The Sensitivity of Balancing in Total Knee Surgery

Background: Balancing is important to outcomes, but has been difficult to achieve due to the high sensitivity of the condylar forces to even small changes in ligament lengths. The first goal was to measure the effects of alignment and component placement errors on balancing. The second goal was to study equivalence between the collateral ligament forces, distraction forces, and condylar contact forces.Methods: The test rig flexed and extended synthetic knees, while condylar forces and ligament forces were measured. For ideal femoral component placement, the sagittal contours exactly matched anatomic. 2mm placement errors relative to anatomic were distal (too little distal femoral resection), proximal, anterior, and posterior. Condylar forces for anatomic placement were defined as reference, while forces for the different placement errors were measured relatively.Results: Condylar forces were significantly influenced by 2mm errors in femoral component placement. For distal placement error in combination with kinematic alignment, there were only small changes in the contact forces. Other placement errors produced large contact errors. For mechanical alignment medial forces were similar to kinematic, but lateral forces reduced to almost zero. The medial and lateral ligament and contact forces, distraction forces, and lift-off moments were highly correlated.Conclusions: The ideal situation was kinematic alignment, which could reproduce the condylar contact forces seen in intact anatomic knees. The least condylar force errors occurred when the femoral component was placed too distally, for the kinematic alignment bone cuts. Different balancing parameters can be used interchangeably.