Last reviewed 01/2018
Radiographically and structurally there is no difference between live and dead bone. However, because dead bone lacks a blood supply, it does not undergo renewal, and after repeated stress it collapses.
Changes occur in four concurrent stages
Stage I : marrow necrosis and cell death occurs within 24 hours of infarction. However there may be no change in macroscopic appearance for weeks or even months.
Stage II : After some days or weeks the living bone surrounding the area of dead bone shows a vascular reaction, and new bone is laid down upon the trabeculae of dead bone. There is an increase in bone density that is visible on x-ray. The newly deposited bone has been laid on a fragile structure and, despite the increased bone mass, small fractures start to appear at the site of osteonecrosis.
Stage III : degeneration of bone architecture becomes more gross and the outline of the bone becomes distorted.
Stage IV : Articular destruction occurs as a result of severe distortion of the bone surface.