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

Abstract

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.

The authors' abstract, as published at the source. BMC Oral Health, 2026 · DOI ↗