Legg-Calvé-Perthes disease (LCPD) is avascular necrosis of the proximal femoral head resulting from compromise of the tenuous blood supply to this area. LCPD usually occurs in children aged 4-10 years. The disease has an insidious onset and may occur after an injury to the hip. In the vast majority of instances, the disorder is unilateral. Both hips are involved in fewer than 10% of cases, and the joints are involved successively, not simultaneously.
LCPD usually occurs in children aged 4-10 years (mean age, 7 years). It occurs more commonly in boys than in girls (male-to-female ratio, 4:1). The condition is rare, occurring in approximately 4 of 100,000 children.
A population-based design study demonstrated that patients with a history of LCPD seem to have a higher risk of severe injury than those without the disease. This risk may be due to a tendency for more hyperactive behavior among LCPD patients, especially females. 
The cause is not known, but children with LCPD have delayed bone age, disproportionate growth, and a mildly shortened stature. LCPD may be idiopathic, or it may result from a slipped capital femoral epiphysis, trauma, steroid use, sickle-cell crisis, toxic synovitis, or congenital dislocation of the hip.
Rapid growth occurs in relation to development of the blood supply of the secondary ossification centers in the epiphyses, causing interruption of adequate blood flow and making these areas prone to avascular necrosis. Interruption of the blood supply to the bone results in necrosis, removal of the necrotic tissue, and its replacement with new bone.
Bone replacement may be so complete and perfect that completely normal bone may result. The adequacy of bone replacement depends on the age of the patient, the presence of associated infection, the congruity of the involved joint, and other mechanical and physiologic factors. Necrosis may occur after trauma or infection, but idiopathic lesions can develop during periods of rapid growth of the epiphyses.
The earliest sign of LCPD is an intermittent limp (abductor lurch), especially after exertion, with mild or intermittent pain in the anterior part of the thigh. LCPD is the most common cause of a limp in the 4- to 10-year-old age group, and the classic presentation has been described as a painless limp. The patient may present with limited range of motion of the affected extremity. The most common symptom is persistent pain.
Hip pain may develop and is a result of necrosis of the involved bone. This pain may be referred to the medial aspect of the ipsilateral knee or to the lateral thigh. The quadriceps muscles and adjacent thigh soft tissues may atrophy, and the hip may develop adduction flexion contracture. The patient may have an antalgic gait with limited hip motion.
Pain may be present with passive range of motion and limited hip movement, especially internal rotation and abduction. Children with LCPD can have a Trendelenburg gait resulting from pain in the gluteus medius.
Laboratory studies and radiography may supplement medical history-taking and physical examination in the assessment of a child with a limp. 
Differential diagnoses include osteomyelitis, pyogenic arthritis, transient synovitis, abscess of the psoas muscle, juvenile rheumatoid arthritis, hemophilia, slipped capital femoral epiphysis, and neoplasm.