Factors leading to hip subluxation and dislocation in femoral lengthening of unilateral congenital short femur.

Congenital short femur is a rare and complex deformity. For it to be treated in the most accurate way, we must know of every possible complication. The purpose of this study was to determine the factors leading to hip subluxation or dislocation in patients with an unilateral congenital short femur who underwent a femoral lengthening procedure. Forty-four patients with congenital unilateral short femora types I, II, and IIIA were seen between 1981 and 1993 at Alfred I. duPont Hospital for Children. Seventeen patients had type I, 9 patients had type II, and 18 patients had type IIIA. Twenty patients underwent simultaneous lengthening of the femur and tibia, and 24 patients had primary lengthening of the femur. Eleven patients had an epiphysiodesis of the contralateral femur for residual leg length discrepancies of <6 cm. We observed that in patients with type IIIA, the acetabular dysplasia is severe, as well as the coxa vara deformity, and is associated with progressive subluxation and dislocation of the hip with lengthening. We believe that the combined abnormality of coxa vara and bow of the femoral shaft contributes to hip subluxation and dislocation in congenital short femur in association with an acetabular dysplasia prior to the lengthening. We recommend correction of the varus bow of the femur and the neck-shaft angle to 120 degrees and the acetabular index to <25 degrees prior to lengthening of type IIIA femora.