Influence of Different Application Methods on the Bonding Effectiveness of Universal Adhesives to Dentin in the Early Phase.

To investigate the changes in the dentin bond strengths of universal adhesives during the early phase and evaluate the effect of a double-layer adhesive application on the performance of the dentin bond. Three universal adhesives and a two-step self-etch adhesive were employed to ascertain the shear bond strengths (SBS) of specimens to bovine dentin with the use of the etch-and-rinse or self-etch mode. The specimens were further divided into two groups based on adhesive application in a single or a double layer. The bonded specimens were stored in distilled water at 37°C for 5 min or 1, 6, 12, or 24 h prior to SBS measurement and the adhesives' Knoop hardness number (KHN). All the adhesives showed increased SBS with prolonged storage periods regardless of the adhesive layer (single or double) or etching mode. Most universal adhesives in the double adhesive layer groups showed significantly higher SBS than single adhesive layer groups for the same storage period. All the adhesives also showed increased KHN with increased storage period. The SBS and KHN values of the adhesives increased with increasing storage duration over a 24-h period. Double adhesive layer application mediated increased dentin bond strength in the early phase.