Effects of multiple coatings of two all-in-one adhesives on dentin bonding

Purpose: Simple changes to bonding technique can improve resin-dentin bond strengths. The objective of this study was to evaluate the effects of multiple coatings of two all-in-one adhesive resins on both microtensile bond strength (μTBS) and nanoleakage. Materials and Methods: The mid-coronal occlusal dentin of extracted human molars was used. Two all-in-one adhesives - iBond (Heraeus Kulzer) and Xeno III (Caulk Dentsply) - were applied to 320-grit abraded dentin surfaces. In groups 1 and 3 during bonding, monomer application and solvent evaporation were done 1, 2, 3, 4 or 5 times on the dentin surface before light curing. In groups 2 and 4 after light curing the first layer, the adhesive was re-applied, the solvent evaporated, and the layer light cured. This was repeated from 2 to 5 times, followed by creation of composite buildups. After 24 h storage in 37°C water, the teeth were sectioned perpendicular to the adhesive interface to produce multiple beams of composite-bonded dentin, approximately 0.9 mm² in area. These were tested to failure in tension. Data were evaluated by three-way ANOVA (material vs coatings vs light curing) followed by multiple comparisons at α = 0.05. Additionally, nanoleakage of silver uptake and adhesive layer thickness were evaluated using transmission electron microscopy (TEM). The results indicated that bond strengths increased with the number of coatings (p < 0.0001) with both adhesives, up to 3 layers, especially if each layer was light cured. Nanoleakage of silver tended to decrease with each coat in both adhesive systems. Conclusion: By simply applying more coats of adhesive, the strength and quality of dentin adhesion can be improved.