Effect of immediate and delayed composite restoration on shear bond strength to remineralized initial enamel lesions: an in vitro study

Dental caries weakens enamel, and remineralizing agents may help restore its structure; however, their effect on bonding to composite resin and the optimal timing of bonding after remineralization remain unclear. This study evaluated the effect of hydroxyapatite (Karex gel) and fluoride-containing hydroxyapatite (Remin Pro) as well as post-remineralization bonding timing on the shear bond strength (SBS) of composite resin to enamel. Eighty extracted human premolars and molars were divided into four groups: non-demineralized enamel (CI), demineralized enamel (CD), hydroxyapatite-treated enamel (KP), and fluoride-containing hydroxyapatite-treated enamel (RP). Except for the CI group, all teeth were demineralized. The RP and KP groups were treated with remineralizing agents for 28 days, while the CI and CD groups were stored in artificial saliva during this period. Each group was further divided based on the timing of composite bonding after remineralization: immediate (24 h) and delayed (7 days). SBS was measured at a crosshead speed of 0.5 mm/min, and failure modes were evaluated under microscopy. Data were analyzed using two-way robust ANOVA, Shapiro–Wilk test, and Bonferroni post hoc analysis. Failure modes were compared using Fisher–Freeman–Halton and McNemar tests (p < 0.05). The group and time interaction was statistically significant (p = 0.004). Immediate bonding (24 h) showed similar SBS values across all groups (n = 40). At 7 days (n = 40), the fluoride-containing hydroxyapatite group (RP) demonstrated significantly higher SBS than the other groups (9.4 MPa), followed by KP group (7.45 MPa), while the lowest values were observed in the demineralized (CD) and control (CI) groups (3.25 MPa and 4.55 MPa, respectively). Most failures were adhesive, while mixed and cohesive failures were more frequent in remineralized groups. These findings suggest that fluoride-containing hydroxyapatite may enhance enamel composite bond strength after adequate post-remineralization time. The observed SBS values are more suitable for orthodontic/low-stress applications, and further maturation or surface conditioning may be required for load-bearing restorations.