Basic calcium phosphate (apatite) and calcium oxalate crystal disorders tend to cause clinical manifestations similar to those of other crystal-induced arthritides. (See also Overview of Crystal-Induced Arthritides.)
Most pathologic calcifications throughout the body contain mixtures of carbonate-substituted hydroxyapatite and octacalcium phosphate. Because these ultramicroscopic crystals are nonacidic calcium phosphates, the term basic calcium phosphate (BCP) is much more precise than apatite. These ultramicroscopic crystals occur in snowball-like clumps in rheumatic conditions (eg, calcific tendinitis, calcific periarthritis, some cases of progressive systemic sclerosis and dermatomyositis). They also occur in joint fluids and cartilages of patients with all degenerative arthropathies sufficiently advanced to cause joint space narrowing on x-ray.
BCP crystals can destroy joints and can cause severe intra-articular or periarticular inflammation.
Milwaukee shoulder syndrome, a profoundly destructive arthropathy affecting predominantly older women that usually develops in the shoulders and (often) knees, is one example.
Besides synovial fluid analysis, x-rays should be taken of symptomatic joints. On x-ray, BCP crystals may be visible as periarticular cloudlike opacities; the crystals often spontaneously resolve over months or occasionally within days. Definitive assay for BCP crystals in synovial fluid is not readily available. Clumped crystals can usually be identified only with special calcium stains or transmission electron microscopy. The clumps are not birefringent under polarized light.
Treatment of BCP crystal deposition disease with oral colchicine, an NSAID, or, if a large joint is involved, intra-articular corticosteroid ester crystal suspension is helpful. Treatment is the same as that for acute gout.
Calcium oxalate crystal deposition is extremely rare. It occurs most often in azotemic patients receiving hemodialysis or peritoneal dialysis, particularly those treated with ascorbic acid (vitamin C), which is metabolized to oxalate.
Crystals may deposit in blood vessel walls and skin, as well as joints. The crystals appear as positively or indeterminately birefringent bipyramidal structures. Synovial fluid may have > 2000 white blood cells/mcL. On x-ray, calcium oxalate crystals are indistinguishable from BCP periarticular calcifications or calcium pyrophosphate dihydrate (CPPD) crystal deposits in cartilage.
Treatment of calcium oxalate crystal deposition disease is the same as that for calcium pyrophosphate arthritis (CPPD crystal deposition disease).