Effect of adhesive strategy on the shear bond strength of monochromatic and nanohybrid composite resins to glass ionomer cements in the sandwich technique

Background The bilayered (sandwich) technique combines the biological benefits of glass ionomer cements (GICs) with the esthetics and mechanical strength of composite resins. Interfacial bonding in this technique is influenced by GIC type, composite resin, and adhesive strategy. This study evaluated the shear bond strength (SBS) of monochromatic and nanohybrid composite resins bonded to resin-modified (RMGIC) and self-cure GIC using etch-and-rinse and self-etch adhesive protocols. Materials and methods Eighty standardized GIC specimens (4 × 3 mm) fabricated from RMGIC and self-cure GIC were randomly assigned to eight groups ( n = 10). Monochromatic (Vitra Unique) and nanohybrid (Filtek Z250 XT) composite resins were bonded using either an etch-and-rinse (Adper Single Bond 2) or self-etch (Clearfil SE Bond) adhesive. SBS was measured using a universal testing machine, and failure modes were analyzed. Data were evaluated using three-way ANOVA and Tukey's post-hoc test ( p < 0.05). Results RMGIC demonstrated significantly higher SBS than self-cure GIC ( p = 0.006), and the nanohybrid composite resin exhibited higher SBS values than the monochromatic composite resin ( p = 0.005). A significant interaction between GIC type and adhesive strategy was observed ( p = 0.033), whereby the etch-and-rinse adhesive enhanced bonding to RMGIC but did not significantly improve bonding to self-cure glass ionomer cement. The highest SBS values were observed when RMGIC was bonded using an etch-and-rinse adhesive and restored with a nanohybrid composite resin, whereas the lowest SBS values occurred when self-cure glass ionomer cement was bonded using a self-etch adhesive and restored with a monochromatic composite resin. Conclusion RMGIC combined with an etch-and-rinse adhesive, particularly when restored with a nanohybrid composite, provided superior immediate bond strength compared with self-cure GIC. These results reflect short-term in vitro performance and do not represent long-term clinical durability.