The effects of temperature and bleaching gels on the properties of tooth-colored restorative materials

Abstract

The effects of bleaching on tooth-colored restorative materials remains controversial. Since previous in vitro research has been performed at different environmental temperatures, it is hypothesized that the temperature differences may be the reason for the conflicting reported results. The purpose of this study was to examine the effect of a simulated at-home bleaching regimen on surface and subsurface microhardness of restorative materials at 2 different environmental temperatures. Seven restorative materials were tested: 4 composite resins, a polyacid-modified composite (compomer), a conventional glass-ionomer cement (CGIC), and a ceramic. For each material, 48 specimens were prepared and divided into 4 groups (n=12): bleaching group at a temperature of 25°C, control group at 25°C, bleaching group at 37°C, and control group at 37°C. The specimens from the bleaching groups were treated with 10% carbamide peroxide for an 8-hour daily test period for 14 days, while the control specimens were stored in artificial saliva for 14 days. The surface and subsurface (0.1-1.0 mm) microhardness values were determined by a Vickers microhardness tester. The surface microhardness data were analyzed with 3-way ANOVA for the effect of different materials, bleaching, and environmental temperature. The subsurface microhardness data were analyzed with repeated measures 4-way ANOVA for the effect of different materials, bleaching, environmental temperature, and subsurface depth as a repeated measure. A post hoc Tukey's Honestly Significant Difference test was used for comparisons among different groups (α=.05). All of the materials showed a loss of surface microhardness, except for the ceramic after bleaching at 37°C. However, only the compomer and CGIC were found to have surface softening after bleaching at 25°C. For compomer and CGIC specimens, bleaching affected microhardness at different subsurface levels ranging from 0.1 mm to 0.5 mm. Bleaching at a higher temperature decreased the subsurface microhardness at deeper levels. The effects of bleaching on restorative materials were material dependent. Environmental temperature influenced the effects of bleaching on surface and subsurface microhardness of restorative materials. Bleaching at increased temperatures showed greater softening effects on the surface and subsurface layers of dental materials.