Effect of Additional Light Curing on Color Stability and Degree of Conversion of Mono-Shade Resin Composites

This study aims to examine the effect of additional light curing on the color stability and degree of conversion (DoC) of mono-shade resin composites cured using different light curing units and irradiation levels. Sixty-six disk-shaped samples were prepared for each of the mono-shade (Omnichroma/OC, Vittra APS Unique/VU) and multi-shade resin (Clearfil Majesty ES-2/CME) composites. The samples were randomly divided into three groups and cured for 20 s according to: (1) QTH at 800 mW/cm² (16 J/cm²), (2) LED at 1000 mW/cm² (20 J/cm²), and (3) 1400 mW/cm² (28 J/cm²). After polishing, half of the samples in each group were exposed to additional light curing. Color parameters were measured at baseline and after 28 days of immersion in a coffee solution. CIEDE2000 color (∆E₀₀) and Whiteness Index (ΔWI_D) changes were used to assess color stability. ∆E₀₀ and ΔWI_D were compared with the perceptibility and acceptability threshold. Mono-shade composites exhibited lower DoC with higher ΔE₀₀ and ΔWI_D changes compared to the multi-shade composite. Mono-shade composites showed clinically unacceptable color and whiteness changes. Additional light curing performed using the same protocol both before and after polishing did not contribute to the color/whiteness stability and DoC of either mono-shade or multi-shade resin composites.