How Frequently Do Four Methods for Mechanically Aligning a Total Knee Arthroplasty Cause Collateral Ligament Imbalance and Change Alignment from Normal in White Patients?

Background Mechanically aligned total knee arthroplasty can create 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 changes in limb and knee alignment from normal. The goal of the present study was to calculate the frequency and range of these undesirable consequences. Methods Four methods of mechanically aligned total knee arthroplasty were simulated on fifty normal three-dimensional bone models of the lower extremity from white subjects. Each method resected the distal aspect of the femur and proximal aspect of the tibia perpendicular to their respective mechanical axes. Setting the posterior joint line perpendicular to the anteroposterior axis of the trochlear groove (Method 1), parallel to the transepicondylar axis (Method 2), externally rotated 3° with respect to the posterior condylar axis (Method 3), and parallel to the tibial resection in 90° of flexion with the use of gap-balancing (Method 4) aligned internal-external rotation of the femoral component. Results The proportion of total knee arthroplasties requiring a ≥2-mm release of a tight collateral ligament was 34% for the medial collateral ligament and 30% for the lateral collateral ligament. The proportion of total knee arthroplasties with ≥2 mm of instability between 0° of extension and 90° of flexion was 56% in the medial compartment and 6% in the lateral compartment for Method 1, 74% and 6% for Method 2, and 42% and 0% for Method 3. Method 4 did not cause ligamentous instability. The proportion of arthroplasties with a ≥2° change from normal was 58% for limb alignment and 58% for knee alignment. Conclusions Surgeons should be aware that, when using the four methods of mechanically aligning a total knee arthroplasty, they will frequently have to manage a wide range of collateral ligament imbalances that are complex, cumulative, and uncorrectable by collateral ligament release, and a wide range of changes 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.