Impact of adhesive application and moisture on the mechanical properties of the adhesive interface determined by the nano-indentation technique

This study assessed the nanohardness (NH) and Young's modulus (YM) of resin-dentin bonding components formed by an ethanol/water-based (Adper Single Bond Plus [SBP]) and an acetone-based system (One Step Plus [OSP]) under different moisture conditions and application methods. On 24 human molars, a flat, superficial dentin surface was exposed by wet abrasion. After acid-etching, two coats of SBP or OSP adhesive were applied on either a dry or rewetted dentin surface under vigorous rubbing action or inactive application. After polymerization of the adhesives (600 mW/cm2/20 seconds), composite buildups were constructed incrementally and the specimens were stored in water (37 degrees C/24 hours). They were cross-sectioned perpendicular to the resin-dentin interface to obtain 1.5 mm-thick slices that were embedded and polished before the test. Nano-indentations were made on the resin composite, adhesive system, hybrid layer and mineralized dentin. The results of NH and YM (GPa) of the adhesive system and hybrid layer were analyzed using three-way repeated measures ANOVA and Tukey's multiple comparison tests (alpha = 0.05). When the dentin was kept wet, the mode of application did not affect the studied properties within the hybrid layer. On the other hand, the vigorous application mode increased the NH and YM of both adhesives applied in air-dried dentin. In the adhesive layer, the highest NH and YM were observed only for SBP, especially when applied in wet dentin under vigorous action. It was concluded that: 1) the vigorous application of both adhesives in dry dentin resulted in high nanohardness and Young's modulus values in the hybrid layer and 2) in the adhesive layer, the moisture associated with the vigorous application mode increased the nanohardness and Young's modulus values of Adper Single Bond Plus.