Adhesion Mechanism of Self-etching Primer Adhesives

Objective: In this study, the adhesion mechanisms of resin to the tooth primed with a self-etching primer was examined by the NMR technique. Methods: Deuterium oxide of 1 g was added to 5 g of Clearfil MEGA Bond Primer (MB) or Unifil Bond Primer (UB). After 400 mg of hydroxyapatite (HAP 200) or dentin particle was suspended in 5 g of the diluted above primer solutions, the suspensions were centrifuged. The liquid-state carbon-13 NMR observations of the supernatant primer solutions were performed by EX 270 spectroscopy (resolution: 0.009 ppm). Furthermore, after the centrifuged residue obtained from the hydroxyapatite or dentin was repeatedly rinsed with ethanol, the solid-state carbon-13 NMR observations were conducted. Results: The addition of hydroxyapatite or dentin to the MB solution resulted in the dramatic decrease in the NMR peak intensity of the carbons assigned to the MDP (Scheffe's test, p<0.005). On the other hand, when the hydroxyapatite or dentin was added to the UB solution, the carbonyl carbon peak attributed to the carboxylic acid in the 4-META shifted to the lower field (p<0.005) by 0.37 ppm or 0.65 ppm, respectively. The decrease in the NMR peak intensity of the MDP and the shift of the carbonyl carbon peak of the carboxylic acid in the 4-META to the lower field were due to the phosphoric acid or the carboxylic acid in these acidic monomers forming an acid-base interaction with the calcium cation in the tooth. Further, the solid-state NMR spectra of the residues showed that the reactivity of the phosphoric acid in the MDP with the tooth component was higher than the carboxylic acid in the 4-META. Conclusions: The NMR technique is useful technique for investigating the adhesion mechanisms. Part of this work was supported by Suzuki Memorial Grant of Nihon University School of Dentistry at Matsudo (02-2006).