The purpose of this study was to prove that an intermediate resin layer (IRL) can increase the bond strength to dentin by reducing the permeability of single-step adhesives.

Flat dentin surfaces were created on buccal and lingual side of freshly extracted third molar using a low-speed diamond saw under copious water flow. Approximately 2.0 mm thick axially sectioned dentin slice was abraded with wet #600 SiC paper. Three single-step self-etch adhesives; Adper Prompt L-Pop (3M ESPE, St Paul, MN, USA), One-Up Bond F (Tokuyama Corp, Tokyo, Japan) and Xeno III (Dentsply, Konstanz, Germany) were used in this study. Each adhesive groups were again subdivided into ten groups by; whether IRL was used or not; whether adhesives were cured with light before application of IRL or not; the mode of composite application.

The results of this study were as follows;

1. Bond strength of single-step adhesives increased by an additional coating of intermediate resin layer, and this increasement was statistically signigicant when self-cured composite was used (p < 0.001).

2. When using IRL, there were no difference on bond strengths regardless the curing procedure of single-step adhesives.

3. There were no significant difference on bond strengths between usage of AB2 or SM as an IRL.

4. The thickness of hybrid layer was correlated with the acidity of adhesive used, and the nanoleakage represented by silver deposits and grains was examined within hybrid and adhesive layer in most of single-step adhesives.

5. Neither thickness of hybrid layer nor nanoleakage were related to bond strength.