Color- and strength-graded zirconia: Strength, light transmission, and composition

Statement of problem Natural teeth are graded in terms of translucency and strength. Graded zirconia materials are now available with a higher yttria content on the top or in the enamel zone to increase surface translucency and a lower yttria content on the bottom or in the dentin zone to increase strength. Such materials could provide multiple advantages over uniform materials and reduce the need for porcelain veneering in anterior artificial crowns; however, studies on the properties of these materials are lacking. Purpose The purpose of this in vitro study was to measure and compare the biaxial flexural strength, percentage light transmission, elemental content, and phase content of zones within and among color graded zirconia blocks and color- and strength-graded zirconia blocks. Material and methods Disks of a color graded material (Katana STML) and a strength- and color-graded material (IPS e.max ZirCAD Prime) were fabricated (Zircom Furnace). Biaxial flexural strength was measured using a piston-on-3-ball test in a universal testing machine (n=10). Absolute light transmission was measured with a spectrophotometer. Elemental compositions were quantified for 3 zones in each of the 2 zirconia materials using X-ray fluorescence analysis. Zirconia phase fractions were quantified for 3 zones using X-ray diffraction analysis (XRD). Where appropriate, 2-way ANOVA, 1-way ANOVA, and Tukey multiple pairwise comparison testing were used to determine which of the 6 zones differed from one another (α=.05). Results The color-graded zirconia exhibited gradients in light transmission and differences in phase content in the 3 zones measured. The color- and strength-graded zirconia exhibited gradients in light transmission, biaxial flexural strength, elemental composition, and phase content in the 3 zones measured. The bottom, dentin, or intaglio layer of a strength graded zirconia material was substantially stronger than all other zones of either material (P<.05). The top, enamel, zones of both materials possessed high light transmissibility (P<.05). The 2 materials differed with respect to biaxial strength (P<.001), light transmission (P<.02), elemental composition, and phase composition overall, as well as in most zone-by zone comparisons (P<.05). The performance and composition of the color graded material was consistent with it being a 5Y material throughout. The performance and composition of the color and strength graded material was consistent with it having a 3Y bottom zone and a 5Y top zone. Conclusions A strength-graded and color-graded zirconia material offers potential advantages in both strength and translucency.