Synovial fluid was aspirated from the Joints of unselected groups of patients suffering from rheumatoid arthritis (30), osteoarthritis (34), pyrophosphate arthropathy (25) and gout (six). Specially selected patients with osteoarthritis (10) and other miscellaneous forms of arthropathy (six) were also Included. Total and differential cell counts were performed, and the fluids were examined by polarized light microscopy and analytical electron microscopy to determine crystal content.fluids from patients with rheumatoid arthritis and gout had a higher total cell count, and greater proportion of polymorphonuclear cells. There was no difference in the cell content in patients with osteoarthritis and chronic pyrophosphate arthropathy. Osteoarthritic fluids containing crystals did not have a significantly different cell count from those without.Polarized light microscopy was confirmed as a good technique for differentiating urate from pyrophosphate crystals. However, pyrophosphate crystals varied greatly in appearance, and were occasionally too small to be visualized. Crystals thought to be brushite, formed in some fluids after aspiration, causing confusion on polarized light examinations, and other unidentified birefringent material was often seen.Analytical and scanning electron microscopy revealed a wide variation in the morphology and elemental content of synovial fluid crystals. Hydroxyapatite crystals were identified in nine out of 34 unselected osteoarthrltlc fluids, in 10 selected cases, and six with synovitis of unknown cause. Hydroxyapatite was seen in combination with pyro phosphate in six cases, and was the only crystal identified in one case of‘pyrophosphate’nrthropathy. Small pyrophosphate crystals, not seen with the light microscope, were identified in two cases. Other crystalline particles containing calcium and phosphorus were seen in some cases, but could not be identified. No crystals were identified in rheumatoid arthritis fluids. Crystal phagocytosis was visualized using scanning electron microscopy.It is concluded that hydroxyapatite crystals are commonly present in synovial fluids in chronic arthritis, with or without pyrophosphate, and that a range of other calcium phosphate salts remain to be identif
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