Effects of sintering time and hydrothermal aging on the mechanical properties of monolithic zirconia ceramic systems

The flexural strength of zirconia restorations is partially dependent on the sintering process. Changes in sintering protocols as well as hydrothermal aging may affect the flexural strength of zirconia materials.

The purpose of this in vitro study was to investigate how changes in sintering parameters and hydrothermal aging affect the biaxial flexural strength of monolithic zirconia.

Specimens were produced from 2 translucent monolithic zirconia ceramics (Zircon X ST, Upcera YZ HT). After coloring, specimens of both ceramics were distributed into groups and subjected to 1 of 6 different sintering protocols. Half were subjected to biaxial flexural strength tests directly after sintering, and the remaining specimens were subjected to hydrothermal aging and then to biaxial flexural strength testing. Biaxial flexural strength data were analyzed by using a statistical software program. Normality of distribution was determined by the Shapiro-Wilk test. Biaxial flexural strength data were compared among groups by using 1-way ANOVA and Tukey post hoc tests, and intragroup data were compared by using paired specimens t tests (α=.05).

The highest overall biaxial flexural strength value was obtained in UW-II. The highest biaxial flexural strength for Zircon X was obtained in ZX-VI and ZX-HTA-VI, whereas the highest biaxial flexural strength for Upcera was obtained in UW-II before hydrothermal aging and in UW-HTA-V after aging (P<.05).

The biaxial flexural strength of Zircon X increased with longer sintering times. Upcera specimens were more fracture-resistant than Zircon X both before and after hydrothermal aging. Based on these findings, longer sintering times are recommended to increase the strength of monolithic zirconia.