Thickness- and composition-driven variations in the optical properties of dental zirconia

The effects of the yttria content and material thickness of dental zirconia on key optical parameters such as color stability, translucency, spectral reflectance, and radiant transmittance remain insufficiently characterized. While translucency improves with increased yttria content, discrepancies between appearance and actual radiant transmittance pose challenges for material selection. Standardized comparative data between zirconia types is insufficient, potentially leading to suboptimal clinical outcomes.

This in vitro study aimed to evaluate how variations in yttria content and specimen thickness influence the optical properties of dental zirconia by focusing on color difference (ΔE₀₀), translucency parameter (TP₀₀), spectral reflectance, and light irradiation transmittance.

Five types of plate-shaped specimens were prepared using 1 type of monolayer zirconia (HT) and 4 types of multilayer zirconia (HM, UM, SM, and YM) with different amounts of yttria. L*a*b* values were measured using a spectrophotometer, and ΔE₀₀ and TP₀₀ were calculated based on the differences in thickness of each specimen. Spectral reflectance was measured across 400 to 700 nm. Irradiation transmittance under 1200 mW/cm² light-emitting diode light was evaluated for polymerization relevance. ΔE₀₀ was analyzed using a 2-way analysis of variance, while TP₀₀ was assessed using a 1-way analysis of variance (α=.05). In addition, correlation analysis was performed to investigate the relationship between TP₀₀ and radiation transmittance.

The ΔE₀₀ between 1-mm and 2-mm thick specimens increased significantly as the thickness difference increased (P<.001), exceeding acceptability thresholds (ΔE₀₀>1.8) in all groups. TP₀₀ and irradiation transmittance decreased as the thickness increased. HT had the highest irradiance at all thicknesses. The reflectance curves for the monolayer HT and the other multilayers were different. The multilayer zirconia showed complex wavelength-dependent behavior. A strong correlation was observed between TP₀₀ and irradiation transmittance (r>0.899, P<.001).

Yttria content and thickness had a significant effect on the optical properties of zirconia. For each zirconia, when the thickness increased by 1 mm or more, ΔE₀₀ exceeded the clinically acceptable threshold. In addition, TP decreased as the thickness increased for all zirconias. Only HT consistently exceeded the critical irradiation transmittance threshold of 400 mW/cm² at thicknesses ≤1.5 mm. Therefore, to achieve sufficient polymerization with light-polymerizing resin luting agents used in combination with zirconia containing different amounts of yttria, it is necessary to understand esthetics and light intensity and to select materials accordingly.