Basic calcium phosphate (BCP) and calcium pyrophosphate dihydrate crystals are the most common types of pathologic crystals, followed by monosodium urate crystals and, in rare cases, calcium oxalate crystals. These crystals have been associated with a variety of quite different rheumatic syndromes. They are responsible for acute synovial inflammation and also contribute to cartilage degradation and bone lesions within the joint. Although understanding of the molecular mechanisms involved in generating the pathologic effects of these crystals has increased, the role of BCP crystals in particular remains poorly understood. The clinical implication of articular deposits of calcium-containing crystals in osteoarthritis is unknown. This review provides an overview of the clinical and pathological changes of these four different types of crystals.
