The whole progression of Perthe's disease takes between 2 and 4 years.
The aetiology is unknown but the following factors may be important:
Firstly there is bone death - this appears normal on a radiograph, but is seen on MRI - in association with loss of vascularisation.
Fragmentation follows; the epiphysis may flatten as a result of poor repair of microfractures.
Lastly there is revascularisation, repair and remodelling of the femoral head.
Note that this process is entirely different from the irreversible death of the femoral head in adult avascular necrosis.
Reference:
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