Octacalcium phosphate, Ca8H2(PO4)6 X 5H2O (OCP), occurs in pathological calcifications frequently as one of the crystalline components of human dental
calculi. OCP has also been presumed a necessary precursor of
biological apatites in both normal (enamel, dentine, cementum, bones) and pathological (e.g., phosphatic renal stones) calcifications. This study investigated the optimum conditions for the direct in vitro formation of OCP in solutions and in gel systems and the factors affecting its formation and transformation or hydrolysis to
apatite. It was observed in both
solution and gel systems that the formation of OCP was dependent on definite conditions of pH and temperature (the higher the temperature the lower the pH at which OCP forms, and vice versa), and on the presence of other
ions. The presence of
pyrophosphate inhibited OCP formation favoring instead the formation of
amorphous calcium phosphate while the presence of
citrate or
carbonate favored the formation of "apatitic"
calcium phosphate at the expense of OCP. The presence of
oxalate ions caused the formation of mixed OCP/
calcium oxalate phases. Hydrolysis of OCP to
apatite was suppressed in the presence of
magnesium or
pyrophosphate, and promoted in the presence of
carbonate or
fluoride ions. In the presence of
oxalate ions, partial hydrolysis of OCP to
calcium oxalate and not to
apatite was observed. Results from this study give insights on the factors (e.g., pH, temperature, presence of
ions besides
calcium and
phosphate) which influence the formation of OCP and its transformation to
apatite and/or
calcium oxalate.
Ions which demonstrated significant effect on the formation and/or transformation of OCP were
magnesium, pyrophosphate,
carbonate,
citrate,
fluoride and
oxalate.