Impact of hydrothermal aging on the light transmittance and flexural strength of colored yttria-stabilized zirconia materials of different formulations.

STATEMENT OF PROBLEM

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) materials of different formulations (3Y-TZP, 4Y-TZP, and 5Y-TZP) can be colored by using color liquids. However, practically and clinically relevant factors such as modifications of sintering protocols and hydrothermal aging might affect the extent of light transmittance and flexural strength of zirconia materials of different formulations; studies on these outcomes, however, are lacking.

PURPOSE

The purpose of this in vitro study was to test the impact of hydrothermal aging on the light transmittance and flexural strength of colored zirconia materials compared with a lithium-disilicate (LiSi) ceramic.

MATERIAL AND METHODS

A total of 210 specimens were prepared from 3Y-TZP (n=30), 3Y-TZP (n=30), 5Y-TZP (n=30), 4Y-TZP (n=60), 4Y-TZP (preshaded, n=30), and LiSi (n=30). All specimens, except for 4Y-TZP and LiSi, were manually colored, predried, and either conventionally sintered at 1450 °C (3Y-TZP, 3Y-TZP, 5Y-TZP, and half of 4Y-TZP) or high-speed sintered at 1580 °C (other half of 4Y-TZP and 4Y-TZP). Light transmittance was measured initially and after 2, 5, 10, 20, 40, and 160 hours of hydrothermal aging (134 °C, 0.2 MPa). Biaxial flexural strength was tested initially and after 160 hours of hydrothermal aging (n=15). The Kolmogorov-Smirnov test, multivariate analysis, and 1-way ANOVA with the Tukey HSD post hoc test, the t test, and linear mixed models were calculated (α=.05).

RESULTS

LiSi showed the highest translucency, followed by 5Y-TZP, 4Y-TZP, 4Y-TZP, 3Y-TZP, and 3Y-TZP. 4Y-TZP was most opaque and matte. The decrease in translucency related to aging hours was higher for LiS and conventional sintered zirconia materials than for 4Y-TZP and 4Y-TZP. Initially, 3Y-TZP had the highest flexural strength, followed by 3Y-TZP, 4Y-TZP, and 4Y-TZP. 4Y-TZP was comparable with 4Y-TZP but significantly higher than 5Y-TZP. LiSi had the lowest biaxial flexural strength. Hydrothermal aging increased biaxial flexural strength for 3Y-TZP and 3Y-TZP (P<.001) but decreased it for 5Y-TZP (P=.005) and 4Y-TZP (P<.001). After aging, 4Y-TZP showed comparable values of flexural strength with 4Y-TZP (P=.06) and higher values than 4Y-TZP after aging (P=.019).

CONCLUSIONS

Manually colored, conventionally sintered 4Y-TZP was resistant to hydrothermal aging regarding flexural strength. High-speed sintering inhibited color development for manually colored 4Y-TZP but did not affect the resistance to hydrothermal aging. The findings were reversed for industrially preshaded 4Y-TZP.