Technique sensitivity of water-free one-step adhesives

Objectives All-in-one adhesives have been associated with water-related problems, such as phase-separation, polymerization-inhibition and reduced shelf life. In this study we investigated whether these problems could be avoided by a water-free self-etch adhesive that requires a ‘wet-bonding’ application procedure. Additionally, the technique-sensitivity of such a system was evaluated. Methods The micro-tensile bond strength (μTBS) of a water-free and a water-containing experimental one-step adhesive (EXP) bonded to either ‘air-dried’, ‘blot-dried’ or ‘overwet’ bur-cut enamel and dentin was determined. Likewise, the μTBS of a commercial water-free adhesive Absolute2 (Denstply-Sankin) was determined. The interfacial interaction of the adhesives was evaluated by SEM and TEM. Results In the blot-dried group, both water-free adhesives obtained the highest bond strength and the bond strength of the water-free EXP was comparable to that of the water-containing EXP bonded to an air-dried surface. When they were applied to overwet surfaces, the water-free adhesives tended towards a lower bond strength. This was partly attributed to the presence of phase-separation (or ‘overwet’) droplets in the adhesive layer. Unexpectedly, even on air-dried dentin, the water-free adhesives were able to demineralize dentin, however to a limited extent, hence impairing the bond strength. Significance Water-free one-step adhesives are a valuable alternative for conventional water-containing one-step adhesives. However, water-related problems in these adhesives are not ruled out, as they are eventually also mixed with water during the application procedure. As such, they partially fail to meet their objective. In addition, their bonding procedure, which involves ‘wet bonding’, is more technique-sensitive than dry bonding.