What is avascular necrosis?

Avascular Necrosis (AVN)

Avascular necrosis is an irreversible condition characterized by bone death due to temporary or permanent loss of blood supply, most commonly affecting the femoral head (>75% of cases), which progresses through distinct stages and requires early intervention to prevent joint collapse and the need for arthroplasty. 1

Definition and Pathophysiology

Avascular necrosis (also known as osteonecrosis or aseptic necrosis) is not a specific disease but rather the final common pathway of various conditions that lead to bone death. The condition occurs when:

• Blood supply to bone tissue is interrupted
• Bone cells die due to lack of oxygen and nutrients
• Dead bone tissue eventually collapses
• Adjacent joint surface becomes damaged

The pathophysiology involves compromised subchondral microcirculation through three primary mechanisms 2:

• Vascular interruption (fractures/dislocations)
• Intravascular occlusion (thrombi/embolic fat)
• Intraosseous extravascular compression (lipocyte hypertrophy/Gaucher cells)

Common Sites of Occurrence

• Femoral head (>75% of cases) 3
• Humeral head
• Knee
• Small bones of wrist and foot 4

Risk Factors

Several well-established risk factors have been identified:

Major Risk Factors

• Corticosteroid use (1.5-fold greater risk with higher doses) 1
• Alcohol abuse
• Trauma
• HIV infection
• Blood dyscrasias (including sickle cell disease)
• Lymphoma/leukemia
• Chemotherapy
• Radiation therapy
• Transplant-related immunosuppression
• Gaucher disease
• Caisson disease 1

Special Populations at Risk

• Transplant patients (particularly kidney transplant recipients)
• Patients with sickle cell disease (AVN is one of the most common causes of severe chronic pain) 5
• Pediatric oncology patients receiving steroid and cytostatic treatments 6

Diagnosis

Clinical Presentation

• Joint pain (often progressive)
• Restricted joint motion
• May be asymptomatic in early stages

Diagnostic Imaging

• MRI is the gold standard, especially for early detection when X-rays appear normal 1
• Important diagnostic principles:
  • Both sides should be imaged when evaluating hip pain, even if symptoms are unilateral
  • X-rays may be normal in early stages
  • Ficat and Arlet classification is most commonly used for femoral head AVN
  • Necrotic volume assessment is critical (≥30% volume has 46-83% risk of collapse vs. <5% risk with <30% volume) 1

Treatment Approach Based on Disease Stage

Early Stage Disease (Ficat and Arlet Stages I and II)

When joint surface is preserved:

• Core decompression (first-line surgical intervention)
  • Can be supplemented with:
    • Autologous bone marrow cells
    • Vascular fibular grafting
    • Electric stimulation 1
• Bisphosphonates may improve bone density and prevent progression
  • Require calcium and vitamin D supplementation 1
• Address modifiable risk factors:
  • Reduce/eliminate corticosteroid use
  • Treat hyperlipidemia (consider statins)
  • Reduce alcohol consumption 1

Advanced Stage Disease (Stages III and IV)

When articular surface collapses:

• Total joint replacement (main treatment)
  • Accounts for 10% of indications for total hip replacements in the US 1
• Resurfacing hemiarthroplasty (alternative for femoral/humeral head AVN) 1
• For talar AVN: talar resection/replacement with arthroplasty or tibiotalar joint fusion 1

Pain Management

• First-line: Regular paracetamol (acetaminophen) and NSAIDs (with caution in elderly or those with renal dysfunction)
• Advanced: Opioids for severe pain (used cautiously with reduced dosing in renal dysfunction) 1

Emerging Therapies

• Mesenchymal stem cell-based therapies show promise for improving functional prognosis, particularly in early stages 3

Important Considerations

• Early diagnosis and intervention significantly improve outcomes
• Despite optimal treatment, nearly 50% of femoral head AVN cases eventually require arthroplasty 4
• Physical therapy and gentle mobilization should be encouraged once acute pain subsides 1
• Routine radiographic monitoring in asymptomatic HIV-infected persons is not recommended 1
• The size and location of bone necrosis significantly influence patient outcomes 4

Common Pitfalls to Avoid

• Delaying diagnosis (consider MRI for unexplained hip pain in young patients with normal X-rays)
• Failing to image both sides when evaluating unilateral hip pain
• Overlooking modifiable risk factors (especially corticosteroid use)
• Using codeine for pain management (high side effect profile including constipation) 1
• Underestimating the importance of necrotic volume assessment in prognosis