Osteonecrosis/Avascular Necrosis (AVN)

Question 1

Define osteonecrosis/avascular necrosis (AVN) [1]

Answer 1

bone infarction (that is, tissue death caused by an interruption of the blood supply) near a joint

Question 2

Which part of the bone does osteonecrosis affect in particular and what are the complications of this? [3]

Answer 2

1. can cause death of subchondral bone (i.e. the bone right under the joint surface)
2. can lead to:
   
   • collapse of the joint surface
   • end stage arthritis

Question 3

Which joints are most likely to get affected by osteonecrosis? [2]

Answer 3

1. hips
2. shoulders

Question 4

What are the typical symptoms of osteonecrosis? [4]

Answer 4

1. Osteonecrosis can be asymptomatic and found incidentally on imaging
2. Most patients present because of pain, either from the infarction itself or from arthritis
3. Rest pain occurs in about 2/3rd of patients
4. Night pain occurs in about 1/3rd of patients

Question 5

What are the typical symptoms of osteonecrosis of the femoral head and what kind of limitations can it cause? [6]

Answer 5

1. groin pain that worsens with:
   
   • weight-bearing and motion
2. less commonly complain of thigh and buttock pain
3. can causes particular limitation in:
   
   • internal rotation
   • abduction

Question 6

The initial physical exam findings on a patient with osteonecrosis are often non-specific.

But after osteonecrosis progresses, joint function deteriorates and the patient will present with the findings of arthrosis.

What are the presenting features of arthrosis? [3]

Answer 6

1. limp
2. tenderness around the affected bone
3. restricted motion

Question 7

Describe the pathophysiology of osteonecrosis [6]

Answer 7

1. In AVN, the necrosis always involves the medullary bone first (the cortex may be spared because it has a collateral blood supply)
2. Likewise, the overlying articular cartilage receives nutrition from the synovial fluid and remains viable
3. The dead bone has empty lacunae and is surrounded by necrotic adipocytes that often rupture and release their fatty acids
4. These fatty acids can bind calcium and form insoluble calcium soaps
5. During the healing process, osteoclasts resorb the necrotic trabeculae, while the remaining ones serve as scaffolding for the deposition of new bone (i.e., creeping substitution)
   
   • Nevertheless, the pace of creeping substitution may not be fast enough to be effective, and, as a result, the necrotic cancellous bone collapses

Question 8

Define bone remodelling [1]

Answer 8

the process by which osteoclasts secrete acid and proteolytic enzymes to digest the bone matrix and osteoblasts synthesize new organic matrix leading to the deposition of newer, better bone)

Question 9

Explain how osteonecrosis leads to arthritis [5]

Answer 9

• If bone becomes ischaemic, it dies and therefore does not remodel
  
  • If bone does not remodel, micro-damage does not get repaired and the mechanical properties of the bone are impaired
    
    • If enough damage accumulates, the subchondral bone can be weakened to the point of collapse
      
      • If the sub-chondral bone collapses, the joint surface becomes irregular and no longer smooth
        
        • If one side of the joint surface is not smooth it will damage the other surface leading to arthritis

Question 10

What x-ray sign indicates preceding subchondral collapse? [1]

Answer 10

pathognomonic crescent sign (subchondral radiolucency)

Question 11

What features can be seen on x-ray in the late stages of AVN? [4]

Answer 11

1. loss of sphericity
2. collapse of the femoral head
3. joint-space narrowing
4. degenerative changes in the acetabulum

Question 12

What imaging modality has the high sensitivity for detecting osteonecrosis? [1]

Answer 12

MRI

Question 13

List the main risk factors of osteonecrosis/AVN [6]

Answer 13

CASH:

Corticosteroid use or Cushing’s disease

Alcohol abuse

Sickle cell disease/haemoglobinopathies

History of trauma, especially a joint dislocation

Question 14

What are the treatment options for osteonecrosis/AVN in the

1. early stages? [2]
2. late stages? [1]

Answer 14

1. Early stages:
   
   • core decompression with or without bone graft is appropriate
2. Late stages:
   
   • total hip arthroplasty

Question 15

What is Keinboch’s Sign? [1]

Answer 15

Blood supply to lunate of the hand eliminated