What is femoral head avascular necrosis?

Femoral Head Avascular Necrosis

Femoral head avascular necrosis (AVN) is a condition characterized by bone death due to inadequate vascular supply to the femoral head, which can lead to subchondral fracture, articular collapse, and secondary osteoarthritis if left untreated. 1

Definition and Pathophysiology

Avascular necrosis of the femoral head occurs through three possible mechanisms:

1. Vascular interruption
2. Vascular occlusion
3. Extravascular intraosseous compression (often from lipid hypertrophy)

The condition results in death of bone cells and marrow elements due to compromised blood supply to the femoral head 1, 2. This vascular compromise leads to bone necrosis, which progresses through stages that can ultimately result in femoral head collapse and joint destruction 2.

Epidemiology

• Most commonly affects adults in their 30s to 50s
• Estimated 10,000-20,000 new symptomatic cases per year in the United States
• Accounts for approximately 10% of total hip replacements in the US
• In non-traumatic cases, bilateral involvement occurs in 70-80% of patients 1

Risk Factors

Major risk factors include:

• Trauma (femoral neck fractures, hip dislocations)
• Corticosteroid therapy
• Alcohol abuse
• HIV infection
• Hematologic disorders (lymphoma, leukemia, sickle cell disease)
• Chemotherapy and radiation therapy
• Gaucher disease
• Caisson disease (decompression sickness)
• Pancreatitis 1, 2, 3

Clinical Presentation

• Initially may be asymptomatic, even with significant necrosis
• Hip pain (often with referred pain to the knee)
• Progressive limitation of hip movement
• Pain typically worsens with weight-bearing activities
• Symptoms often develop after significant necrosis has occurred 2, 3

Diagnostic Imaging

Early diagnosis is crucial to:

1. Exclude other causes of hip pain
2. Allow for early intervention to prevent articular collapse and need for joint replacement 1

Imaging modalities in order of diagnostic utility:

1. MRI: Most sensitive imaging method for early detection

   • Shows a low signal intensity band that clearly delimits the necrotic area
   • Can detect AVN before radiographic changes appear

2. Plain Radiography: Initial screening tool

   • May not show changes until 6 months after onset
   • Shows sclerosis, cystic changes, and subchondral collapse in later stages

3. CT Scan: Particularly useful to evaluate subchondral fractures and extent of collapse

4. Bone Scintigraphy: Limited use, reserved for exceptional cases 1, 2

Staging and Prognosis

Several staging systems exist, with Ficat and Arlet being the most commonly used despite not accounting for size or location of the necrotic lesion. Other systems include University of Pennsylvania (Steinberg), ARCO, and Japanese Orthopedic Association systems 1.

Prognosis depends on:

• Size of the necrotic lesion: Femoral heads with >30% necrotic volume progress to collapse in 46-83% of cases, while those with <30% progress in <5% of cases
• Location of the lesion
• Presence of risk factors
• Stage at diagnosis
• Patient age (>40 years increases risk of collapse)
• BMI (>24 kg/m² increases risk of collapse)
• Presence of joint effusion or bone marrow edema 1, 4

Treatment Options

Treatment approach depends on the stage of disease:

Non-surgical (Early pre-collapse stages)

• Limited weight-bearing
• Pharmacotherapy options with limited supporting evidence:
  • Statins
  • Bisphosphonates
  • Anticoagulants
  • Extracorporeal shock wave therapy
  • Hyperbaric oxygen 1, 3

Surgical (Early to late stages)

• Early stages (pre-collapse):

  • Core decompression (with or without bone marrow cells)
  • Vascular fibular grafting
  • Electric stimulation

• Late stages (post-collapse):

  • Resurfacing hemiarthroplasty
  • Total hip arthroplasty for severe secondary osteoarthritis 1, 3

Complications

• Subchondral fracture
• Femoral head collapse (particularly with necrotic volume >30%)
• Secondary osteoarthritis
• Joint destruction requiring total hip replacement 1, 4

Key Points for Clinical Management

• Early diagnosis is critical for preserving the femoral head
• MRI is the most sensitive diagnostic tool for early detection
• Patients with risk factors should be monitored closely
• The size of the necrotic lesion is the most important prognostic factor
• Bilateral evaluation is essential as non-traumatic cases are often bilateral (70-80%)
• Screening for other sites of osteonecrosis may be warranted in high-risk patients 1, 2