Adhesion/decalcification mechanisms of acid interactions with human hard tissues

In order to study adhesion/decalcification mechanisms of acid interactions with human hard tissues such as bones and teeth, the chemical interaction o f five carboxylic acids (acetic, citric, lactic, maleic, and oxalic) and tw o inorganic acids (hydrochloric and nitric) with enamel and two synthetic h ydroxyapatite (HAp) powders with, respectively, a high and a low crystallin ity were analyzed using X-ray photoelectron spectroscopy (XPS), atomic abso rption spectrophotometry (AAS), and spectrophotometry (S). X-ray diffractio n revealed that the crystallinity of the highly crystallized HAp was consid erably higher than that of enamel while the crystallinity of the poorly cry stallized HAp was similar to that of dentin and bone. XPS of acid-treated e namel demonstrated for all carboxylic acids ionic bonding to calcium of HAP . AAS and S showed for both HAps that all carboxylic and inorganic acids ex cept oxalic acid extracted Ca significantly more than P, leading to a Ca/P ratio close to that of synthetic HAp (2.16 w/w). Oxalic acid extracted hard ly any Ca, but substantially more P, leading to a significantly smaller Ca/ P ratio than that of HAp. AAS showed that the calcium salt of oxalic acid h ardly could be dissolved, whereas the calcium salts of all the other acids were very soluble in their respective acid solution. These results confirm the adhesion/decalcification concept (AD-concept) previously advanced. Depe nding on the dissolution rate of the respective calcium salts, acids either adhere to or decalcify apatitic substrates. It is concluded that the AD-co ncept that originally dictated the interaction of carboxylic acids with hum an hard tissues can be extended to inorganic acids, such as hydrochloric an d nitric acid. Furthermore, HAp crystallinity was found not to affect the a dhesion/decalcification behavior of acids when interacting with apatitic su bstrates, so that the AD-concept can be applied to all human hard tissues w ith varying HAp crystallinity. (C) 2001 John Wiley & Sons, Inc.