Effect of bleaching on fracture toughness of resin composites.

This study determined the effect of bleaching agents on the fracture toughness of composite materials. Four nanofilled resin composites were evaluated: Filtek Supreme Plus, Tetric EvoCeram, Premise and Esthet-X. Four concentrations of bleaching agents were tested: Opalescence PF 10%, 20%, 35% and 45%. Fifty specimens of each composite conforming to ASTM guidelines for the single edge notch bar-shaped specimen were fabricated in a metal mold. The specimens were stored in artificial saliva at 37 degrees C for a minimum of 24 hours prior to exposing them to the bleaching agents. Ten specimens of each material were used as controls. Forty specimens of each brand were evenly divided into four groups for bleaching agent application (n = 10). The resin composites were bleached on both sides for 14 days. The specimens were then subjected to a three-point bending test with a crosshead speed of 0.2 mm per second. The fracture toughness (K(Ic)) was calculated. The control group K(Ic) value of Esthet-X was significantly higher than that of the other composites in the controlled groups. Bleaching agents significantly improved the fracture toughness values of Filtek Supreme Plus. The application of bleaching agents did not significantly change the fracture toughness values of the other nanofilled resin composites tested.

Failure analysis of clinically failed all-ceramic fixed partial dentures using fractal geometry.

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.