Abstract
The interrelationship between crystals and osteoarthritis (OA) has been reviewed twice in recent years.
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Evidence for a causal role of crystals in cartilage degeneration is primarily inferential, based on correlative data; however, the thesis that articular crystals promote cartilage degeneration is supported by clinical observations and experimental evidence of their in vitro effects. More definitive investigations of causality are impeded by the lack of a suitable animal model for studying noninflammatory aspects of crystal deposition, the slow pace of degeneration, and problems with detection of early degeneration and small crystal deposits.
The bulk of this article deals with calcium-containing crystals within joints, the class of crystals most common in the osteoarthritic joint. These crystals may cause acute attacks of inflammatory arthritis or periarthritis but, more often, they are associated with an exaggerated form of osteoarthritis (OA). Two general types have been studied best, calcium pyrophosphate dihydrate (CPPD) and basic calcium phosphate (BCP).
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The latter term encompasses at least three major species: tricalcium phosphate, carbonate-substituted apatite, and octacalcium phosphate.
33
There is compelling in vitro evidence that these crystals engender multiple biological effects that promote joint degeneration, and clinical observations support a relationship of both crystal types to OA.
