Interaction of Phosphoric Acid Ester Monomer with Hydroxyapatite

Objectives: Among functional monomers used in contemporary dental adhesives, 10-methacryloxydecyl dihydrogen phosphate (MDP) was found to chemically interact most intensively and stably with hydroxyapatite (HAp). This effect was thought to be at the basis of the superior bonding effectiveness of MDP-based self-etch adhesives to enamel/dentin. The aim of this study was to fully elucidate the chemical interaction and reactivity of MDP with HAp. Methods: 2000 mg of HAp particles were dispersed, while stirring, in 10000 mg of a mixed solution of MDP, absolute ethanol and distilled deionized water in a MDP:EtOH:H2O ratio of 15:45:40 wt%. The MDP-coated HAp particles were separated from the mixed solution by centrifugation and decantation, after 5 min, 1 hour and 24 hours, respectively. The separated HAp particles were then washed three times with absolute ethanol, and dried at room temperature in ambient atmosphere, prior to identification with 31P CP-MAS NMR spectroscopy and powder X-ray diffraction. Results: In an aqueous ethanol solution, Ca ions were leached from HAp to form at short term a MDP-calcium salt (CaMHP2) layered structure on the HAp surface. When MDP was allowed to interact for longer time (<24 hours), CaHPO4•2H2O precipitated on top of this MDP-calcium salt layered structure. Conclusion: The intense chemical interaction of MDP with HAp must be ascribed to superficial dissolution of HAp and subsequent deposition of calcium phosphate salts with lower solubility.