Effects of Double Application of Contemporary Self-Etch Adhesives on Their Bonding Performance to Dentin with Clinically Relevant Smear Layers

Purpose: To evaluate the effects of double application of self-etch adhesives on their microtensile bond strength (μTBS) to dentin, as well as the hardness (H) of resin-dentin interfacial structures. Materials and Methods: Midcoronal dentin surfaces (n = 45) were polished with 180-grit SiC paper and randomly allocated to 9 groups based on three adhesives - Scotchbond Universal Adhesive (SB), G Premio Bond (GP), and Clearfil Megabond 2 (MB) - and three application modes: single application (S), double application with light curing after each application (DL), and double application with light curing only at the end (D). Following composite-resin build up and water storage (37°C; 24 h), μTBS were obtained. The hardness of adhesive, resin-dentin interface and dentin were evaluated by nanoindentation. The μTBS and H data were analyzed by two-way ANOVA to demonstrate the effects of adhesive and application mode as well as their interaction, followed by Tukey's test (α = 0.05). Fracture modes were determined using a stereomicroscope. Results: μTBS and H were significantly affected by the adhesives and their application modes (p ˂ 0.001). Double application significantly increased the µTBS and H of adhesive and resin-dentin interface (p ˂ 0.05). SB and MB showed significantly higher µTBS than GP (p ˂ 0.05). Conclusion: Double applications during bonding of dentin having clinically relevant smear layers were beneficial for three contemporary self-etch adhesives. The improvement in bonding performance is believed to be the result of increased chemical interaction, better solvent removal, and improved resin infiltration, leading to improved mechanical properties of the resin-dentin interface or thicker adhesive layers providing improved stress distribution. Keywords: clinically relevant smear layer, dentin bonding, double application, hardness, microtensile bond strength, self-etch adhesive, single application