Improving Bonding Protocols: The Effect of Selective Dentin Etching with Two Different Universal Adhesives—An In Vitro Study

Universal adhesives can be applied in versatile bonding strategies, with selective dentin etching (SDE) emerging as a promising approach for enhancing dentin–adhesive interfaces. This study evaluated the immediate adhesive interface to dentin of two universal adhesives (Optibond^TM Universal and Futurabond M+) with an SDE strategy. Sixty human molars were randomly assigned to six experimental groups (n = 10): control (self-etch strategy), SDE, and SDE3 (SDE with three adhesive layers). After dentin exposure and smear layer simulation, adhesives were applied, followed by composite resin restoration. Microtensile bond strength in 1 mm² beams was performed in a universal testing machine (1 kN; 0.5 mm/min) after 24 h water storage. Failure modes were classified, and the adhesive interfaces were characterized by scanning electronic microscopy. SDE was higher for both adhesives compared to the control group, but was this change statistically significant in Futurabond M+ only (Mann–Whitney, p < 0.001). On the other hand, in Optibond^TM Universal, only SDE3 significantly increased bond strength (Mann–Whitney, p < 0.001). Adhesive failures predominated across all groups. Microscopy images revealed longer and more numerous resin tags in SDE and SDE3 specimens. The SDE strategy enhanced immediate bond strength of these universal adhesives, with product-specific variations, suggesting that application strategies should be tailored to each universal adhesive’s unique formulation to optimize dentin bonding effectiveness.