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Radiographic Evaluation

- See:
      - Ficat Classification
      - Enneking's Stages of Osteonecrosis

- Discussion:
    - combination of osteoclastic resorption and new bone formation in revascularized areas creates x-ray appearance of mottled density;
    - when collapse of segment of necrotic bone occurs, the compression of more bone into smaller area also produces increased x-ray 
           - there are several anatomic reasons for the increased density of necrotic bone as seen radiographs;
    - if radiographic evaluation is consistent w/ diagnosis of AVN, no additional diagnostic tests are necessary;
    - note however, that plain radiographs are often inadequate to make the diagnosis;

- AP View:
    - usually demonstrate the principal area of involvement;
    - because anterior and posterior acetabular margins overlap the superior portion of femoral head, subtle 
           evidence of osteosclerotic or cystic changes in subchondral regions may be missed;

- Lateral View:
    - because AP views of the hip may be difficult to interpret, it is necessary to evaluate frog-leg lateral x-rays of the femoral head;
    - cross-table lateral x-ray is not satisfactory because architectural details
           of the femoral head are obscured by the soft tissues that overlie this area;
    - lateral radiographs also allow staging purposes since it is often anterior segment
           of the femoral head that first collapses or exhibits the crescent sign

- Cresent Sign: