What are the primary causes of hip avascular necrosis (AVN)?

Causes of Hip Avascular Necrosis

Primary Causes

Hip avascular necrosis results from bone death due to inadequate vascular supply, with corticosteroid therapy, chronic alcohol abuse, and trauma being the three most common causes. 1, 2

Major Iatrogenic and Substance-Related Causes

• Corticosteroid therapy is the leading iatrogenic cause, particularly with high-dose and prolonged treatment 1, 2, 3

  • The mechanism involves lipid hypertrophy causing extravascular intraosseous compression and elevated marrow pressure 4
  • Risk increases with cumulative dose and duration 4, 3
  • In kidney transplant patients, high-dose glucocorticoids resulted in at least 1.5-fold greater risk compared to low-dose regimens 1, 4

• Chronic alcohol abuse is a major independent risk factor for bilateral AVN 2, 5, 6, 7

Traumatic Causes

• Hip trauma including femoral neck fractures and posterior hip dislocations are critical causes 7, 8
  • Type of fracture (displaced versus undisplaced) significantly affects AVN risk 7
  • Time between injury and hip reduction is the most critical factor: reduction after 24 hours results in AVN in 21.73% of cases, compared to 4.34% when reduced within 6-12 hours 8
  • Acetabular fracture-dislocation carries higher AVN risk (34.78%) compared to isolated hip dislocation (4.76%) 8

Hematologic and Metabolic Causes

• Sickle cell disease and other hemoglobinopathies are particularly important causes, especially in younger patients 1, 2, 5, 4

• Blood dyscrasias including lymphoma and leukemia predispose to bilateral AVN 1, 4

• Hyperlipidemia contributes through fat emboli formation 2, 5, 4

• Hypercoagulability states cause microvascular thrombosis leading to AVN 2, 5, 4

Infectious and Treatment-Related Causes

• HIV infection increases AVN risk independent of treatment, with approximately 5% of HIV patients having asymptomatic bilateral AVN detectable on MRI 1, 5, 4

  • The combination of HIV plus corticosteroids creates synergistic risk 4

• Chemotherapy damages vascular endothelium 1, 4

• Radiation therapy causes radiation-associated arteritis affecting vessels of any size 1, 4

Other Systemic Causes

• Gaucher disease causes bilateral AVN through marrow infiltration and vascular compromise 1, 4

• Caisson disease produces nitrogen bubble emboli 1, 4

• Prior dialysis in transplant patients increases risk, with longer duration of dialysis prior to kidney transplantation increasing AVN risk post-transplant 1, 4

Pathophysiological Mechanisms

Three primary mechanisms lead to AVN 1, 2, 4:

1. Vascular interruption through direct vessel injury
2. Vascular occlusion from thrombosis or emboli
3. Extravascular intraosseous compression from elevated marrow pressure due to lipid hypertrophy

Critical Clinical Pitfall

In nontraumatic cases, femoral head osteonecrosis is bilateral in 70-80% of patients, distinguishing it from traumatic AVN which is typically unilateral 4. Many patients with bilateral AVN are asymptomatic in early stages, with positive MRI findings in approximately 5% of at-risk patients 2, 5, 4. When bilateral femoral head AVN is present, other sites are frequently involved, including the knee (44%), ankle (17%), and shoulder (15%) 4.