The gross appearance and radiologic changes of bones affected by osteonecrosis may divided into 4 stages.
Appearance
Stage
external examination of the bone and joint is normal; on cut section a wedge-shaped area of chalky or dull yellow bone may be seen that is separated from normal bone by a hyperemic zone
I
diffuse osteoporosis, sclerosis and cyst formation, resulting in a mottled appearance in X-rays; a band of sclerotic bone involves the hyperemic zone (creeping substitution)
II
linear fracture below the end-plate with subchondral bone collapse resulting in a "crescent sign" on X-ray; the joint space is still normal
III
abnormal contour to bone with changes of degenerative joint disease; the joint space is narrowed, irregular and deformed
IV
MRI is able to detect changes earlier than routine X-rays, with a low density band seen in T1-weighted images.
Features separating Stage IV osteonecrosis from osteoarthritis:
(1) absence of eburnated bone
(2) presence of bony and cartilaginous debris in the surrounding synovial and capsular tissue.
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