Abstract
The cornerstone to correct ligament balancing is correct varus and valgus alignment in flexion and extension. For alignment in the extended position, fixed anatomic landmarks such as the intramedullary canal of the femur and long axis of the tibia are accepted. When the joint surface is resected at an angle of 5° to 7° valgus to the medullary canal of the femur and perpendicular to the long axis of the tibia, the joint surfaces are perpendicular to the mechanical axis of the lower extremity, and roughly parallel to the epicondylar axis. In the flexed position, anatomic landmarks are equally important for varus-valgus alignment. Incorrect varus-valgus alignment in flexion not only malaligns the long axes of the femur and tibia, but also incorrectly positions the patellar groove both in flexion and extension.
Finding suitable landmarks for varus-valgus alignment has led to efforts to use the posterior femoral condyles, epicondylar axis, and anteroposterior (AP) axis of the femur. The posterior femoral condyles often are not reliable rotational alignment landmarks because the femoral joint surface has been worn or otherwise distorted by developmental abnormalities or the arthritic process. As with the distal surfaces, the posterior femoral condylar surfaces sometimes are damaged or hypoplastic (more commonly in the valgus than in the varus knee) and cannot serve as reliable anatomic guides for alignment. The epicondylar axis is anatomically inconsistent and in all cases other than revision total knee arthroplasty with severe bone loss, is unreliable for varus-valgus alignment in flexion just as it is in extension. The AP axis, defined by the centre of the intercondylar notch posteriorly and the deepest part of the patellar groove anteriorly, is highly consistent, and always lies within the median sagittal plane that bisects the lower extremity, passing through the hip, knee, and ankle. When the articular surfaces are resected perpendicular to the AP axis, they are perpendicular to the AP plane, and the extremity can function normally in this plane throughout the arc of flexion. Once the knee is set up in correct alignment, ligament balancing can be done with simple procedures based on basic anatomy. Anterior structures tighten in flexion, and posterior structures tighten in extension. Release of tight structures in a controlled fashion completes the aligned and balanced knee.
