Three-dimensional shape of the dysplastic femur: implications for THR

This study evaluates the three-dimensional anatomy of the femur with congenital dysplasia of the hip (CDH) in comparison with healthy controls. Computed tomographic scans were obtained from 207 women (154 with dysplasia; 54 healthy controls) with an average age of 51.6 years (range, 18-82 years). Most of the dysplastic joints were classified as Crowe I (43%), or Crowe II or III (48%), with 9% Crowe IV. Individualized three-dimensional computer models of the femur were generated by reconstruction of the CT scans. Dimensional and morphometric parameters were derived by computer analysis of each of the femoral reconstructions. The dysplastic femurs had shorter necks and smaller, straighter canals than the controls. The shape of the canal became more abnormal with increasing subluxation. Detailed analysis showed that the primary deformity of the dysplastic femur is rotational, with an increase in anteversion of 5 degrees to 16 degrees, depending on the degree of subluxation of the hip. The rotational deformity of the dysplastic femur arises within the diaphysis between the lesser trochanter and the isthmus and is not attributable to a torsional deformity of the metaphysis. This study shows that there is a significant difference in the geometry of the normal femurs and those with CDH, even in mild cases. In CDH cases, we recommend the use of modular or specially-designed components to accommodate the shape of the dysplastic canal.