Effect of the Outer Composite Layer Thickness in Different Layering Techniques on the Color of Dark Shade Composite Resin Restoration

Abstract

This study was conducted to investigate the effect of variation of the thickness of the outer composite layer in two different shade correction layering techniques on the color of light shade composite restoration. A total of 40 resin composite discs of 12 mm diameter were used in this study. The discs were equally divided into 8 groups (n = 5) according to the two levels of the study, Level 1: Opaque resin composite shade, 2 groups (light opaque shade (O3) or dark opaque shade (O5) and Level 2: The variation in translucent surface layer thickness, 4 groups (0.25mm, 0.5mm, 0.75mm and 1mm). The opaque O4 shade, 2 mm in thickness covered with 0.5mm TN translucent shade served as a control (n = 5). Twenty O3, twenty O5 and five O4 opaque shades discs were prepared in a circular split copper mold with central hole of 10 mm diameter and a thickness of 2 mm. The cured O3, O4 and O5 discs were removed and inserted to another split copper mold with 10 mm diameter and a varying thickness of either 2.25 mm, 2.5 mm, 2.75 mm or 3 mm over which TD, TN and TL composite were layered respectively and light cured. The cured discs were then stored dry in light proof plastic containers for 24 hours before color measurement.

VITA Easyshade Compact (Vita Zahnfabrik, Bad Säckingen, Germany) was used for color measurement of the specimens. The Clarke’s formula20 of ΔE*ab = [(Δ L*) 2 + (Δ a*) 2 + (Δ b*) 2]1/2 as was applied to calculate ΔE. A ΔE*ab of 3.7 units was utilized as the acceptability threshold beyond which color mismatch between two specimens is considered.51

Results of color measurements showed that ΔE with control and all CIE color parameters of L*, a* and b* values of the prepared resin composite discs was affected by both the resin composite shade combination and the outer composite layer thickness. Increasing the thickness of the TD layer results in decreasing the L* value and also results in decreasing a* and b* values. While increasing the thickness of the TL layer result in increasing the L* value but results in decreasing both a* and b* values. Thickness of the TD layer 0.5 mm or 0.75 mm is advisable to correct the shade of a light opaque increment while only 0.5 mm thickness of the TL layer is advisable to correct the shade of a dark opaque increment. Under the limitations of this study, several conclusions could be suggested: 1. The final color of layered resin composite restorations is dependent on the thickness of both TD and TL layers to control. 2. TD layer possibly corrects shade by decreasing the lightness of the opaque layer. 3. TL layer possibly corrects shade by increasing the lightness and decreasing both a* and b* of the opaque layer. 4. Shade correction by darkening a lighter shade gives better match than lightning of the darker shade. 5. An outer enamel layer of 0.5 mm and 0.75 mm is mandatory for shade correction of light shade and 0.5mm for dark shade.