Crystal populations in human synovial fluid. Identification of apatite, octacalcium phosphate, and tricalcium phosphate.

Defatted joint fluid pellets from 4 patients with the "Milwaukee shoulder" syndrome, an aspirate of milky fluid from subcutaneous tissue of a girl with dermatomyositis/calcinosis, and 2 rabbit synovial membranes calcified by calciphylaxis were studied by Fourier transform infrared analysis. This method permits sequential electronic subtraction ("stripping") of known patterns of highly characterized reference compounds from an unknown sample pattern. Collagen and hydroxyapatite, with carbonate partially (1-3%) substituted for phosphate, and to a lesser extent hydroxyl, were found in all samples. Octacalcium phosphate (OCP) was found in all samples except that from a patient with synovial chondromatosis which contained tricalcium phosphate (TCP). Sulfate was found in 2 of the shoulder joint fluid specimens. An unidentified organic phase was present in all samples. This study confirms recent observations by others that pathologic calcifications contain several mineral phases. These data raise the possibility that hydroxyapatite nucleation may be controlled by hydrolytic alteration of precursor calcium phosphate phases such as OCP or TCP. Studies of the biologic effects of synthetic calcium phosphate crystals must take into account the existence of several crystalline phases in natural pathologic calcifications.