Idiopathic avascular necrosis of the femoral head in a young child: a small, dense, then fragmented epiphysis with a subchondral crescent — the lateral pillar and head-at-risk signs predict outcome; MRI shows it before the radiograph.
Orient first
- Age 4–8 years, boys more; bilateral in a minority (asynchronous).
- Stages: initial (dense, small epiphysis), fragmentation, reossification, healed.
- Lateral pillar (Herring) classification during fragmentation predicts outcome (verify).
Acquire the study
- AP pelvis and frog-leg lateral.
The manoeuvre
- Epiphysis: smaller and denser than the other side on the AP view.
- Frog-leg lateral: subchondral lucent crescent.
- Fragmentation; lateral pillar height compared with the other side in %.
- Head-at-risk signs: lateral subluxation, calcification lateral to the epiphysis, horizontal physis.
What confirms it
- Epiphyseal sclerosis, crescent or fragmentation, or non-perfusion on MRI.
What licenses you to exclude it
- A normal MRI with contrast excludes Perthes at that time.
The classic misread
- Meyer dysplasia (irregular, bilateral, younger children) mistaken for Perthes.