Effect of enamel shade thickness and dentin shade base on optical properties of a BisGMA-based composite

Objectives: To evaluate the effect of enamel shade thickness on color parameters and translucency of a commercial microhybrid BisGMA-based composite.
Methods: Composite LC MH Ceramic AP (S&C Polymer GmbH, Germany) was used to prepare rectangular-shaped specimens of the following thickness: 0.2 mm (dentin A2 shade), 0.6 mm, 1 mm and 2 mm (enamel A2 shade). A high-intensity Bluephase polywave (Ivoclar Vivadent) light-curing unit was used to cure each sample for 40 s at 1 mm distance. Color and translucency (TP) of uncured and cured dentin (D) and enamel (E) specimens and cured combinations D2+E0.6, D2+E1 and D2+E2 were measured using a spectrophotometer VITA Easyshade Advance 4.0 (Vita) under the standard illuminant D65 against a white and black background. Three specimens were prepared in each group. Color values were expressed using the CIEL*a*b* system. Data were analyzed using one-way ANOVA with Tukey's post-hoc test at α=0.05.
Results: TP values of uncured E specimens were 16.56±0.13, 6.29±0.36 and 2.93±0.05 (E0.6, E1, E2, respectively) and significantly increased to 25.15±1.58, 18.08±0.28 and 8.57±1.91 (E0.6, E1, E2, respectively) after polymerization (p<0.05). No differences in TP were observed between D+E groups (p>0.05). Color differences between uncured E specimens were not significantly different (p>0.05). Cured E specimens became significantly "whiter" (p<0.05) which was confirmed by the greatest Δb* between uncured and cured groups. After polymerization, color differences between E groups exceeded the acceptability threshold (ΔE≥3.3), while those between D+E groups were clinically acceptable (ΔE≤3.3).
Conclusions: Translucency ​​decreased with increasing thickness of the enamel shade layer. Although the thickness of the enamel shade layer did not affect color parameters before curing, it significantly affected color changes after curing. Curing mostly affected b*, followed by L* whereas a* was the least affected color parameter. In general, dentin shade base stabilized color and translucency of the enamel shade layer.