Effect of different sintering protocols on the optical properties of dental zirconia

Monolithic zirconia materials, despite their high mechanical performance, may present certain aesthetic limitations. In response to increasing esthetic demands, new generations of zirconia stabilized with 4 mol% and 5 mol% yttria have been developed, providing both high strength and enhanced translucency. Multilayer zirconia ceramics have also been introduced to improve the color gradients of monolithic restorations. Although CAD/CAM systems allow the fabrication of highly accurate restorations, clinical adjustments are frequently required, making surface finishing procedures essential. The aim of this study was to evaluate the effects of different sintering protocols and polishing on the optical properties, surface roughness, and phase composition of multilayer monolithic zirconia materials. A total of 96 disc-shaped specimens (10 mm in diameter, 1 mm in thickness) were prepared from Katana UTML, VITA YZ ST, and VITA YZ XT blocks in accordance with ISO 6872. The specimens were subjected to three different sintering protocols (conventional, fast, and high-speed) followed by polishing procedures. Optical properties (color change ΔE₀₀ and translucency parameter TP₀₀) were measured using a spectrophotometer, surface roughness (Ra) was evaluated with a profilometer, and phase composition was analyzed by X-ray diffraction (XRD). Data were statistically analyzed with a significance level set at p<0.05. The results indicated that sintering protocol had no significant effect on color change (ΔE₀₀), and all values remained within clinically acceptable thresholds. TP₀₀ was mainly influenced by material type, with VITA YZ XT showing the highest and VITA YZ ST the lowest translucency. Polishing significantly reduced surface roughness in all groups to clinically acceptable levels. XRD analysis confirmed that tetragonal ZrO₂ remained the dominant phase, with no evidence of monoclinic phase formation, indicating structural stability before and after polishing. In conclusion, the material type was the primary determinant of esthetic outcomes, whereas the sintering protocol mainly influenced production time as a secondary factor. Polishing improved surface quality across all materials, while fast sintering protocols demonstrated potential advantages for streamlining clinical workflows.