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ABSTRACT
Among functional monomers used in contemporary dental adhesives, 10-methacryloyloxydecyl dihydrogen phosphate (MDP) has been found to interact chemically with hydroxyapatite (HAp) most intensively and stably. This effect was thought to be the basis of the superior bonding effectiveness of MDP-based self-etch adhesives to enamel/dentin. To elucidate fully the chemical interaction and reactivity of MDP with HAp, we used ^sup 31^P CP-MAS NMR spectroscopy and powder x-ray diffraction. In an aqueous ethanol solution, Ca ions were leached from HAp to form, at short term, a MDP-calcium salt (CaMHP^sub 2^) layered structure on the HAp surface. When MDP was allowed to interact for longer time (< 24 hrs), CaHPO^sub 4^.2H^sub 2^O precipitated on top of this MDP-calcium salt layered structure. In conclusion, the intense chemical interaction of MDP with HAp must be ascribed to superficial dissolution of HAp induced by the MDP adsorption and subsequent deposition of MDP-calcium salt with a solubility lower than that of CaHPO^sub 4^.2H^sub 2^O.
KEY WORDS: acid monomer, NMR, XRD, adsorption, demineralization.
INTRODUCTION
A mong contemporary dental adhesives, self-etch adhesives are clinically most easy to use and therefore very attractive for routine use in a busy daily practice. The so-called 'mild' self-etch adhesives demineralize dentin only partially, leaving some hydroxyapatite (HAp) crystals around collagen within a submicron hybrid layer (Van Meerbeek et cil., 2003). Such residual HAp may serve as a template for additional chemical interaction with the adhesive's functional monomer, and is especially regarded as essential to the long-term stability of the bond (Inoue et al., 2005). Among functional monomers contained in contemporary self-etch adhesives, the monomer 10-methacryloyloxydecyl dihydrogen phosphate (MDP) was found to adhere to HAp most readily and intensively (Yoshida et ai, 2004). On the basis of Yoshida's adhesion-decalcification concept (AD-concept; Yoshida et al., 2001), the bond between MDP and HAp should be stable, because the MDP-calcium salt hardly dissolves in water (Yoshida et ai, 2004).
However, the actual chemical interaction of MDP with HAp has not been elucidated in full. Recently, Fu et al. (2005) reported that the reaction of phosphoric acid esters (PAEs) with HAp produced PAEs-HAp complexes that do not result from simple acid-base reactions, like those occurring with Ca(OD)^sub 2^ (deuterated calcium hydroxide) in either liquid or solid. Based on liquid-state ^sup...