Crystal-associated arthropathies: what's new in old joints

Degenerative arthritis in the aged includes two major disease categories--osteoarthritis and the crystal-associated arthropathics. The crystals chiefly involved are monosodium urate (gout) and calcium pyrophosphate dihydrate (CPPD), although several others (e.g., cholesterol, brushite and apatite) have been implicated. This report illustrates how the newer diagnostic techniques such as polarized-light microscopy, analytical electron microscopy and x-ray microdiffraction have augmented knowledge concerning diseases associated with articular crystal deposition. For example, diffraction techniques are required for accurate identification of the apatite crystals found in synovial fluid effusions and in the matrix vesicles of degenerate cartilage. According to ultrastructural studies, monosodium urate crystals found in tophi, joint surfaces and effusions show a distinct morphology. Present in inactive joints, the crystal surfaces are bare; in acute gout, the crystals are covered with mucin, confirming the observation that protein binding to crystals is necessary for inflammation to proceed. CPPD disease is by far the most common crystal-associated arthropathy affeting the aged. The incidence of CPPD deposits in articular tissues increases with age but, in contrast to gout, affects both men and women. The pathogenesis of CPPD disease is a mystery, but factors under investigation include matrix abnormalities, ionic imbalances, and enzyme disorders.