Prognostic factors and outcome measures in Perthes disease.

The shape of the femoral head at the time when Perthes disease heals is the most important determinant of the risk for degenerative arthritis; hence, the shape of the femoral head and congruence of the hip are the most useful outcome measures. Although several prognostic factors that predict femoral head deformation may be identified during the course of Perthes disease, only two prognostic factors may be identified early enough to institute preventive intervention: femoral head extrusion and the age at onset of the disease. Femoral head extrusion is the only factor that may be influenced by treatment.