Critical Reviews in Oral Biology & Medicine, Vol 3, 61-82, Copyright © 1992 by International & American Associations for Dental Research
The role of brushite and octacalcium phosphate in apatite formation
M. S. Johnsson and G. H. Nancollas
Chemistry Department, State University of New York, Buffalo 14124.
Studies of apatite mineral formation are complicated by the possibility of
forming several calcium phosphate phases. The least soluble, hydroxyapatite
(HAP), is preferentially formed under neutral or basic conditions. In more
acidic solutions phases such as dicalcium phosphate dihydrate (Brushite,
DCPD) and octacalcium phosphate (OCP) are often found. Even under ideal HAP
precipitation conditions the precipitates are generally nonstoichiometric,
suggesting the formation of calcium-deficient apatites. Both DCPD and OCP
have been implicated as possible precursors to the formation of apatite.
This may occur by the initial precipitation of DCPD and/or OCP followed by
transformation to a more apatitic phase. Although DCPD and OCP are often
detected during in vitro crystallization, in vivo studies of bone formation
rarely show the presence of these acidic calcium phosphate phases. In the
latter case the situation is more complicated, since a large number of ions
and molecules are present that can be incorporated into the crystal lattice
or adsorbed at the crystallite surfaces. In biological apatite, DCPD and
OCP are usually detected only during pathological calcification where the
pH is often relatively low. In normal in vivo calcifications these phases
have not been found, suggesting the involvement of other precursors or the
formation of an initial amorphous calcium phosphate phase (ACP) followed by
transformation to apatite.
