The effect of rehardening methods/storage environment on the delayed bond strength of composite resin to bleached enamel

Rehardening/remineralizing agents have been suggested to reverse the adverse effects of bleaching on the enamel. This research compared the effects of different nano-hydroxyapatite (NHA) combinations, artificial saliva alone, or with pH cycling on delayed shear bond strength (SBS) to in-office bleached enamel. In this in-vitro experimental study, ninety extracted intact premolars with the flattened enamel surface were randomly divided into nine groups: Group 1 (No-bleach) served as negative control; the others were bleached with 40% hydrogen peroxide; Group 2 (bleach/Immediate bond) immediate resin bonding; in groups 3 to 9, resin bonding was carried out after three weeks; Group 3 (bleach/Distilled Water), storage in distilled water; Group 4 (bleach/Saliva), storage in artificial saliva; Groups 5 (bleach/Removal), after storage, 0.5 mm removal of enamel; Group 6 (bleach /Dem-Rem) one-week pH-cycling followed by two-week storage in saliva; Group 7 (bleach/NHA), one-week application of nanohydroxyapatite followed by two-week storage in saliva; Group 8 (bleach/NHA-RI), resin infiltration after NHA treatment; Group 9 (bleach/NHA-CH), one-week application of nanohydroxyapatite-chitosan combination followed by two-week storage in saliva. After 24 h, SBS was measured. Immediate bonding resulted in the lowest SBS (13.67 ± 1.97 MPa; P < 0.001). Among the 3-week delayed groups, Dem-Rem (27.61 ± 2.5) and NHA (26.91 ± 1.23) showed significantly higher SBS than distilled water (19.79 ± 1.76), removal (23.86 ± 1.48), and NHA-RI (21.19 ± 1.73) groups ( P < 0.01 for all comparisons). No significant SBS difference was observed between the tested NHA and NHA–chitosan formulations (25.06 ± 2.24 vs. 26.91 ± 1.23 MPa; P = 0.37). Furthermore, the SBS of the Saliva group (25.47 ± 1.69 MPa) was significantly higher than that of the distilled water and NHA-RI groups ( P < 0.001), with no significant differences compared with the remaining delayed-bleached groups. Delayed bonding after NHA or NHA–chitosan treatment resulted in bond strength comparable to that obtained with artificial saliva storage alone or with demineralization–remineralization cycling.