Perthe's disease

1-Is avascular necrosis of capital femoral epiphysis in children.

2-Children with hip pain should be examined for hip pathology.

3-Stages:

-Stage 1: Anterior aspect or part of the epiphysis is affected(in the form of fissures or any appearance with or without air inside the fissure).

-Stage 2: The above + metaphyseal reaction(irregularities).

-Stage 3: Complete affection of epiphysis with metaphyseal reaction.

-Stage 4: Flattening and collapse with osteoarthritis changes(If you find changes in acetabulum in addition to hip disease, you must know that you are confronting with a joint disease.
The difference between primary and secondary osteoarthritis(as in case of avascular necrosis and rheumatoid ) is that in the former the osteophytes and cystic changes are extensive while in the latter are minimal but with joint space narrowing.
If the hip is massively destructed, we cannot evaluate if it is due to primary or secondary osteoarthritis.