Hydrogen Peroxide Diffusion and Optical Response in a Universal Composite Resin: A Spectrophotometric In Vitro Investigation with Clinical Implications

Background: The aim of this in vitro study was to evaluate the extent to which a hydrogen peroxide–based whi- tening agent penetrates a universal composite resin and induces optical changes detectable by spectrophotometry. Materials and Methods: 60 rectangular-shaped composite specimens were fabricated and finished using a standar- dized protocol, then divided into two groups based on the polishing method used: 30 specimens were polished with a single-step polishing system (Enhance), and the remaining specimens were polished with a two-step polishing system(Astropol). Each group was further divided into two subgroups based on the ageing solution in which they were submerged after polishing: red wine (study groups) or physiological solution (control groups). Specimens then underwent ageing and were exposed to a 40% peroxide gel at predefined intervals. Color measurements (L*, a*, b*) were recorded before and after treatment and overall color difference (ΔEab*) was computed according to the CIE L*a*b* formula. Statistical analysis was performed using software and applying Wilcoxon’s W test and Welch’s t test. Results: In both study groups, a statistically significant difference in ΔEab* values was found between the measu- rement points after bleaching (p < 0.05). Between-group comparisons showed statistically significant differences in lateral ΔEab* after bleaching (p < 0.05). All measurement points achieved final ΔEab* values ≤ 3.3 following whitening treatment. Conclusions: Within the limitations of this study, hydrogen peroxide–based systems appear capable of diffusing into universal composite resins and altering their optical properties. Furthermore, the bleaching effect appears to be greater in specimens polished using a single-step method.