Background:​ In mechanically aligned total knee arthroplasty (TKA), there are two strategies for aligning the components in the coronal plane. The first strategy references the mechanical axes of the femur and tibia and sets the limb at a 0° hip-knee-ankle angle. The second strategy references the anatomic axes of the femur and tibia and sets the knee at 5° or 7° valgus. However, these strategies may cause undesirable consequences recognized as a tight collateral ligament in 0° of extension, instability in a compartment between 0° of extension and 90° of flexion that is uncorrectable by collateral ligament release, and a change in limb and knee alignment from normal. The goal of this research project was to determine the frequency and range of these undesirable consequences.

Methods:​ The TKA was simulated on fifty three-dimensional bone models of normal lower extremities from white subjects. To establish a repeatable projection from which to make the measurements, the simulations were performed on the limb projected in the sagittal, coronal, and axial kinematic planes. In the coronal kinematic plane, mechanically aligned TKA was simulated using each of the two strategies:​ (1) setting the limb at a 0° hip-knee-ankle angle, and (2) setting knee at 5° or 7° valgus. In the axial kinematic plane, the internal-external rotation of the femoral component was set using each of the four methods:​ (Method 1) perpendicular to the anteroposterior axis of the trochlear groove), (Method 2) parallel to the transepicondylar axis, (Method 3) 3° externally rotated to the posterior condylar axis, and (Method 4) parallel to the tibial resection at 90° flexion with the use of gap-balancing.

Results:​ The percentage of simulated TKAs that required ≥ 2 mm release of a tight collateral ligament at 0° of extension was 58% with the limb at a 0° hip-knee-ankle angle, 60% with the knee set at 5° valgus, and 64% with the knee set at 7° valgus. For Methods 1-3, the percentage of simulated TKAS that had ≥ 2 mm ligament instability in a compartment between 0° of extension and 90° of flexion that is uncorrectable by collateral ligament release was 56%, 74%, and 42% respectively with the limb set at a 0° hip-knee-ankle angle, 60%, 58%, and 54% respectively with the knee set at 5° valgus, and 64%, 74%, and 48% respectively with the knee set at 7° valgus. Gap balancing Method 4 did not cause ligament when used with any of the coronal alignment strategies. The percentage of simulated TKAS that had ≥ 2° change in limb and knee alignment was 58% with the limb set at a 0° hip-knee-ankle angle, 60% with the knee set at 5° valgus, and 64% with the knee set at 7° valgus.

Discussion:​ Surgeons should be aware when they mechanically align a TKA with the limb set at a 0° hip-knee-ankle angle and the knee set at 5° or 7° valgus with four different methods for setting the internal-external rotation of the femoral component, that they will frequently have to manage a wide range of instabilities that are complex, cumulative, and uncorrectable by collateral ligament release, and a wide range of change in limb and knee alignment from normal. Patients who perceive these changes in stability, limb alignment, and knee alignment may be dissatisfied and require counseling.