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
- 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)
- 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.
- 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
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