Imaging

As a rule, radiographic changes in Perthe's disease are usually firmly established before symptoms develop although few changes are apparent in very early disease (1):

• collapse
• sclerosis - due to reduced resorption and relative osteopenia of the rest of the bone
• subchondral fractures as the earliest sign of collapse
• increased joint space
• femoral head ossification centre is smaller on the affected side and can appear fragmented
• coxa magna - spherical enlargement of the femoral head probably due to increased vascularity

Waldenström has divided radiological stages which all children with Perthes disease pass through into initial, condensation, fragmentation, repair, and healing stages (1)

MRI can detect avascular areas before there is collapse. MRI is therefore indicated in cases of recurrent irritable hip (3).

Bone scans are useful in early disease when diagnosis can be prompted by demonstration of decreased uptake in the avascular phase and increased uptake in the healing phase.

Reference:

1. Poul J. Diagnosis of Legg-Calvé-Perthes disease. Ortop Traumatol Rehabil. Oct 30 2004;6(5):604-6
2. Van Campenhout A, Moens P, Fabry G. Serial bone scintigraphy in Legg-Calvé-Perthes disease: correlation with the Catterall and Herring classification. J Pediatr Orthop B. Jan 2006;15(1):6-10
3. Sales de Gauzy J, Briot J, Swider P. Coxa magna quantification using MRI in Legg-Calve-Perthes disease.Clin Biomech (Bristol, Avon). Jan 2009;24(1):43-6