Layer-Dependent Degradation of Surface-Coated PMMA: Interplay Between Fabrication Method, Simulated Toothbrushing on Surface Roughness and Color Stability

Background Surface coatings are used to improve the surface and optical properties of provisional restorations, but how the number of coating layers affects their durability across PMMA fabrication methods remains unclear. This in vitro study evaluated the effects of coating-layer number and fabrication method on surface roughness, coating thickness, and color change after simulated toothbrushing. Methods Eighty-one PMMA specimens (10 × 10 × 3 mm) were fabricated by milling, 3D printing, or auto-polymerization and coated with one, two, or three layers of Optiglaze Color (n = 9 per subgroup). Surface roughness (Ra), incremental layer thickness, and color (CIELab) were evaluated before and after 10,000 toothbrushing cycles, and coating cross-sections were examined by scanning electron microscopy (SEM). Data were analyzed using ANOVA (α = 0.05). Results Baseline Ra was highest in auto-polymerized (2.50 ± 0.74 µm), intermediate in 3D-printed (1.36 ± 0.35 µm), and lowest in milled (0.70 ± 0.33 µm) specimens. SEM showed that cumulative coating thickness increased from approximately 100 to 200 µm with additional layers. Coating application reduced Ra in all groups except the one-layer milled group. After brushing, Ra increased significantly in most groups, most markedly in the one- and two-layer milled groups, whereas the three-layer coating maintained the lowest Ra values. Thickness of the C Plus and Clear HV layers decreased significantly in the milled and auto-polymerized groups but not in the 3D-printed group. Δ E * remained ≤ 3.3 except in the three-layer 3D-printed and auto-polymerized groups, and coating-layer number, but not fabrication method, significantly affected Δ E *. Conclusions Within the limitations of this in vitro study, multilayer coatings were associated with lower Ra after toothbrushing than the single-layer coating, although Ra still increased significantly in several multilayer groups. Thickness loss varied according to the fabrication method and layer, and the three-layer coating exceeded the acceptable color threshold in two methods. The benefit of additional layers should be weighed against both surface durability and color stability.