Bucholz and Ogden classification of osteonecrosis of femoral head in congenital dislocation of hip. A, Normal femoral head at 2 months (a), 1 year (b), and 9 years (c) of age. B, Type I: a, sites of temporary vascular occlusion; b, irregular ossification in secondary center; c, normal epiphyseal contour, slight decrease in height of capital femoral ossification center. C, Type II: a, probable primary site of vascular occlusion; b, metaphyseal and epiphyseal irregularities; c, premature fusion of lateral metaphysis and epiphysis. D, Type III: a, sites of temporary vascular occlusion; b, impaired longitudinal growth of capital femoral epiphysis; c, irregularly shaped femoral head. E, Type IV: a, sites of temporary vascular occlusion; b, impaired longitudinal and latitudinal growth; c, premature epiphyseal closure.