Effect of the double-application of one-step universal adhesives on bonding strength to bur-cut enamel

One-step universal adhesives form a thinner adhesive layer, which is more susceptible to oxygen-induced polymerization inhibition, thereby weakening the bond strength. Some studies have suggested strengthening the adhesive layer by applying multiple layers of adhesive. Thus, this study aimed to examine the effect of double application on the bond strength of universal adhesives to bur-cut enamel. The enamel surfaces were prepared using a medium-grit diamond bur. The specimens were randomly distributed into eighteen groups based on three factors: the adhesive used (Optibond Universal, G-Premio Bond, Clearfil S3 Bond Universal), the application method of adhesives (single application and double-application with or without light activation between layers), and the type of composite resin (Clearfil Majesty Flow and Clearfil Majesty Esthetic). The adhesive agents were applied in self-etch mode, and composite blocks were fabricated. After 24 h, the composite blocks were subjected to a shear bond strength test, and the fractured surfaces were examined using a stereomicroscope. Additionally, the thickness of the adhesive layers was assessed using scanning electron microscopy. The data were evaluated by a three-way ANOVA, Weibull analysis, and Kruskal-Wallis tests (α = 0.05). The double-application of the adhesives did not improve the bonding potential to enamel (p > 0.05). The use of flowable composite resin did not enhance the bond strength. The adhesives demonstrated similar reliability. Optibond Universal exhibited higher bond strength to bur-cut enamel compared to G-Premio Bond and Clearfil S3 Bond Universal, regardless of application method and composite type (p < 0.05). The double-application of the adhesives and composite type did not affect the bonding strength to bur-cut enamel, regardless of whether light activation was applied between the applications.