Color Changes Caused by Grinding Adjustments of Individually Stained Zirconia

Objectives: To meet aesthetic requirements, monolithic zirconia can be stained by color infiltration prior to the sintering process. Discolorations may occur due to mechanical removal, as a result of occlusal adjustment, leading to damasking effects. The objective was to analyze the effect of step-wise material reduction on the color stability of individually stained monolithic zirconia.

Methods: Uncolored zirconia specimen (Dentsply Sirona, Cercon ht, white, n=36) were stained by color infiltration prior to the sintering process. Target colors were Vita A2, A3.5 and A4. A staining protocol for each target color was developed and standardized. After sintering, the zirconia plates were grinded with diamond discs (220grit, reduction 10µm) to avoid reflections and other surface effects on the color measurement. A spectroradiometer (Photoresearch, SpectraScan PR670) was used to control the target color at baseline and after each step of subsequent removal of material from the zirconia specimen, simulating occlusal adjustment by step-wise grinding of the colored surface. The material was removed with a precision grinding machine (Struers,Tegramin-25) in steps of 20µm, 50µm, 100µm, 150µm, 200µm, 250µm, 300µm, 400µm and 500μm, and controlled with a micrometer screw (Mahr, Mikromar40EWR).

Results: Color measurements were given in CIEL*a*b*coordinates and the color differences were calculated as ΔE and ΔE00 in relation to the baseline values. Mechanical removal showed a significant color change from 20μm to 500µm (p≤0.05). After 100μm reduction, color differences (ΔE00) of 2.8 for A2, 1.9 for A3,5 and 1.7 for A4 were observed. A reduction of 500µm resulted in color differences (ΔE00) of 13.7 for A2, 13.5 for A3.5 and 14.5 for A4.

Conclusions: Assuming a 50:50% acceptability threshold of ΔE00=1.8, occlusal adjustments and other grinding procedures result in demasking effects if 100µm have to be removed. Further research is necessary to improve the infiltration depth of staining solutions.