New concept of resin-dentin interfacial adhesion: the nanointeraction zone

The purpose of this study was to evaluate the morphological appearance of resin-dentin interface produced by 1-step adhesive systems in order to classify them, using TEM. It was hypothesized that all interfaces produced by each 1-step adhesive show characteristics similar to that produced by the conventional self-etching system rather than that produced by etch-and-rinse system. The flattened dentin surfaces were subjected to bonding treatment using either a 2-step adhesive: single bond, mega bond, unifil bond; or a 1-step adhesive: Prompt L-Pop, absolute, xeno CF, AQ bond, one-up bond F, G bond, and SSB-200. A flowable resin was then applied to these surfaces. The results revealed that resin-dentin interfaces produced by 1-step systems were classified into three categories. Additionally, the interface produced by G bond and SSB-200 showed a different morphological appearance from that of the others. The interface produced by these adhesives was thinner and less demineralized, with the presence of hydroxyapatite-coated collagen fibrils. It is proposed that "Nanointeraction Zone (NIZ)" is a new complementary classification for partially demineralized resin-dentin interdiffusion zones. Thus, one-step self-etching adhesive systems can be classified into three categories, etch-and-rinse type, mild self-etching type, and NIZ type, by their morphological appearance of the interface.