Mathematical precision in rotational corrective osteotomy of the femur

We used a mathematical model to determine whether the approach to derotation of the malrotated femur can be improved for use in corrective osteotomy of the femur. Rotational corrective osteotomy of the femur (RCOF) is a procedure performed by orthopedic surgeons to correct congenital femoral torsion deformities and posttraumatic femoral shaft malunions. In the conventional technique, osteotomy is performed at the malrotation site, and correction is achieved by rotating the distal segment of the femur so that the patella and toes point upward, symmetric with the normal leg when the patient is in the supine position. This approach does not ensure the rotational position of the proximal segment of the femur, and malrotation can still persist. Intraoperative muscle spasm or preoperative soft tissue contracture may alter the proximal segment rotation immediately after osteotomy. Although marked improvement in the accuracy of measuring the malrotation angle for RCOF has been made possible by the computed tomography and magnetic resonance imaging, the orthopedic surgeon does not have a defined approach for transforming this value from degrees to millimeters so that derotation of the femur can be achieved intraoperatively with precision. We carried out studies using ten cadaver femurs with malrotation angles of 15 to 44 degrees to see if this surgical technique can be improved by a mathematical model that converts the malrotation angle value (in degrees) to an equivalent value on the circle circumference (in millimeters). Our results show errors of 0.5 to 4.5 degrees in the rotation angle and virtually eliminate the error of malrotation associated with RCOF. We suggest that our model is clinically applicable and that its definitive value will arise from clinical applications.