Influence of Adhesive Application Modes and Dentin Depth on the Micro-Shear Bond Strength of Single Shade Resin Composites

Because superficial and deep dentin differ in their structural characteristics and bonding behavior, optimizing adhesive strategies for both substrates is clinically important. The aim of this in vitro study was to evaluate the effects of different application strategies of a universal adhesive system and single-shade resin composites on the micro-shear bond strength (μSBS) to superficial and deep dentin. A total of 240 extracted human third molars were prepared to obtain superficial or deep dentin specimens. A universal adhesive system (Gluma Bond Universal) was applied using etch-and-rinse (E&R), self-etch (SE), and selective dentin etch (SDE) strategies. Four commercially available single-shade resin composites (Omnichroma, Zenchroma, Charisma Diamond One, and Vittra APS Unique) were evaluated. After 24 h of water storage, μSBS was evaluated and failure modes were analyzed. Dentin depth, adhesive strategy, and composite type significantly affected μSBS values (p < 0.05). Superficial dentin exhibited significantly higher bond strength than deep dentin (p < 0.001). The effect of adhesive strategy depended on dentin depth; in deep dentin, SDE provided significantly higher bond strength than both E&R and SE applications (p < 0.001). Significant interactions were observed among dentin depth, adhesive strategy, and composite type (p < 0.05). The predominant mode of failure across most experimental groups was adhesive failure. Within the limitations of this in vitro study, dentin depth, adhesive strategy, and single-shade composite type significantly influenced dentin bond strength. Selective dentin etching was associated with increased bond strength in deep dentin under the experimental conditions of this study.