Femoral head shape in Perthes' disease. Is the contralateral hip abnormal?

Femoral head shape has been assessed in 47 children with unilateral Perthes' disease and 41 control children. Measurements were taken from radiographs and arthrographs by a computerized method. In controls the femoral head was round, particularly in lateral outline, and characteristically asymmetric in anteroposterior views at all ages studied. In children with Perthes' disease the bony femoral head was less round than in controls and flattened anteriorly. Mediolateral symmetry differed from that of controls, reflecting lateral flattening of the cross section of the femoral epiphysis. Age-related changes correlated with disease duration but not with treatment. The "unaffected" hip showed some evidence of anterior and lateral flattening from the time of diagnosis of disease in the other hip. No subject developed bilateral disease during the follow-up period. This early involvement of the "unaffected" hip could indicate a constitutional abnormality. The cartilaginous femoral head was rounder than the underlying bony epiphysis, particularly in anteroposterior views. The cartilage of the hip with Perthes' disease was less round than that on the "unaffected" side. There was no evidence of anterior or lateral flattening of either cartilaginous femoral head.