Legg-Calve-Perthes disease is idiopathic avascular necrosis of the femoral head. It occurs most commonly in children between the ages of 5-8 who present with knee or hip pain or a limp. Boys are more commonly affected than girls (5:1). The disease is bilateral in approximately 15% of cases. A good prognostic indicator in Legg-Calve-Perthes disease is the age of onset because if it occurs by age six, then restoration of the spherical femoral head is likely and degenerative osteoarthrits can be avoided.

Early radiographic findings:

• widening of the joint space: secondary to effusion or hypertrophic synovium
• crescent sign: subchondral linear lucency representing a fracture through necrotic bone (best seen on frogleg view)
• MRI: high-signal marrow edema on T2WI
• bone scan: decreased activity in the capital femoral epiphysis early and increased activity during healing

Chronic findings:

• fragmentation and/or collapse of the femoral epiphysis with areas of increased sclerosis and lucency
• coxa magna = broad overgrown femoral head
• coxa plana = flat femoral head
• short femoral neck
• arrest of physeal growth

	Legg-Calve-Perthes disease in a 6-year-old boy. AP pelvic radiograph shows irregularity in contour of the right femoral head and widening of the right joint space as compared with the normal findings on the left.
	Legg-Calve-Perthes disease in a 6-year-old boy.Coned-down frogleg view of the right hip shows a crescentic subchondral lucency consistent with a fracture through necrotic bone.

Legg-Calve-Perthes disease in a 6-year-old boy. Coned-down AP view of the right hip demonstrates the progression of this disease. Note how the femoral head has become markedly flattened (coxa plana) and enlarged (coxa magna).