Shear bond strength and adhesive remnant index of metal brackets bonded after different post-bleaching waiting periods in enamel bleached with two bleaching agents: An in vitro study.

Background: Successful fixed orthodontic therapy using metal brackets depends on reliable bracket-to-enamel bonding. However, many patients undergo tooth bleaching before fixed orthodontics, which may alter enamel and compromise shear bond strength (SBS). The present in vitro study aimed to compare the SBS and the adhesive remnant index (ARI) of metal brackets bonded after different post-bleaching waiting periods to enamel bleached with two different agents. Material and Methods: An in vitro experimental study was performed using eighty-four human premolars extracted for orthodontic reasons and randomly allocated to seven groups: an unbleached control group and six experimental groups bleached with 22% carbamide peroxide or 35% hydrogen peroxide and evaluated after three post-bleaching intervals (24 hours, 7 days, and 14 days). Brackets were bonded using Orthocem. SBS was tested in a universal testing machine, and failure mode was assessed using the adhesive remnant index (ARI) under stereomicroscopy. SBS was analyzed using robust one-way ANOVA with Games-Howell post hoc comparisons (p < 0.05), and ARI distributions were compared using Fisher's exact test (p < 0.05). Results: SBS varied according to bleaching agent and post-bleaching interval (p < 0.001). With 35% hydrogen peroxide, SBS peaked at 7 days (12.63 ± 4.26 MPa) and decreased at 14 days (4.84 ± 2.08 MPa) (p < 0.05). With 22% carbamide peroxide, SBS also peaked at 7 days (9.73 ± 3.76 MPa) and decreased at 14 days (3.23 ± 1.46 MPa) (p < 0.05). Between agents, SBS was similar at 24 hours and 7 days (p = 0.936 and p = 0.091) but higher with hydrogen peroxide at 14 days (p = 0.039). ARI distributions did not differ across intervals for either bleaching agent (p > 0.05). Conclusions: The SBS of metal brackets depended on the type of bleaching agent and the time elapsed, suggesting that bleached enamel may behave variably during the immediate post-treatment period. In addition, after 14 days, a significant deterioration in adhesion was observed, especially with 22% carbamide peroxide, which could translate into a higher risk of detachment if cementation is performed at that time. On the other hand, the distribution of the IRA did not show significant changes over time, indicating that bleaching would mainly affect the magnitude of bond strength rather than the failure pattern. These findings support orthodontic planning that considers the bleaching agent used and the waiting time before cementation, although clinical studies are needed to confirm its applicability in patients.