To evaluate the translucency, opalescence and light transmission characteristics of resin composites with different thicknesses.
Disks of three resin composites (Estelite∑, Beautifil II, Clearfil Majesty) of A2 shade were prepared in diameter of 10 mm with various thicknesses (0.5 mm, 1.0 mm and 2.0 mm). Color was measured according to CIELAB color scale on a reflection spectrophotometer and a color haze meter, and translucency parameter (TP) and opalescence parameter (OP) were calculated. Using the distribution graphs of transmitted light intensity on a goniophotometer, diffusion factor (DF) as an indicator for a diffuse transmission property and peak gain (G0) for a straight-line transmission property were calculated.
The TP and G0 values significantly decreased in the order: 0.5 mm > 1.0 mm > 2.0 mm thickness ( p < 0.05). The OP value increased as the thickness of resin composite increased from 1.0 mm to 2.0 mm ( p < 0.05), while was similar for the 0.5 mm- and 1.0 mm-thick groups. The DF value increased from 0.5 mm to 1.0 mm, but was similar for the 1.0 mm- and 2.0 mm-thick groups. There was a significant correlation between TP and DF ( R 2 = 0.96, p < 0.001). On the other hand, there was a weak correlation between OP and DF ( R 2 = 0.21, p < 0.005), however within the values of the 0.5 mm-thick group, OP was highly correlated with DF ( R 2 = 0.84, p < 0.001).
At the 0.5 mm thickness, the translucency (TP) and opalescence (OP) of resin composites had a significant correlation with a diffuse transmission property (DF). When more than 1.0 mm thickness of resin composites, translucency and opalescence were influenced by the thickness, in which translucency significantly decreased and opalescence significantly increased.
Light-cured resin composites have been widely used as esthetic restorative materials for anterior and posterior teeth. However, there are difficulties in accurately matching the color of resin composites and to that of the surrounding teeth because their color appearances are influenced and perceived by various factors; color and its elements of hue, value and chroma; translucency, fluorescence and opalescence; light transmission and diffusion; texture and luster of the surface .
The translucency of resin composite has usually been evaluated by its translucency parameter (TP) or contrast ratio (CR) . The TP value corresponds directly to the common visual assessments of translucency , and is regarded as an indicator for masking ability . It has been shown that the TP value increased exponentially with a reduction in thickness of resin composite regardless of the shade . In addition, based on the thickness at which the TP value became 2.0 – the threshold of color distinction by human eyes, the minimum thickness which can avoid an unfavorable color change by the background color was over 2.56 mm for body-shades and 1.88 mm for opaque-shades . However, the thickness of the restoration may be limited in the clinical situation. Therefore, other properties such as opalescence and fluorescence that can mask underlying color should be investigated .
Opalescence occurs where there is light scattering of the shorter wavelengths of the visible spectrum, giving a material a bluish appearance under reflected light and an orange/brown appearance under transmitted light . Since the enamel of natural teeth is opalescent, resin composite should be also exhibit opalescence. The opalescence of esthetic restorative materials has usually been determined using the opalescence parameter (OP) . The OP does not correlate with the masking effect, however the OP contributes to masking of the background color along with TP . It was demonstrated that the masking effect for resin composites correlates with the translucency when the translucency was obviously different; however, when the translucency was in a similar range, opalescence influenced the masking effect of resin composites . Opalescence as well as translucency would be therefore be influenced by the light transmission characteristics through the material .
When light illuminates resin composite, it scatters at the surface of the filler particles and diffuses in multiple directions. Light then emerges as diffuse transmission and as straight-line transmission depending on the thickness of the material . Blue light is preferentially deviated from straight-line transmission through the material, unlike the longer red-orange wavelength. A two-dimensional goniophotometer which consists of a metal halide lamp as a light source and an angular goniometer, is useful for measuring the light reflecting and transmission characteristics, in which the intensity of specular and diffuse light in the material can be measured using a light source rotated around the fixed specimen at different angles to the specimen’s surface. Using a goniophotometer, previous studies have evaluated effects of hairdye on specular and diffuse reflectance of hair fibers . However, there have been few studies, which have evaluated the light transmission characteristics (straight-line and diffusion transmission) of resin composites using a goniophotometer . In addition, the relationship between translucency, opalescence and light transmission characteristics (straight-line and diffusion transmittances) of different thicknesses of resin composite has not been clarified.
Therefore, the purpose of this study was to determine the translucency and opalescence of resin composites with a reflection spectrophotometer and a color haze meter, and to evaluate the straight-line and diffusion transmissions of resin composites using a goniophotometer. We also investigated their relationships with different thicknesses of resin composite.
Materials and methods
Three light-cured resin composites of A2 shade, Estelite∑ (Tokuyama Dental Corp., Tokyo, Japan), Beautifil II (Shofu Dental Corp., Kyoto, Japan) and Clearfil Majesty (Kuraray Medical Inc., Tokyo, Japan) were used in this study ( Table 1 ). Resin composite disks (10 mm in diameter) were made and covered with celluloid strips on glass plates, which were separated by spacers of 0.5 mm, 1.0 mm and 2.0 mm thick. Five specimens were prepared for each material. After curing with a light-curing unit (Optilux 500, Demetron, Danbury, CT, USA, 600 mW/cm 2 ) for 60 s each from the top and bottom sides, the strips and glass plates were removed. All specimens were stored at 37 °C in 100% relative humidity for 24 h before measurement.
|Estelite∑||Filler: 82 wt% (71 vol%) silica–zirconia spherical of 0.1–0.3 μm (average 0.2 μm)
Base resin: Bis-GMA, TEGDMA
|Tokuyama Dental Corp., Tokyo, Japan||006067|
|Beautifil II||Filler: 83.3 wt% (68.6 vol%) multi-functional glass and S-PRG filler based on fluoroboraluminosilicate glass
Particle size range: 0.01–4.0 μm (mean 0.8 μm)
Base resin: Bis-GMA, TEGDMA
|Shofu Dental Corp., Kyoto, Japan||040718|
|Clearfil Majesty||Filler: 78 wt% (66 vol%) silanated barium glass filler, pre-polymerized organic filler
Micro-filler (glass filler): mean 1.5 μm
Nano-filler: mean 20 nm
Base resin: Bis-GMA, TEGDMA, hydrophobic aromatic dimethacrylate
|Kuraray Medical Inc., Tokyo, Japan||0028AA|
Measurements of translucency and opalescence parameter by spectrophotometer
The color reflectance of resin composite over white and black backgrounds was measured using a reflection spectrophotometer (SE6000, Nippon Denshoku, Tokyo, Japan), according to the CIELAB color scale relative to the standard illuminant C2 over a zero calibrating box (CIE L * = 0.0, a * = 0.0, and b * = 0.0), excluding ultraviolet light. The translucency parameter (TP) was obtained by calculating the color difference of the specimen over a black and white backing. The formula is:
TP = [ ( L B − L W ) 2 + ( a B − a W ) 2 + ( b B − b W ) 2 ] 1 / 2
Subscript B refers to the color coordinates over a black background and the subscript W refers to those over a white background.
The color transmittance of resin composite was measured using a color haze meter (COH 400, Nippon Denshoku, Tokyo, Japan), according to the CIELAB color scale as previously mentioned. Illuminating and viewing configurations were CIE diffuse/8° geometry. The opalescence parameter (OP) was calculated as the difference in blue-yellow and red-green coordinates between the transmitted and reflected colors. The formula is:
OP = [ ( CIE a T − CIE a R ) 2 + ( CIE b T − CIE b R ) 2 ] 1 / 2