Immediate vs. delayed bonding to bleached enamel: an in-vitro evaluation of nanoscopic changes and bond strength

High-concentration hydrogen peroxide bleaching can induce structural and compositional changes in enamel that adversely affect adhesive bonding. While delayed bonding after bleaching has been investigated, existing evidence is largely based on bond strength outcomes alone, with limited insight into the associated nanoscale and surface alterations that influence adhesive performance. This study evaluated tensile bond strength and corresponding nanoscopic enamel changes in unbleached and bleached enamel bonded with orthodontic brackets after 24 h and a two-week delay. Ninety extracted human premolars were divided into three groups (n = 30): unbleached (control), bleached with immediate bonding (after 24 h), and bleached with delayed bonding (14 days). After cleaning and polishing, the Everbrite Advanced Tooth Whitening System (CA91748USA) was employed for bleaching the coronal tooth surface, and dual-cure self-adhesive RlyxU200 was used to bond orthodontic brackets on the buccal surface of the teeth. Tensile bond strength testing was conducted using a universal testing machine. The type of bond failure was analyzed under an inverted microscope through the ARI index. Enamel surface topography and elemental changes were analyzed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), respectively, while atomic force microscopy (AFM) was used for 3D nanoscopic characterization of surface roughness. Data were analyzed using one-way ANOVA with Tukey post hoc tests. The highest bond strength was observed in the unbleached group (8.91 ± 2.56 MPa), which was significantly greater than both immediate (2.72 ± 0.93 MPa) and delayed (4.54 ± 2.37 MPa) bonding groups (p < 0.001). Immediate bonding showed more adhesive failures, while delayed bonding exhibited partial recovery of bond strength. SEM and AFM revealed increased surface porosity, grooves, and reduced resin penetration in bleached groups. EDX analysis demonstrated reduced calcium and phosphorus levels in bleached enamel compared to controls. Bleaching significantly compromises the enamel morphology, mineral composition, and adhesive performance at the enamel-adhesive interface. Immediate bonding after bleaching results in the weakest adhesion, whereas a two-week delay leads to partial, but incomplete recovery of bonding performance under in vitro conditions.