Transformation-toughened zirconia for dental inlays, crowns and bridges: chemical stability and effect of low-temperature aging on flexural strength and surface structure

Abstract

Objectives. One concern, for transformation-toughened zirconia (Y-TZP) is their liability to low-temperature aging with accompanying alterations of properties such as strength. The loss in strength is attributed to the transformation of tetragonal grains to monoclinic. The transformation is related to loading of the dental inlays, crowns and bridges (reconstructions), temperature and time of exposure to surrounding media (aging) and the manufacturing process of Y-TZP. The purpose of this study was to determine chemical stability and effect of aging (4% acetic acid at 80 °C for 168 h) on flexural strength, surface and crystalline structures of two shades, P0 and P17, of a Y-TZP ceramic used for dental reconstructions. The hypotheses to be tested were that both shades of the dental Y-TZP ceramic have high flexural strength and chemical stability compared to other dental ceramics, and that the strength, surface and crystal structures of the ceramic were not affected by aging. Methods. Forty specimens of Y-TZP, 20 of the shade P0 and 20 of the shade P17 were ground and polished. Ten specimens of each shade were exposed to low-temperature aging. The flexural strength of all 40 specimens was registered. Surfaces of the specimens were evaluated by using scanning electron microscopy (SEM), X-ray diffractometry and roughness recorder. The chemical solubility in 4% acetic acid was recorded by weight loss, and SEM was used to evaluate the surfaces of Y-TZP and dental feldspathic porcelain samples immersed in 8% SnF. Wilcoxon Signed Rank test was used for statistical analysis. Results. As expected, the two shades, P0 and P17, of the studied dental Y-TZP had high strengths that were not affected by aging, and high chemical stability in the tested solutions. Contrary to what was assumed the crystal and surface structures of P0 and P17 were affected. Transformation from tetragonal to monoclinic structures occurred and small elevations on the ceramic surfaces were observed after aging. Significance. The transformed-toughened Y-TZP ceramic used for dental inlays, crowns and bridges has high flexural strength compared to other dental ceramics and is chemically stable. The chemical solubility is well below the limit values of ISO 6872:1995 for dental ceramics.