الفهرس 
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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 10 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 (O1) or dark opaque shade (O3)) and Level 2: The variation in translucent surface layer thickness, 4 groups (0.25mm, 0.5mm, 0.75mm and 1mm). The opaque O2 shade, 2 mm in thickness covered with 0.5mm TN translucent shade served as a control (n = 5). Twenty O1, twenty O3 and five O2 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 O1, O2 and O3 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.32
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 but has no effect on both the a* and b* values. While increasing the thickness of the TL layer has no effect on the L* value but results in decreasing both the a* and b* values.
An increased thickness of the TD layer of 0.75 – 1 mm is advisable to correct the shade of a light opaque increment while a decreased thickness of the TL layer of 0.25 – 0.5 mm is advisable to correct the shade of a dark opaque increment. Shade correction of a light opaque increment by TD gives a lower color difference to control than shade correction of a dark opaque increment by TL.
Under the limitations of this study, the following conclusions could be deduced:
1. The final color of layered resin composite restorations is dependent on the thickness of the outer enamel composite layer.
2. Shade correction with dark translucent shade (TD) requires an outer composite layer thickness of 0.75 - 1 mm to occur while shade correction with light translucent shade (TL) requires an outer composite layer thickness of 0.25 - 0.5 mm.
3. Mechanism of shade correction by TD is by decreasing the lightness while that by TL is by decreasing both a* and b* values.
4. Shade correction by darkening with TD gives better color match than shade correction by lightening with TL.