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We report a new arthrographic index to quantitate the loss of sphericity of the femoral head during early Perthes' disease. The technique has been applied to 272 arthrograms of 131 normal and 141 affected hips. It measures marginal or segmental collapse as well as central collapse of the epiphysis, thus providing a more accurate assessment of deformity in 57.4% of anteroposterior and 46.8% of lateral radiographs as compared with the previous standard measurement (caput index). The measurement principle applies equally to other recent radiological developments such as magnetic resonance imaging.

A review at maturity of 42 patients with Perthes' disease has allowed a correlation to be made between femoral head deformity during the initial phases of fragmentation and reparation, as quantified arthrographically, with the final shape and size of the femoral head measured on plain radiographs. Cases with loss of sphericity or coxa magna at maturity had significantly lower arthrographic indexes of deformity (p = 0.001 and p = 0.005 respectively), regardless of the age at presentation. All were treated uniformly by containment in recumbency and a daily exercise program.