Effect of 25% in-office hydrogen peroxide bleaching on flexural strength of resin-based composites: an in vitro study

Resin-based composites (RBCs) are commonly used in restorative dentistry, but bleaching agents may compromise their mechanical integrity. The effect of in-office bleaching with 25% hydrogen peroxide (HP) on the flexural strength (FS) of contemporary RBCs remains unclear. An in vitro study was conducted using three resin composites: Beautifil II (giomer), Spectrum (microhybrid), and Omnichroma (universal). Seventy-two bar-shaped specimens (25 × 2 × 2 mm) were allocated into six groups (n = 12 each). Three groups served as unbleached controls representing giomer, microhybrid, and single-shade universal resin composites stored in distilled water. The other three groups were the corresponding bleaching subgroups, treated with 25% hydrogen peroxide gel activated in three 15-minute light-assisted cycles (total 45 min). Bleaching was performed with 25% HP gel (Philips Zoom) applied in three 15-min cycles with light activation. The FS was measured by three-point bending according to ISO 4049. The data were analyzed using two-way ANOVA and Tukey’s post hoc tests (p < 0.05). Bleaching with 25% HP had no significant effect on FS across materials (p = 0.618). The resin composite type significantly influenced the FS (p < 0.001). Spectrum exhibited higher FS values than Beautifil II and Omnichroma, which were statistically similar. In-office bleaching with 25% HP did not reduce the FS of the tested RBCs. The material composition had a greater impact on FS than bleaching, with the microhybrid resin composite (Spectrum) performing better than the giomer and universal resin composites.