A Basic Study on the Background Color Masking Effect of Light-cured Composite Resins : Influence of Layering on Lightness of Composite Resin

Abstract

Natural teeth vary in color, transparency and other properties depending on the site of the tooth. When light-cured composite resin is used for fabricating crowns, the restoration needs to blend in with the natural tooth. However, because of the semi-transparent nature of light-cured composite resin, it is extremely difficult to mask background elements such as stained dentin and tooth discoloration. In the present research, a laminated filling consisting of an incisor shade (I), a standard shade (S) and an opaque shade (O) of various light-cured composite resins was assessed in order to decide the best method of laminated filling based on differences in the degree of brightness of the underlying dentin. A fundamental study was performed aimed at masking the underlying dentin color with the various light-cured composite resins. Samples of the incisor shade, standard shade (A3) and opaque shade (OA3) were prepared using three types of light-cured composite resins, Estelite Σ Quick (Tokuyama Dental), Clearfil® Majesty® (Kuraray Medical) and Beautifil II (Shofu). The samples were prepared at thicknesses of 0.5mm, 1.0mm, 1.5mm and 2.0mm. Each laminated sample was 2.0mm thick, and the lamination was done by changing the thickness of each sample to create laminated samples of O/S, S/I and O/S/I. The samples were placed on a white board, a black board and on mats of various shades of gray (L^* values ranging from 10 to 80) and measured with a Spectra Scan PR650 non-contact spectrophotometer (Photo Research, USA). The XYZ values and CIELAB values were calculated from the measurement results, and the following were determined: (1) the L^* values and C^*ab values on the white and black boards, (2) the translucency parameter (TP value), (3) the contrast ratio (CR value) and (4) the L^* values on the various gray mats.The results indicated that the transparency of Beautifil II was lower than that of Estelite Σ Quick and Clearfil® Majesty®, and that it effectively masked the underlying dentin color. In comparisons of individual shades, the opaque shade was less transparent and less affected by the underlying color, but when the thickness was 1.0mm or more, the color of the sample itself was more likely to have an effect than the underlying color. It was found that laminated samples with a thickness of 2.0mm masked the underlying color even without using the opaque shade, and a high degree of brightness could be obtained. The results confirmed earlier findings showing that with different types of composite resins, the brightness, color and transparency may differ even for the same shade. The color and transparency of natural teeth change as a person ages. Moreover, the brightness, color and transparency also differ depending on the site of the tooth and the type of tooth. In order to achieve more aesthetically pleasing crown restorations, it is first necessary to be aware of the color of the tooth being restored, the cavity preparation form and the optical properties of the composite resin, and then to select the appropriate material and lamination method.