Comparative in-situ evaluation of surface roughness and surface-subsurface microhardness of bovine and human enamel after hydrogen peroxide bleaching.

Abstract

To compare bleaching-induced changes in surface roughness, surface microhardness, and cross-sectional microhardness between bovine and human enamel under in-situ conditions. Forty enamel/dentin specimens (20 bovine and 20 human) were allocated to non-bleached and bleached groups. Ten volunteers wore palatal appliances containing four specimens each. Bleaching was performed with 35% hydrogen peroxide during three weekly sessions. Surface roughness and surface microhardness were evaluated at baseline (T0), immediately after bleaching (T1), and 15 days later (T2). Cross-sectional microhardness was measured at T2 at depths from 5 to 200 μm. Surface morphology was qualitatively assessed by scanning electron microscopy. Data were analyzed using linear mixed-effects models, with volunteer included as a random effect to account for clustering of specimens within participants (α = 0.05). No significant differences between bovine and human enamel were detected for surface roughness, surface microhardness, or cross-sectional microhardness under the evaluated conditions (p > 0.05). Bleaching significantly increased surface roughness and reduced surface microhardness immediately after treatment for both substrates (p < 0.001). Surface microhardness recovered to baseline values after 15 days, whereas surface roughness remained significantly increased relative to baseline. Cross-sectional microhardness changes were depth-dependent, with bleaching-related reductions detected up to approximately 20 μm in bovine enamel and 10 μm in human enamel. Qualitative SEM analysis showed subtle superficial irregularities following bleaching, with no clear qualitative differences between substrates. Under the evaluated in-situ conditions, bovine and human enamel showed no consistent differences in their mechanical response to 35% hydrogen peroxide bleaching. Bleaching predominantly affected the superficial enamel, with surface microhardness recovering but surface roughness remaining increased after 15 days.