Objective: To evaluate the marginal adaptation of resin composite restorations, bonded in butt-joint dentin cavities with self-etching adhesives, either polymerized under ambient air or nitrogen. Methods: Four self-etching primer adhesives, AQ Bond (AQB, Sun Medical), iBond (IBO, Heraeus), One-up Bond F Plus (OUB, Tokuyama) and Prompt L-Pop (PLP, 3M-ESPE) were evaluated. Peripheral, approximal dentin of human molars was exposed by wet grinding on SiC paper, grit 600. Cylindrical cavities, entirely located in dentin, were cut with a medium grained diamond bur under copious water spray. The cavity diameters and depths were 4 mm and 1.5 mm, respectively. Eight cavities were prepared for each adhesive and each of the two curing modes, light-activation either under ambient laboratory atmosphere or under nitrogen (60 l/hour). Application of the adhesives followed the manufacturers instructions. For light activation the tungsten-halogen curing unit Translux Energy (Heraeus) was used. The cavities were bulk filled with the resin composite Venus (Heraeus) from preloaded tips. The free surface was covered with a transparent matrix strip, pressed flush with the peripheral dentin surface, and light-activated for 60 seconds with Translux Energy. The specimens were immediately immersed into 23° C water for 15 minutes. Excess resin composite was removed on wet SiC papers 600 and 4000 until the entire cavity margin was exposed for microscopic evaluation at 500x magnification. When a gap was detected the maximum width was measured with an ocular screw micrometer. Data analysis: Kruskal-Wallis ANOVA and Mann-Whitney U-Test at a<0.05.Results: The numbers of gap-free specimens with AQB, IBO, OUB, and PLP, polymerized under air/nitrogen together with significant differences indicated by letters were: 5a/7a, 4a/8a, 0b/2a, and 0b/0b, respectively. Conclusion: Apart from PLP, curing under nitrogen had a remarkably positive effect on gap prevention or reduction with the adhesives AQB, IBO, and OUB.

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