Description

Childhood femoral head osteonecrosis of unknown etiology, which can weaken the femoral head and produce a permanent deformity in some patients, predisposing them to early arthritis

Epidemiology

• Incidence varies depending on the region: 0 to 15 per 100,000 children <15 years of age
• In United States and Canada, about 5 per 100,000
• Rare in persons of African descent
• Most frequent in children 4 to 8 years old
• 3 to 5 times more common in boys than girls
• 10–15% develop bilateral disease in asynchronous fashion.
• Associated factors: delayed bone age, hyperactivity, exposure to smoking, genitourinary anomalies (hypospadias, undescended testis, and inguinal hernia)

Pathophysiology

• A partial or complete disruption of femoral head blood supply produces a partial or total femoral head osteonecrosis.
• Greater the head involvement, worse prognosis
• Bone necrosis and subsequent resorption of necrotic bone weaken the femoral head.
• Weight bearing worsens the femoral head deformity.
• Chronic hip joint synovitis also develops producing pain and restriction of motion.
• Necrotic head goes through four stages of healing over 3 to 5 years:
  • 1. Stage of avascular necrosis: smaller femoral head epiphysis with increased radiodensity
  • 2. Stage of fragmentation: Necrotic epiphysis shows fragmentation. Necrotic bone is resorbed, weakening the head. Most deformity occurs during this stage, which lasts 1 to 2 years.
  • 3. Stage of reossification: New bone begins to fill the epiphysis; longest stage, lasting up to 3 years
  • 4. Healed: Head is completely reossified. Not all heads heal back in round shape, and deformed heads are at risk for developing arthritis later.

Etiology

• Unknown
• Unlikely genetic transmission, as <5% have family history
• Many theories:
  • Multifactorial (genetic predisposition with environmental trigger)
  • Hyperactivity and subclinical trauma
  • Type II collagenopathy
  • Thrombophilia (factor V Leiden)
  • Smoke exposure