The authors investigated the deformity of the cartilaginous femoral head in 30 patients who had unilaterally affected Catterall groups III and IV Legg-Calvé-Perthes disease. The extent of collapse and the sphericity of cartilaginous head were determined using coronal and sagittal magnetic resonance images. The maximal femoral head collapse was larger in the sagittal plane than in the coronal plane; however, the instant radii of curvature varied significantly more in the coronal plane than in the sagittal plane. These data suggest that the sphericity of the femoral head is better maintained, despite a more extensive collapse, in the sagittal plane. The authors assume that the physiologic, wider range of hip motion in the sagittal plane in daily life helps to maintain the sphericity of the femoral head in the sagittal plane by a molding mechanism. These observations support the current concept of containment and motion therapy.