Statement of problemConcern has been raised with regard to the low-temperature degradation (LTD) of translucent yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) for monolithic zirconia restorations.PurposeThe 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.Material and MethodsThin 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 (σf) and m/t. The Spearman rank correlation coefficient was used to assess the relationship between σf and monoclinic phase fraction (α=.05).ResultsArtificial 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).ConclusionsThe 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.

Statement of problemConcern has been expressed with regard to hydrothermal aging of yttria-stabilized tetragonal zirconia polycrystalline.PurposeThe purpose of this study was to assess the accelerated aging characteristics of a new yttria-stabilized tetragonal zirconia polycrystalline material and 2 commercially available yttria-stabilized tetragonal zirconia polycrystalline materials by exposing specimens to hydrothermal treatments in steam at 134°C, 0.2 MPa, and at 180°C, 1.0 MPa.Material and methodsThin bars of zirconia: Prettau, Zirprime, and a new zirconia, ZirTough, n=55 for each brand (22×3×0.2 mm) were cut and ground from blocks sintered according to the manufacturer’s specifications. The control specimens for each group were evaluated in the nonaged condition, and their chemical composition was measured with energy dispersive spectroscopy. The experimental specimens were artificially aged under standard autoclave sterilization conditions, 134°C at 0.2 MPa (n=5 per group at 5, 50, 100, 150, and 200 hours), and under standard industrial ceramic aging conditions, 180°C at 1.0 MPa (n=5 per group at 8, 16, 24, and 48 hours). The tetragonal to monoclinic transformation was measured by using x-ray diffraction for all groups. Flexural strength was measured with a 4-point bend test (ASTM 1161-B) for all the groups, and the fracture surfaces were examined with scanning electron microscopy. The data were analyzed as a function of aging time. To test for an aging effect on the flexural strength and the monoclinic-tetragonal ratio, a 1-way ANOVA (with heteroscedasticity-consistent standard errors) was used to test for a general time effect. For the analyses of the monoclinic-tetragonal ratio, the same specimens were used at 0 hours and after aging, and the data were analyzed with an ANOVA for an incomplete block design. The relationship between flexural strength and monoclinic-tetragonal ratio was assessed with the Spearman rank correlation coefficient based on the average value at each aging.ResultsAfter 200 hours at 134°C and 0.2 MPa, the flexural strength decreased from a mean (standard deviation) of 1328 ±89.9 MPa to all fractured during aging for Prettau (P<.001); 1041 ±130 to 779 ±137 MPa for Zirprime (P=.<.001) and 1436 ±136 to 1243 ±101 MPa for ZirTough (P=.017). After 200 hours at 134°C and 0.2 MPa, a portion of the tetragonal crystals transformed to the monoclinic phase in all specimens. The mean (standard deviation) monoclinic phase fraction increased from 3.08% ±0.28% to 78.8% ±2.0% for Prettau, 1.95% ±0.48% to 74.8% ±0.52% for Zirprime, and 12.4% ±0.60% to 31.4% ±4.4% for ZirTough (all P<.001). After 16 hours at 180°C and 1.0 MPa, all Prettau specimens had spontaneously fractured during aging. The Zirprime and ZirTough specimens were intact after 48 hours at 180°C and 1.0 MPa, and the mean (standard deviation) flexural strength had decreased from 1041 ±130 MPa to 595 ±88.4 MPa for Zirprime and 1436 ±136 MPa to 1068 ±76.8 MPa for ZirTough (all P<.001). The mean (standard deviation) monoclinic phase fraction increased from 3.08% ±0.28% to 79.0% ±0.13% for Prettau, from 1.95% ±0.48% to 68.1% ±4.4%, for Zirprime, and from 12.4% ±0.60% to 39.5% ±5.56% for ZirTough (all P<.001). The flexural strength decreased with an increase in the monoclinic phase for all the groups (Spearman rank correlation coefficients, –0.71 to –1.0). Scanning electron microscope micrographs revealed a transformed layer on the fracture surfaces. The decrease in flexural strength was related to the increase in monoclinic phase from long-term degradation.ConclusionHydrothermal aging of zirconia caused a statistically significant decrease in flexural strength of thin bars of zirconia, which was the result of the transformation from tetragonal to monoclinic crystal structure. ZirTough exhibited the least decrease in strength and smallest amount of monoclinic phase after aging.