Effect of bleaching–remineralization sequence on enamel microhardness and surface properties during laser-assisted bleaching

This in vitro study evaluated whether the sequence of bleaching and remineralization procedures and the type of remineralizing agent influence enamel microhardness, surface roughness, and color change following diode laser-assisted bleaching. One hundred twenty human enamel specimens were randomly assigned to two treatment sequences: Bleach-First (BF) and Remineralize-First (RF), each further subdivided into four groups (n = 15) representing different remineralization mechanisms (CPP-ACP–based, calcium–phosphate–magnesium, hydroxyapatite-based, and fluoride varnish). Bleaching was performed using a 35% hydrogen peroxide gel (Whiteness HP, FGM) activated with a 940-nm diode laser (Epic X, Biolase; 7 W, continuous-wave mode; total irradiation time: 120 s). Surface microhardness (VHN; 200 g load for 15 s), surface roughness (Ra), and color (ΔE00) were measured at baseline and after each procedural stage. Data were statistically analyzed using repeated-measures and two-way ANOVA (α = 0.05). A significant difference between treatment sequences was observed at the initial stage (p < 0.001), with bleaching reducing and remineralization increasing microhardness. A significant time × sequence interaction (p < 0.001) indicated that these changes depended on the order of application, while no significant main effect of remineralizing agent or sequence × agent interaction was detected (p > 0.05). Surface roughness values showed no significant differences between sequences or agents, except for a transient increase in the fluoride varnish group in the RF sequence. Similarly, all groups exhibited clinically acceptable color change, with no significant differences between sequences or agents. SEM/EDS analyses further revealed sequence-dependent surface alterations, supporting the microhardness findings. The sequence of bleaching and remineralization procedures was identified as the primary factor influencing enamel microhardness, whereas the type of remineralizing agent had no significant effect. Remineralization applied after bleaching appears more effective for restoring enamel microhardness, without affecting surface roughness or color outcomes. Within the limitations of this in vitro study, post-bleaching remineralization demonstrated favorable effects on enamel microhardness without compromising bleaching efficacy.