Biological apatite vs hydroxyapatite at the atomic level.

Known differences in chemical content of biological apatites, specifically the dense (sp.g. greater than 2.95) portion of human tooth enamel (TE), and of stoichiometric hydroxyapatite (OHAp) imply atomic-scale crystal-structural differences, via major and minor substitutions, which can impart very different characteristics to the two materials. Further, some substitutions are shown to produce important effects not predictable from study of pure materials alone. A number of crystal-detail differences between TE and OHAp, both at room temperature and in response to heating (which occurs to some degree locally in hard tissue in vivo as a result of grinding, laser action, etc.), are tabulated. They include CO3 content and incorporation, lattice parameters, water content, non OHAp phases formed on heating, structural OH deficiency, and hydrogen-bonding of F and Cl "impurities" to OH. The significance of these differences is such that it is clearly misleading to speak of tooth enamel, and by inference other biological apatites, as "hydroxyapatite" without considerable qualification of the statement. The differences between biological apatite and OHAp are, in fact, of greater current interest and probable biological significance than are the similarities.