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This condition is also known as Legg-Calve-Perthes Disease, Legg-Perthes disease, Perthes disease, coxa plana, and aseptic or avascular necrosis of the femoral head.

What is Legg-Calve-Perthes disease?
The hip is a "ball and socket" joint. Necrosis or death of the femoral head means that the "ball" part of the joint is no longer properly functioning.