MATERIALS AND METHODS
Two commercially available resin composites, Durafil VS (Heraeus Kulzer, Wehrheim, Germany) and TPH3 (Dentsply Caulk, Milford, USA), were used in this study. Durafil VS (DF) is a microfilled composite composed of 40% volume silicon dioxide fillers and BisGMA matrix, while TPH3 is a microhybrid composite composed of 58% volume barium-alumino-boro-silicate, fluoro-boro-silicate fillers, and a matrix of BisGMA and TEGDMA. Twenty-four specimens, 15 mm in diameter and 2 mm thick, were fabricated from each material (12 of shade A2 and 12 of shade A4) using flexible molds. Each specimen was prepared as one increment and light-polymerized from each side for 40 seconds using a QTH light unit with intensity of 850 mW/cm2 (Optilux 501, Kerr, USA). Specimens were polished with silicon carbide paper and each group of 12 specimens was subdivided into three subgroups (n=4), with each assigned to a bleaching agent. Specimens were stored in distilled water at 37°C for 24 hours and then subjected to bleaching using one of three in-office agents. Hydrogen peroxide concentration and pH of the bleaching gels as well as mode of application recommended by the manufacturers are listed in Table 1.
All subgroups were subjected to an initial bleaching session, after which color and surface roughness were measured. The specimens were then stained by placing them in a coffee solution prepared by boiling 3.6 grams of coffee powder (Nescafe Classic, Nestle, Switzerland) in 300 ml of distilled water for 10 minutes, then filtering it. Immersion in the coffee solution was maintained for 48 hours at 37°C, after which the color of specimens was determined before subjecting them to a second bleaching session. Color assessment was performed using a colorimeter (Chroma Meter CR 300, Minolta Co. Ltd., Japan), which was calibrated before every session following the manufacturer’s instructions. The colorimeter displayed the different color parameters (L*, a* and b*) according to the CIELab color system, where L* describes the luminance reflectance, while a* and b* describe the red-green and yellow-blue color coordinates, respectively.
The change in color from baseline was calculated after the first bleaching session (ΔE1), after staining (ΔE2), and after the second bleaching session (ΔE3). The change in color after the second bleaching compared to the color after staining (ΔE3S) was also calculated. ΔE values were obtained using the Hunter’s equation:18 ΔE=[(ΔL)2+(Δa)2+(Δb)2]1/2, Where: ΔL=Lvisit−Lbaseline              Δa=avisit−abaseline                    Δb=bvisit−bbaseline
For surface roughness measurements, the specimens were examined for topographical quality using an Environmental Scanning Electron Microscope (ESEM) (Quanta 200, FEI Company, Philips, Netherland). Specimens were photomicrographed at 1000 times magnification and the images were analyzed quantitatively using image analysis software. A three-dimensional surface roughness profile was automatically plotted. At the Z-axis, the peaks or surface elevations were marked, and the height of each peak was automatically computed. Mean surface roughness values (Ra) were calculated for each specimen. Ra describes the arithmetic mean of all values of the roughness profile (R) over the evaluated length.
Data were statistically analyzed using SPSS 15.0 package (Chicago, Illinois). Regression models with two-way ANOVA and Tukey’s post-hoc tests were used to test significance for the effects of composite material and bleaching agent on color and surface roughness at P≤.05. Pearson’s correlation coefficient was used to determine significant correlations between color and surface roughness measurements.