3. Avascular necrosis

Question 1

Definition of avascular necrosis

Answer 1

Also known as osteonecrosis or aseptic necrosis; cellular death of bone components due to the interruption of blood supply

Question 2

Causes of avascular necrosis

Answer 2

1. Post-trauma
   - Tranverse fracture of scaphoid causing avascular necrosis of the proximal segment
   - Transcervical fracture of proximal femur causing avascular necrosis of the femoral head
2. Corticosteroids
3. Nitrogen bubbles in dysbarism
4. Vasculitis
5. Radiation
6. Vascular compression
7. Venous hypertension, thrombosis
8. Gaucher’s disease
9. Alcoholism

Question 3

Morphology of avascular necrosis

Answer 3

Grossly:

1. Necrotic area in cross section appears yellow, opaque & chalky with hyperemic fibrous tissue at its margin
2. Classically well-demarcated & wedge shape
3. Adjacent bone may be thickened
4. Late disease: osteoarthritic changes (loss of smooth contour of femoral head, destruction of articular cartilage, loose bodies, osteophytes)

Histologically:

1. Necrosis of components of bone (mainly medullary)
   - Cortical bone typically unaffected due to collateral circulation
   - Articular cartilage overlying subchondral infarcts typically unaffected due to provision of nutrients by synovial fluid (but will be involved eventually with collapse of the necrotic subchondral bone)
2. Fat & marrow necrosis (adipocytes in ischemic areas undergo necrosis & release fatty acids which bind calcium to form calcium soaps)
3. Granulation tissue at the periphery of the infarcted area & layering of non-resorbed necrotic bone by woven bone (creeping substitution)

Question 4

Clinical features of avascular necrosis

Answer 4

Xray findings:

1. Early stages: unremarkable
2. Mid-moderate stages: sclerosis & bone density changes
3. Advanced stages: bone deformities such as flattening, subchondral radiolucent line (crescent sign) & collapse of femoral head are evident
4. Note: MRI is the most sensitive study & is the imaging procedure of choice in AVN

Question 5

Complications of avascular necrosis

Answer 5

1. Deformity (femoral head deformity & collapse)
2. Secondary osteoarthritis (especially in cases of subchondral infarcts)
3. May require total hip replacement in severe cases