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


OBJECTIVE
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.


METHODS
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 (σf) and m/t. The Spearman rank correlation coefficient was used to assess the relationship between σfand monoclinic phase fraction (α=.05).


RESULTS
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).


CONCLUSIONS
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.
Research Associate Professor, Department of Material Science and Engineering, University of Washington, Seattle, Wash. Electronic address: bflinn@u.washington.edu.
Brian D
BD
Flinn
Professor, Department of Restorative Dentistry, School of Dentistry, and Adjunct Professor Department of Materials Science and Engineering, University of Washington, Seattle, Wash.
Ariel J
AJ
Raigrodski
Research Associate Professor, Department of Oral Health Sciences, University of Washington, Seattle, Wash.
Lloyd A
LA
Mancl
Postdoctoral Research Associate, Department of Material Science and Engineering, University of Washington, Seattle, Wash.
Ryan
R
Toivola
Research Engineer, Department of Material Science and Engineering, University of Washington, Seattle, Wash.
Tuesday
T
Kuykendall
D002516:Ceramics / Q000737:chemistry* / Q000627:therapeutic use; D003793:Dental Restoration, Permanent / Q000379:methods*; D003799:Dental Stress Analysis; D013718:Tensile Strength; D013997:Time Factors; D015019:Yttrium / Q000737:chemistry* / Q000627:therapeutic use; D015040:Zirconium / Q000737:chemistry* / Q000627:therapeutic use
C499362:yttria stabilized tetragonal zirconia; D015019:Yttrium; D015040:Zirconium