Fracture toughness of yttria-stabilized zirconia sintered in conventional and microwave ovens.

Statement of problem. The fabrication of zirconium dioxide (ZrO2) dental prosthetic substructures requires an extended sintering process (8 to 10 hours) in a conventional oven. Microwave sintering is a shorter process (2 hours) than conventional sintering. Purpose. The purpose of this study was to compare the fracture toughness of 3 mol %Y2O3-stabilized ZrO2 sintered in a conventional or microwave oven. Material and methods. Partially sintered ZrO2-specimens from 3 manufacturers, KaVo, Lava 3M, and Crystal HS were milled (KaVo Everest engine) and randomly divided into 2 groups: conventional sintering and microwave sintering (n=16 per group). The specimens were sintered according to the manufacturers' recommendations and stored in artificial saliva for 10 days. Fracture toughness was determined by using a 4-point bend test, and load to fracture was recorded. Mean fracture toughness for each material was calculated. A 2-way ANOVA followed by the Tukey HDS post hoc test was used to assess the significance of sintering and material effects on fracture toughness, including an interaction between the 2 factors (alpha=.05). Results. The 2-way ANOVA suggested a significant main effect for ZrO2 manufacturer (P<.001). The post hoc Tukey HSD test indicated that mean fracture toughness for the KaVo ZrO2 (5.85 MPa.m(1/2) +/- 1.29) was significantly higher than for Lava 3M (5.19 MPa.m(1/2) +/- 0.47) and Crystal HS (4.94 MPa.m(1/2) +/- 0.66) (P<.05) and no significant difference was observed between Lava 3M and Crystal HS (P>.05). The main effect of the sintering process (Conventional [5.30 MPa.m(1/2) +/- 1.00] or Microwave [5.36 MPa.m(1/2) +/- 0.92]) was not significant (P=.76), and there was no interaction between sintering and ZrO2 manufacturer (P=.91). Conclusions. Based on the results of this study, no statistically significant difference was observed in the fracture toughness of ZrO2 sintered in microwave or conventional ovens. (J Prosthet Dent 2013;109:165-171)