Following previous research (Van Landuyt et al., JDR 2008), the bonding effectiveness of the functional monomers HAEPA (2-[4-(dihydroxyphosphoryl]-2-oxabutyl]acrylic acid), EAEPA (ethyl 2-[4-dihydroxyphosphoryl]-2-oxabutylacrylate), and MAEPA (2,4,6 trimethylphenyl 2-[oxabutyl]acrylate) (Ivoclar-Vivadent) appeared to depend on their chemical structure, with MAEPA being most effective. Objectives: To chemically unravel the adhesion mechanisms of these monomers by studying their molecular interaction with hydroxyapatite (HAp) and dentin. Methods: The interaction of HAEPA, EAEPA and MAEPA with synthetic HAp particles (Pentax, Japan) and dentin was examined by X-ray diffraction (XRD). 15wt% monomer/ethanol solutions were applied on HAp for 5min, 1hr, and 24hrs (referred to as ‘Monomer-HAp_5min/1h/24h'), after which HAp was washed with ethanol and air-dried. The same monomer/ethanol solutions were applied on dentin for 20s, and subsequently either air-dried without washing (referred to as ‘Monomer_D'), or additionally washed with distilled water (referred to as ‘Monomer_D-dw'). Results: XRD showed the formation of CaHPO4•2H2O (DCPD) on HAp, but only after 24hrs. Monomer-HAp_5min and Monomer-HAp_1h samples showed peaks representing HAp only. The intensity of the XRD peaks representing DCPD differed for the monomers: HAEPA=EAEPA>MAEPA. On dentin, characteristic peaks indicating the formation of Ca-monomer salts could be observed, but only for EAEPA and MAEPA. The XRD pattern for HAEPA_D was not different from that of (untreated) dentin. After washing with water, only MAEPA_D-dw revealed peaks typical of MAEPA_Ca salt that resisted washing. Conclusion: The bonding performance of functional monomers is clearly associated with their capacity of forming stable Ca-monomer salts. While MAEPA formed DCPD the least, it did also form chemically stable MAEPA_Ca crystals that could not be washed away. The chemical stability of the Ca-monomer salts is thought to contribute to the long-term stability of the bond to tooth tissue.
This research was supported by a Grant-in Aid for Scientific Research ((B)21390514) from the Japan Society for the Promotion of Science