Methods: We examined the chemical interaction of functional phosphoric acid monomers (Phenyl-P,10-MDP) and a hydroxyapatite (HAp) particle or dentin by XRD and solid-state NMR. TEM was used for further evaluation of the resultant interfacial ultrastructure.
Results: XRD and NMR analysis revealed that DCPD readily formed when Phenyl-P interacted with HAp for only 5min, after which this calcium-salt deposition gradually increased. Ionic bonding of Phenyl-P to HAp was hardly detected. On the contrary, 10-MDP bonded to HAp was detectable after 5min and clearly intense after 1h- and 24h-exposure. DCPD was clearly detected after the 24h-exposure. On natural dentin the nano-layering of 10-MDP_Ca was also observed using TEM and thin-film XRD, but could not be observed for Phenyl-P.TEM of adhesive-dentin interfaces produced by the Phenyl-P-based adhesive (Clearfil Liner Bond II) disclosed that almost all the apatite around collagen was demineralized, while the 10-MDP-based adhesive (Clearfil SE) only partially demineralized dentin, leaving abundant apatite crystals around the collagen within the submicron hybrid layer.
Conclusion: The initial chemical reaction involves the formation of Phenyl-P_Ca bindings, its dissociation, accompanied by superficial dissolution of HAp, because Phenyl-P_Ca is chemically unstable and soluble. On the contrary, the initial chemical reaction involves the formation of 10-MDP_Ca bindings that are chemically stable, resulting in the formation of 10-MDP_Ca crystals at the HAp surface.The advanced adhesive-tooth interaction model gives not only insight in the bond degradation mechanisms, but also provides a nanometer-scale basis to develop functional monomers for more durable tooth reconstruction.