Translucency of High-Translucency Zirconia versus Lithium Disilicate at Varying Thicknesses: A Systematic Review and Exploratory Meta-analysis

Abstract

Background: High-translucency zirconia has been developed to improve the esthetic limitations of conventional zirconia while retaining favorable mechanical properties. Lithium disilicate remains an established esthetic ceramic, but the relative translucency of the two material families is strongly influenced by thickness and zirconia composition.

Objective: To systematically synthesize comparative evidence on the translucency of true polycrystalline zirconia and lithium disilicate and to examine the influence of material thickness.
Methods: Comparative primary studies evaluating at least one true polycrystalline zirconia and one lithium disilicate group were eligible when a quantitative optical translucency outcome was reported at a defined thickness. We treated the translucency parameter (TP), CIEDE2000-based translucency parameter (TP00), contrast ratio (CR), and direct/total transmittance as distinct outcome constructs. Studies using only zirconia-reinforced lithium silicate without a true zirconia arm were excluded. We included 17 traceable comparative primary studies in the qualitative synthesis. Quantitative pooling was restricted to directly comparable thickness-specific TP datasets with extractable sample size, mean, and standard deviation.

Results: Across the included evidence, lithium disilicate generally demonstrated greater translucency than conventional 3Y zirconia and most high-translucency zirconias at equal thickness. Increasing yttria content improved zirconia translucency, with 5Y/5.5Y materials approaching lithium disilicate in several studies. Thickness consistently reduced translucency. In an exploratory random-effects meta-analysis of directly comparable 5Y/5.5Y zirconia versus lithium disilicate datasets, the pooled mean difference in TP at 0.8 mm was -3.26 (95% CI -4.35 to -2.16; I²=95.5%), favoring lithium disilicate. At 1.5 mm, the pooled mean difference was -0.81 (95% CI -2.27 to 0.65; I²=98.6%), showing no clear pooled difference. The very high heterogeneity and small number of studies require cautious interpretation.
Conclusion: Lithium disilicate remains more translucent than most zirconia formulations at equal thickness, although newer high-yttria zirconias substantially narrow this difference. Thickness, yttria content, shade, specimen geometry, and optical measurement method are major determinants of apparent translucency. Material selection should therefore be based on both composition and clinically intended restoration thickness rather than material name alone.