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 (α=.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.

Human perception of dental porcelain translucency correlated to spectrophotometric measurements.

PURPOSE: This study evaluated the relationship between instrumental measurements and subjective visual assessment of differences in dental porcelain translucency. MATERIALS AND METHODS: Unshaded feldspathic porcelain was used with controlled amounts of tin oxide to create two groups of 12-mm diameter disks with incremental changes in opacity. Contrast ratio (CR = Yb/Yw) was determined with a spectrophotometer, and used as a measure of porcelain translucency (Group A = 0.20 to 0.40; Group B = 0.6-0.8). Within each group, there were 14 specimens with 11 CRs. Three observer groups (first year dental students, residents, faculty with >10 years of shade matching experience) were recruited to assess the translucency between porcelain disks under two lighting conditions (reflected light, transmitted light). Each subject's ability to distinguish between specimens of differing translucency was determined. Descriptive statistics and three-way ANOVA followed by a post-hoc Tukey-Kramer test were used to evaluate the translucency perception threshold (TPT) of subjects (alpha= 0.05). RESULTS: The overall mean TPT (DeltaC) was 0.07, while 50% of the subjects could perceive a 0.06 CR difference between porcelain specimens. Three-way ANOVA revealed a significant difference in translucency perception among the observer groups (p < 0.0001), whereas the main effects for porcelain opacity (p= 0.3038) and lighting condition (p= 0.0645) were not significant, and no significant interactions were found. Post-hoc Tukey-Kramer test indicated that the mean TPT observed in the faculty group (DeltaC = 0.04) was significantly lower than those observed in student (DeltaC = 0.09) and resident groups (DeltaC = 0.08), while there was no significant difference between students and residents. CONCLUSIONS: The overall mean TPT of all subjects was 0.07, and 50% of the study population perceived a 0.06 CR difference in translucency. Increased shade matching experience (> or =10 years) significantly improved the ability to perceive differences in translucency; however, neither the viewing condition nor porcelain opacity affected the perceived translucency threshold.