Influence of aging on flexural strength of translucent zirconia for monolithic restorations

Concern has been raised with regard to the low-temperature degradation (LTD) of translucent yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) for monolithic zirconia restorations.

The purpose of this in vitro study was to assess the LTD behavior of 4 commercially available translucent Y-TZP materials by accelerated aging specimens in steam at 134°C, 0.2 MPa.

Thin bars (22×3×0.2 mm) of Y-TZP, including Katana ML (Kuraray Noritake Dental Inc), Katana HT13 (Kuraray Noritake Dental Inc), Prettau (Zirkonzahn), and BruxZir (Glidewell Laboratories) (n=30 for each group), were machined from sintered blocks. Control specimens were assessed in the nonaged condition. Artificially ageing (n=5 per group at 5, 50, 100, 150, and 200 hours) was conducted in steam at 134°C at 0.2 MPa. The specimens were characterized, tested in 4-point flexure, and the fracture surfaces were analyzed. The monoclinic-to-tetragonal (m/t) peak intensity ratio measured by x-ray diffraction was used to calculate the monoclinic phase fraction and monitor LTD. Linear regression with heteroscedasticity-consistent robust standard errors was used to test for the effect of LTD (aging time) on (σ_f) and m/t. The Spearman rank correlation coefficient was used to assess the relationship between σ_f and monoclinic phase fraction (α=.05).

Artificial aging resulted in LTD as shown by an increase in the monoclinic phase fraction for all specimens. After aging for 200 hours, the mean ±SD monoclinic phase fraction increased from 2.90 ±0.34% to 76.1 ±0.64% for Prettau, 2.69 ±0.18% to 76.0 ±0.26% for BruxZir, 4.6 ±0.19% to 35.8 ±0.80% for Katana HT13, and 3.57 ±0.35% to 33.2 ±1.1% for Katana ML (all P<.001). Flexural strength changed from a mean ±SD of 1612 ±197 MPa to all fractured during aging for Prettau (P<.001); 1248 ±73.5 MPa to all fractured during aging for BruxZir (P<.001); 1052 ±84.2 to 1099 ±70 MPa ±130 for Katana HT13 (P=.45); and from 875 ±130 to 909 ±70 MPa (P=.82) for Katana ML. The mean flexural strength values of Prettau and BruxZir decreased with an increase in the monoclinic phase with Spearman rank correlation coefficients of -0.80 (P=.001) for Prettau and -0.63 (P=.022) for BruxZir. No significant changes in flexural strength were measured for Katana ML or Katana HT13 (P>.05).

The LTD of Y-TZP resulted in a significant decrease in flexural strength of Prettau and BruxZir, whereas Katana ML and Katana HT13 exhibited less LTD and no significant decrease in flexural strength.