Assessing Total Knee Arthroplasty Implant Balance with a Passive Knee Drop Test

This study investigated the ability of a modified PKD test to quantify the relative change in the stiffness of a TKA when changing the thickness of tibial inserts. Comparing the stiffness ratios between test constructs demonstrated that the modified PKD test was sensitive to variations in stiffness caused by thicker implants. A significant increase in knee stiffness was observed with as little as a 2mm incremental insert thickness, resulting in nearly twice the stiffness of the TKA, as documented by the increase in log decrement ratios. The lack of impact on implant size or design type suggests that these variables, when the TKA is appropriately sized and the procedure appropriately performed, do not impact stiffness. The ability of the modified PKD test to produce a sensitive and reproducible measure of relative construct stiffness is a promising new tool to help the surgeon in the operating room assess appropriate insert thickness for soft-tissue balancing.