zirconia-based fixed partial dentures (FPDs) is not significantly different from that of the in vitro group and to determine the potential reasons for their failures.
Methods: Fracture toughness values of the veneer layers in clinically failed zirconia core/glass veneer FPDs (n=4) and laboratory prepared glass veneer bar specimens (n=6) were determined using fractal analysis. A modified slit island technique was employed to measure the fractal dimensional increment (D*) of the two studied groups. The fracture toughness (K(C)) values were estimated using equation K(C)=Ea(o)(1/2)D*(1/2), where E is the elastic modulus and a(o) is a characteristic length parameter. Fracture toughness (K(C)) values of the specimens calculated using fractal analysis and fractography were statistically compared using a paired t-test.
Results: The average fracture toughness of the veneer in clinically FPDs (0.5+/-0.05 MPam(1/2)) is not significantly different (p>0.05) from that of the bar specimens (0.6+/-0.1 MPam(1/2)). The reasons for the early failures in FPDs could be occlusal overloading, stress corrosion, fatigue or improper structure design.
Significance: Fractal analysis is shown to be an alternative analytic tool for clinically failed ceramic restorations, especially for those with fracture origins chipped off during mastication and hence could not be analyzed using other techniques, such as fractography.