The aim of this study was to evaluate the effect of single- and double-layer application of two different universal adhesive systems on the bond strength of coronal and root dentin. Human dentin surfaces of 80 extracted maxillary incisors were used in this study. Coronal and root dentin surfaces were exposed, and standardized smear layers were created by polishing with silicon carbide abrasive paper for 30 s. The specimens were randomly divided into eight groups (n = 10). Gluma Bond Universal and Single Bond Universal adhesives were applied in single- and double-layer modes to both coronal and root dentin according to the manufacturers’ instructions. A nanohybrid resin composite (G-ænial A’Chord, GC Corporation, Tokyo, Japan) was then applied. After 24 h of storage, the specimens were subjected to thermocycling, followed by microshear bond strength testing at a crosshead speed of 1 mm/min. The microshear bond strength of Gluma Bond Universal to both coronal and root dentin was statistically significantly higher than that of Single Bond Universal. Double-layer application resulted in significantly higher bond strength values compared with single-layer application for both adhesive systems and dentin types. In the double-layer application groups, the bond strength of coronal dentin was statistically significantly higher than that of root dentin for both adhesive systems (p = 0.001; p < 0.05). Within the limitations of this in vitro study, double-layer application of universal adhesives increased bond strength, with higher values observed in coronal dentin than in root dentin. This application strategy may be considered a simple clinical approach to enhance the bonding effectiveness of universal adhesive systems.
Bulut et al. (Mon,) studied this question.