Effects of bleaching agents containing nano-hydroxyapatite on sound and demineralized enamel: an in vitro study

This in vitro study aimed to evaluate the effects of office and home bleaching agents containing nano-hydroxyapatite (n-HAP) on the surface roughness, microhardness, and micromorphology of sound and demineralized enamel. A total of 120 enamel specimens were obtained from extracted human third molars, half of which underwent a demineralization cycle to simulate initial enamel demineralization. The specimens were divided into five subgroups (n = 12) based on the bleaching agent applied: no bleaching (control), 40% hydrogen peroxide (HP), 16% carbamide peroxide (CP), 40% HP + n-HAP, and 6% HP + n-HAP. Surface roughness was measured using non-contact optical profilometry, and microhardness was assessed via the Vickers hardness test. Micromorphological changes were examined by scanning electron microscopy (SEM) at × 1,000 and × 4,000 magnifications. Statistical analyses were performed using the Shapiro-Wilk, Mann-Whitney U, Kruskal-Wallis H, and Bonferroni post hoc tests (p < 0.05). In the demineralized groups, surface roughness significantly increased while microhardness decreased (p < 0.05). The highest surface roughness was observed in the demineralized group treated with 16% CP, whereas microhardness significantly increased in the group treated with 6% HP + n-HAP. SEM findings revealed pronounced structural degradation in groups treated with high-concentration HP, while n-HAP-containing groups exhibited mineral deposition and preservation of the prismatic enamel structure. Although no remineralization protocol was used before bleaching, the findings indicate that low-concentration HP agents containing n-HAP may protect enamel with initial demineralization. These results highlight the potential clinical benefit of such formulations in reducing bleaching-related enamel damage.