Anatomic and radiographic studies reveal that the normal joint line is oriented horizontally. An average of 6 degrees of overall tibiofemoral valgus is produced by an average of 8 to 9 degrees of distal femoral valgus combined with an average of 2 to 3 degrees of proximal tibial varus (range: 0 to 6 degrees),32 and the joint line is parallel to the floor. Following this orientation during total knee arthroplasty provides an anatomic alignment. This places the mechanical axis slightly into the medial compartment providing an even distribution of forces across an asymmetric tibial tray. No external rotation of the femoral component is required for this method.
Most total knee instrumentation produces a slightly different joint line, which is oriented perpendicular to the mechanical axis (from the center of the femoral head to the center of the ankle), due to a tibial resection that is perpendicular to the long axis of the tibia. The joint line produced is generally 2 to 3 degrees from parallel to the floor. Krakow has referred to this alignment approach as classical alignment.33 Externally rotating the femoral component 3 degrees is recommended to compensate for the iatro-genic soft tissue imbalance that this creates (Fig. 7.3).
Our preference is to reestablish the normal anatomy as closely as possible, in order to achieve the goal of normal kinematics. Correct positioning of the implants is usually accomplished by cutting the tibia perpendicular for the valgus knee, or in slight varus in the frontal plane for the varus knee, and by cutting the distal femur in 6 degrees of valgus from the anatomic axis. This accomplishes an overall alignment of 4 to 6 degrees of valgus with better patellar tracking. A standard 6-degree valgus cut of the femur is recommended, although the instruments allow 4, 6, or 8 degrees. The anatomic-mechanical axis angle can be measured from a radiograph, but it may be inaccurate by 1 to 2 degrees because of rotational inconsistency. The true anatomic axis may be off with all intramedullary instruments if the starting point on the distal femur is too medial or lateral, or if the medullary rod is not perfectly centered in the medullary canal.
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