Femoral fracture mimicking acute arthritis in Down's syndrome.

We describe a 38-year-old white woman with Down's syndrome with a history of chronic arthritis and hyperuricemia who presented with acute left knee pain, patellar tenderness and patello-femoral instability. Findings appeared due to a fracture of the lateral femoral condyle, which responded to conservative therapy with a spica cast. Patellofemoral instability in Down's syndrome can be associated with significant morbidity including femoral condyle fracture. Patients with Down's syndrome who present with acute knee arthritis responding poorly to antiinflammatory agents and other conventional therapy should be assessed for fractures related to patellar dislocation, as treatments for the 2 disorders differ.