A 65-year-old male develops the sudden onset of severe knee pain. The knee is red, swollen, and tender. He has a history of diabetes mellitus and cardiomyopathy. An x-ray of the knee shows linear calcification. Definitive diagnosis is best made by

(Braunwald, 15/e, pp 1995-1996.)The acute mono- aicular ahritis in association with linear calcification of the cailage of the knee suggests the diagnosis of pseudogout, also called calcium pyro-phosphate dihydrate deposition disease. The disease resembles gout. Positive birefringent crystals (looking blue when parallel to the axis of the red com- pensator on a polarizing microscope) can be demonstrated in joint fluid. Serum uric acid and calcium levels are normal, as is the rheumatoid factor. Pseudogout is about half as common as gout but becomes more common after age 65. Calcium pyrophosphate dihydrate deposition disease is diagnosed in symptomatic patients by characteristic x-ray findings or crystals in synol fluid. The disease is treated with NSAIDs or colchicine. Linear calcifications or chondrocalcinosis are often found in the joints of elderly patients who do not have symptomatic joint problems; such patients do not require treatment.