External rotational positioning of the leg after intertrochanteric combined varus-derotational osteotomy in Perthes' disease

A 13-year-old boy underwent an intertrochanteric combined varus derotational osteotomy (VDO) for Perthes' disease of his right hip. After surgery he walked with his right leg externally rotated. Computed tomography scans of both knees and hips were obtained with the legs in their spontaneous position to evaluate rotation and anteversion. The femoral condyles were in 28 degrees of external rotation on the operated side compared with 2 degrees on the non-operated side. The angle between the femoral neck and the horizontal plane was 32 degrees on both sides. We conclude that there may be a risk that a derotational osteotomy may produce an external rotational position of the leg instead of altering the angle between the femoral neck and the horizontal plane.