Surface treatment-induced changes in optical characteristics of additive- and subtractive-manufactured zirconia...

This study evaluated the effects of various surface treatments on the optical properties of zirconia frameworks fabricated by additive and subtractive manufacturing. Eighty-eight rectangular specimens (14 × 12 × 1 mm³) were prepared, comprising 44 subtractively milled from prefabricated CAD/CAM blocks and 44 additively produced by 3D printing a zirconia slurry. Each group was divided according to surface treatment: control (untreated), air-borne particle abrasion (APA), tribochemical silica coating (TSC), and diamond bur grinding (DBG). Color parameters-lightness (L*), chromatic coordinates (a*, b*), chroma (C*), and hue angle (h°)-were measured to determine color difference (ΔE₀₀) and relative translucency parameter (RTP₀₀). Data were statistically analyzed (α = 0.05). All color parameters were significantly influenced by the manufacturing method, surface treatment, and their interaction (P < 0.001). L* decreased after surface treatments except in the additive-TSC group. RTP₀₀ was affected by both main factors and their interaction (P < 0.001), showing the highest value in additive control (11.16 ± 1.60) and the lowest in subtractive control (8.69 ± 1.02). ΔE₀₀ was significantly affected by surface treatment and the interaction term (P < 0.001), but not by manufacturing method alone (P = 0.079). ΔE₀₀ ranged from 1.16 ± 0.71 (additive-TSC) to 2.59 ± 0.77 (subtractive-TSC). APA produced moderate color changes, while DBG yielded the lowest ΔE₀₀ values in both manufacturing types. Both DBG and additive-TSC groups remained within the clinically acceptable threshold (ΔE₀₀ ≤ 1.8). Subtractive zirconia exhibited higher lightness and more neutral hues, whereas additive zirconia appeared darker, warmer, and more chromatic owing to layer-by-layer structural heterogeneity. Among the treatments, tribochemical silica coating best preserved translucency and minimized color alteration.