Purpose: This laboratory-based study evaluated the effects of sintering protocols, yttria content, and zirconia thickness on the optical properties of monolithic zirconia.

Methods: Three partially stabilized zirconia (PSZ) materials, one monolayer (HT) and two multilayer (GE, FX) samples, with thicknesses of 0.5-1.5 mm were tested under conventional and speed sintering protocols. Translucency parameter (TP), color difference (ΔE₀₀), and spectral reflectance were measured using a spectrophotometer against standardized black and white backgrounds. Statistical analyses were performed using a one-way analysis of variance, Tukey's post-hoc tests, and correlation analysis.

Results: The TP values decreased with increasing zirconia thickness; significant differences were observed between the sintering protocols. Speed sintering resulted in lower transparency in certain materials, particularly GE. Conversely, HT exhibited consistent optical properties across the sintering protocols. The ΔE₀₀ values for multilayer zirconia exceeded clinically acceptable thresholds, with greater variations observed for thicker specimens. Spectral reflectance curves were minimally influenced by the sintering protocol and thickness for monolayer zirconia, whereas they showed significant variations for multilayer zirconia.

Conclusion: The results indicate that sintering protocols and material composition significantly influence the optical properties of zirconia, underscoring the necessity of optimizing processing conditions to enhance both esthetic and functional performance in clinical applications.