Bonding efficiency of contemporary adhesives to the dentinoenamel junction zone

The aim of this study was to investigate the bonding of dental adhesives to the dentinoenamel junction (DEJ) zone. Bonding of four adhesives [two etch-and-rinse adhesives (ERAs) and two self-etching adhesives (SEAs)] to enamel, dentin, and the DEJ zone was evaluated using a micro-shear test. Based on the measured bond strengths of dentin/enamel and on the area percentages of dentin in the DEJ zone, predicted bond strengths for the DEJ zone were calculated and compared with those measured. The DEJ zone was analyzed, using scanning electron microscopy, after conditioning, resin infiltration, and debonding. Regardless of the adhesive, bond strengths were significantly influenced by substrates, exhibiting the following order of bond strength (strongest to weakest): dentin >DEJ zone > enamel. The predicted values of the DEJ zone for the ERA groups were significantly higher than the measured values. Analysis of the ERA specimens using scanning electron microscopy showed distinct etching textures of enamel and dentin, which outlined the DEJ and increased the adhesive thickness at the enamel side of the DEJ. Those characteristics could not be detected in the SEA groups. The DEJ zone displayed bond properties that were stronger than enamel but weaker than dentin and therefore may be considered as transitional bond properties. Enamel/dentin within the DEJ zone might bond more weakly to ERAs than its counterpart of the bulk tissue. The presence of the DEJ in the bond area might compromise the bonding efficiency of ERAs.