Heterotopic ossification presenting as acute arthritis

We describe a patient with acute myeloblastic leukemia in remission whto developed painful, swollen knees 6 weeks after being ventilated for acute respiratory distress syndrom. Synovial fluid analysis was nondiagnostic and initial radiographs of his knees were normal. Repeat radiographs 5 weeks later showeed periarticular calcification and the diagnosis of heterotopic ossification was made. Heterotopic ossification should be part of the differential diagnosis of acute arthritis in critically ill patients