STATEMENT OF PROBLEM: Advanced high-strength dental ceramics based on zirconium oxide are widely used for dental restorations. However, their durability in liquids has been questioned.
PURPOSE: The purpose of this study was to compare the flexure strength of zirconium dioxide materials designed for different sintering techniques before and after surface exposure to an acidic solution and to correlate the dissolution of the materials with strength data.
MATERIAL AND METHODS: An accelerated aging test, immersion in hot (80°C) 4 vol% acetic acid was used to study the solubility of elements from the surface and the effect on the strength of 3 dental zirconia products: DY: Y-TZP material, milled in HIPed state (Denzir); DM: Mg-PSZ material, milled in dense-sintered state (Denzir M); and CY: Y-TZP material, milled from presintered block and then dense-sintered (Cercon base). Disks were prepared with a surface conditioning as for copings provided for dental restorations. After immersion for 1 week, the liquid was analyzed for a range of elements with ICP-MS. Biaxial flexure strength was measured for 3 samples (n=15) of each material: ground on a 20 µm diamond disk without immersion; after 1 week’s immersion in acetic acid; and after 1 week’s immersion, grinding, and 1 more week in acid. Fracture probability was analyzed by maximum likelihood attribution of individual measurements to 1 or 2 Weibull distributions, each with 2 free parameters.
RESULTS: Dissolution was found to be limited. The total mass loss was less than 0.7 µg/cm(2) for Y-TZP and 3.5 µg/cm(2) for Mg-PSZ. The mean strength was reduced for all materials after immersion in the fluid. Weibull statistics revealed 2 fracture mechanisms in Y-TZP milled in fully sintered condition. For DY only, 2 distributions significantly (P>.99) improved the description of individual treatment data.
CONCLUSIONS: Y-TZP materials showed the highest biaxial flexure strength. Immersion for 1 week in hot 4 vol% acetic acid weakened all 3 zirconia materials by 100 to 200 MPa. Milling in the presintered state resulted in less variability than milling in the HIPed state. Mg-PSZ exhibited the least variability.
