The effects of femoral shaft malrotation on lower extremity anatomy.

OBJECTIVE To determine how axial rotation around the anatomic axis of the femur, as would occur with malrotation of a femoral fracture, affects frontal and sagittal plane alignment and knee joint orientation. DESIGN Computer-generated models of the lower extremity were constructed using standardized dimensions. To simulate a malrotated fracture, these models were rotated in the shaft around the anatomic axis in 15 degrees increments from 60 degrees internal to 60 degrees external rotation. Rotation was performed at the proximal fourth, mid-shaft, and distal fourth. MAIN OUTCOME MEASUREMENTS At each rotational position, the mechanical axis deviation in millimeters and the changes in mechanical lateral distal femoral angle in degrees were measured to quantify frontal plane malalignment and malorientation, respectively. The mechanical axis deviation in millimeters in the sagittal plane was also measured at each rotatory position. RESULTS Femoral shaft malrotation greater than 30 degrees internal rotation of a subtrochanteric fracture or more than 45 degrees of a midshaft fracture or external rotation of 30 degrees or greater of a supracondylar fracture resulted in frontal plane malalignment. External rotation of a supracondylar fracture of 45 degrees or more results in knee joint malorientation. Any external rotation at all 3 fracture levels caused posterior displacement of the weight-bearing axis in the sagittal plane. CONCLUSIONS Malrotation of a femoral shaft fracture is not just a cosmetic problem. Internal and external rotation causes malalignment and malorientation in the frontal plane, depending on the level of the fracture and the magnitude of malrotation. External rotation of any degree at the proximal fourth, mid-shaft, and distal fourth causes a posterior shift of the weight-bearing axis in the sagittal plane.