Objectives . The purpose of this study was to evaluate the effect of the intermediate resin layer (IRL) on bond strength and durability of single-step adhesive by examining microtensile bond strengths to dentin. In order to gain more information, the debonded beams of each group were prepared for FE-SEM to observe the fracture patterns, and the change of resin–dentin interface nanoleakage were observed using TEM.

Materials and methods . Two single-step self-etch adhesives: G-Bond (GC Corporation, Japan) and Clearfil Tri-S Bond (Kuraray, Japan); and two intermediate resin: D/E Resin in All-Bond2 (Bisco, USA) and experimental hydrophobic resin (Bisco, USA) were used in this study. In the control group, each single-step adhesive was used according to manufacturer’s instructions. In the experimental groups, the two intermediate resins were used over the single-step adhesives, which had not been light-cured. Each group was further divided into 3 subgroups according to thermocycling (0, 5000 and 20,000 cycles). The results of bonding strength were statistically analyzed using ANOVA and multiple comparisons are made using Turkey’s test at p < 0.05 level.

Results . The additional application of intermediate resin layer did not increase the immediate microtensile bond strengths of single step adhesives. In the control groups of two single-step adhesives, the bond strengths decreased significantly after thermocycling ( p < 0.05). In the experimental groups, the bond strengths decreased after thermocycling, but they were not significant. The decrease of bond strengths was not influenced by hydrophilicity of IRL ( p > 0.05). After thermocycling, FE-SEM and TEM images of control groups showed most of resin tags excluded from dentinal tubules and more severe nanoleakage appeared in hybrid layer than experimental groups.

Conclusions . The durability of single-step adhesives can be improved by the additional application of intermediate resin layer.