Osteonecrosis (avascular necrosis)
- Lesion
When the blood supply to a piece of bone is cut, the bone and marrow die. Under joint cartilage the dead bone collapses and the joint wears out.
What it is
Osteonecrosis is ischaemic death of bone and marrow. Most cases follow fracture or vascular disruption, glucocorticoid treatment, heavy alcohol use, sickle cell disease and other haemoglobinopathies, decompression sickness, radiation or Gaucher disease; many are idiopathic. Medullary infarcts in the shaft are often silent, but subchondral infarcts under a joint surface, typically the femoral head, cause pain and collapse leading to secondary osteoarthritis. Repair proceeds by creeping substitution: new bone is laid on dead trabeculae, which are slowly removed.
Structure
- Subchondral infarct: pale yellow wedge with its base on the joint surface; overlying cartilage intact early.
- Dead trabeculae with empty osteocyte lacunae, necrotic fat and dystrophic calcification of marrow.
- Border of hyperaemia, fibrous tissue and new bone (creeping substitution) around the necrotic zone.
- Late: crescent sign (subchondral fracture), flattening and collapse of the femoral head, secondary osteoarthritis.
- Medullary infarct: geographic dead zone in the shaft, often calcified on X-ray.
Step by step
- Blood flow is cut by fracture, thrombosis or sickling, vessel compression (steroid-swollen fat cells, Gaucher cells) or gas bubbles.
- Osteocytes and marrow cells die within hours to days; dead bone keeps its shape and looks normal at first on X-ray.
- Inflammation and granulation tissue form at the edge of the infarct.
- Osteoclasts remove dead trabeculae while osteoblasts add new bone on top, weakening the area during repair.
- Under a joint, the weakened subchondral plate fractures and collapses; the joint degenerates.
Clinical notes
- Hip pain on weight bearing in a patient on steroids, with heavy alcohol use or with sickle cell disease: think of femoral head avascular necrosis; MRI detects it before plain X-rays.
- Scaphoid waist fractures can cut off the proximal pole's blood supply, because it enters distally.
- Medication-related osteonecrosis of the jaw follows antiresorptive drugs and dental procedures.
- Advanced femoral head collapse often needs hip replacement.
Test yourself
Think of the answer, then tap the question to see it.
Why does the articular cartilage survive a subchondral infarct?
Because cartilage is nourished by synovial fluid, not by the bone's blood supply.
What is creeping substitution?
Repair of dead bone in which osteoclasts resorb dead trabeculae while osteoblasts lay new bone on them.
What are three classic causes of osteonecrosis?
Glucocorticoids, alcohol, sickle cell disease (also fracture, decompression sickness, radiation, Gaucher disease).
Which imaging test detects early avascular necrosis best?
MRI, which shows marrow changes before plain X-rays change.
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Femoral Head Avascular Necrosis (StatPearls (NCBI Bookshelf), 2025); Avascular Necrosis | Osteonecrosis | MedlinePlus (MedlinePlus, US National Library of Medicine, 2025); Legg-Calve-Perthes Disease (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.