Avascular Necrosis of the Hip

Pathophysiology

  • Final common pathway is microvascular coagulation, leading to:
    • Venous congestion
    • Retrograde arterial occlusion
  • Exact mechanism for each risk factor is unknown

Causes

Common Causes:

1. Corticosteroids – Most frequent

2. Post Traumatic

3. Alcohol

4. Sickle cell disease

5. Caissons disease

Uncommon Causes:

1. Irradiation

2. Smoking

3. HIV

4. Pregnancy

5. Coagulopathies

6. Inflammatory conditions (e.g., SLE)

7. Statins

- Many others: Types of viruses or any state causing increased cortisol

Classification and Management Factors

Ficat System:

1. Stage 1: XR normal, AVN on MRI (pre-collapse)

2. Stage 2: Sclerosis and cyst formation (pre-collapse)

3. Stage 3: Subchondral collapse (crescent sign)

4. Stage 4: Extensive femoral head collapse & acetabular degeneration

Steinberg Classification (Modification of Ficat):

1. Stage 0: Normal XR, MRI & Bone Scan

2. Stage 1: Normal XR, Abnormal MRI or Bone Scan

3. Stage 2: XR features – sclerosis, cysts (pre-collapse)

4. Stage 3: Crescent sign – subchondral collapse

5. Stage 4: Femoral head deformation & flattening

6. Stage 5: Joint space narrowing

7. Stage 6: Advanced degenerative disease

Comparison of Ficat and Steinberg: - Essentially the same, focusing on Stage 3 (crescent sign) as the critical stage. - Steinberg quantifies the degree of collapse and arthrosis better in stages 4, 5, and 6.

Quantification of Head Necrosis

  • Multiply the % involvement on AP view by % involved on lateral view
  • 50% on both views equates to 25% total volume involved
  • (0.5 = 0.25) (25%)

Important Factors in Management Decisions:

- **Size of necrotic lesion**
- **Presence of the crescent sign**
- **Degree of head collapse**
- **Acetabular changes**

Diagnosis

Clinical Presentation: - Groin pain, risk factors, limp, insidious chronic onset

Imaging:

- XR:

  • Sclerosis & cysts

  • Crescent sign

  • Joint space narrowing

  • Severe arthrosis

- MRI:

  • 99% sensitivity and specificity for early diagnosis

  • Pathognomonic sign: Double line sign (present in 80%)

  • MR appearances correspond to stage of AVN:

  • Fat > Blood > Fluid > fibrous tissue = Early > Late disease

  • High T1, Intermediate T2

  • High T1, High T2

  • Low T1, High T2

  • Low T1, Low T2

Management

Non-Surgical:

  • NWB (Non-Weight Bearing), USS (Ultrasound), and ECSW (Extracorporeal Shock Wave Therapy):
    • Not proven to help but useful to try if unfit for surgery
  • Bisphosphonates:
    • May be helpful in slowing down the rate of head collapse but not currently recommended as first-line treatment

Surgical:

  • The treatment of choice as it is proven to alter natural history
  • Bone conserving versus Arthroplasty:
    • Presence of the crescent sign indicates arthroplasty (unless very small)
  • Core Decompression:
    • Ficat 1 or 2
    • Small to medium necrotic lesion (30% max)
    • Reduction of pressure relieves venous congestion
    • Encourages neovascularisation
    • Cannulated drill or DHS reamer without the proximal large flute
    • Drill into the lesion but avoid joint penetration
    • Pre-op MRI to outline the area to aim for
    • Send reamings for histology
    • Some surgeons then insert non-vascularised bone graft up the channel ± BMP to encourage healing
    • Not shown to be better than decompression alone
    • Results:
      • 70% good at up to 8 years
      • Smaller the lesion, the better the results
      • Not as good for steroid-induced AVN
  • Osteotomy:
    • Ficat 1-3
    • Small localised lesions <50%
    • Valgus, varus, or rotational osteotomy depending on location
    • Will not work in collapse >50%
    • Rarely done in the UK due to issues with:
      • Non-union of osteotomy
      • Technical difficulty
      • Makes THR salvage more difficult
    • Results:
      • Good reported in Japan and Korea
      • Not reproduced in the West
  • Vascularised Bone Graft:
    • Ficat 1-3
    • Lesions <50% in size
    • Harvest mid-portion fibula
    • Drill and debride the necrotic lesion
    • Introducing the fibula into the drill track
    • A microvascular anastomosis is then performed between the peroneal vessels and the ascending lateral circumflex artery
    • Results:
      • Up to 90% success at 5 years in type 1-3 lesions
      • Useful for larger size lesions without significant collapse
      • Best results still in stage 1 or 2 pre-collapse
    • Time-consuming technique requiring a highly skilled team
  • Arthroplasty:
    • Ficat 3 or 4
    • If stage 3 lesion should be of significant size
    • Older Patients – single operation
    • Results of THR:
      • As good for AVN as in OA (Osteoarthritis)
      • Avoid if possible in young but still has good outcomes
    • Resurfacing contra-indicated
  • Arthrodesis:
    • An option for very young patients
    • Position of fusion is:
      • 30 deg flexion
      • 0-5 deg external rotation
      • 0-5 deg adduction
    • Results:
      • Not as good as for fusion in OA (Osteoarthritis) – more non-union
      • 5% have back pain
      • 40% have ipsilateral knee pain
      • 20% have contralateral hip pain
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