الفهرس 
Introduction .Only 14 pages are availabe for public view
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Abstract
This study was conducted to compare the effect of variation of the outer enamel composite layer thickness on the final shade for the bilaminar technique versus the experimental trilaminar technique and to evaluate the effect of variation of the core layers thicknesses on the final shade for the experimental trilaminar technique using three methods of evaluation which are spectrophotometry by VITA Easyshade® V (Vita Zahnfabrik, Bad Säckingen, Germany), cross-polarized photography by eLAB_prime software (Emulation, Freiburg, Germany) and subjectively by evaluation of shade differences in color calibrated cross polarized photographs.
A total of 216 resin composite discs of 10 mm diameter were prepared in this study. The discs were equally divided into 36 groups (n = 6) according to the three levels of the study, Level 1: Shade combination, 4 groups bilaminar technique with a core layer of 3 mm dentin shade, Experimental trilaminar technique (trilaminar-1) with a core layer of 1 mm body shade + 2 mm dentin shade, first modification of experimental trilaminar technique (trilaminar-2) with a core layer of 1.5 mm dentin shade + 1.5 mm body shade, and second modification of experimental trilaminar technique (trilaminar-3) with a core layer of 2 mm body shade + 1 mm dentin shade) and Level 2: Final shade, 12 groups (A1, A2 and A3). Level 3: Enamel shade thickness, 36 groups (0.25 mm, 0.5 mm, 0.75 mm).
Resin composite discs were prepared using circular split copper molds with a central hole of 10 mm diameter and a thickness of 3 mm (M-3) and 2 mm (M-2) and a specially constructed device composed of a cylindrical mold with an internal diameter of 10 mm and length of 4 cm with a central piston of the same dimensions snugly fit into the internal chamber of the mold. The piston can be depressed by turning a knob. Each full turn of the knob depresses the central piston by 0.5 mm. Each full turn is divided into 50 equal divisions.
Results of color measurements by both spectrophotometry and cross-polarized light photography adopting ΔE00 color differences formulas showed that thickness of the outer enamel composite, core layers thicknesses demonstrated a significant effect on the final shade to be obtained. Non dental observers were found to be more tolerant to shade difference compared to dental observers.
Under the limitations of this study the following conclusions could be deduced:
1. The bilaminar technique is superior to the trilaminar technique in shade reproduction. However, the trilaminar technique is less sensitive to minor variation in outer enamel composite layer thickness.
2. Using equal thicknesses of body and dentin composites in the trilaminar technique produces the least change in color upon minor variation in outer enamel composite layer thickness.
3. Detectability of color difference is highly affected by the method of color measurement. Therefore, supplementing instrumental color measurement with subjective evaluation is important to obtain a clinically relevant shade match.
4. Tolerance to shade difference is affected by profession but not affected by gender.