Effects of Surface Polishing of Monolithic Zirconia on Tooth Color

Objectives: The objective was to investigate the influence of polishing on the color of monolithic zirconia.
Methods: 72 monolithic zirconia specimens (Dentsply Sirona, Cercon ht) were prepared and divided into 6 groups (n=12). Three groups included uncolored blanks, while the other three groups included pre-colored blanks. Prior to sintering all specimens underwent additional staining with the target colors Vita A2, A3.5 and A4. Polishing was performed with a precision grinding machine (Struers, Tegramin-25) in three steps, using diamond grinding discs of 220grit, 500grit and 1200grit. In a standardized experimental set-up, color was measured at baseline and after each polishing step using a spectroradiometer (SpectraScan PR670). Measurements were given in CIEL*a*b* coordinates and the color differences were calculated as ΔE and ΔE00 in relation to baseline. The amount of material reduction during the polishing process was controlled with a micrometer screw (Mahr, Mikromar 40EWR).
Results: Polishing caused significant color changes (p≤0.05). The mean color changes (ΔE00) of all 6 groups were 2.2 (after 220grit), 2.5 (after 500grit) and 2.8 (after 1200grit). The material reduction combined with the polishing steps was 10μm (220grit), 7μm (500grit) and 4μm (1200grit).
Conclusions: Polishing of zirconia after sintering leads to a substantial color change. In clinical practice, evaluation of correct shade match should therefore be carried out after final polishing. The influence of surface finish on color measurement should be considered.