Benefits of Using Functional Joint Coordinate Systems in In Vitro Knee Testing

To measure knee joint kinematics, coordinate systems (CS) must be assigned to the tibia and femur. Functional CS have been shown to be more reproducible than anatomical. This study aims to quantify the benefits of using functional CS in in vitro testing. Seven cadaveric knee joints were loaded in a six-degree-of-freedom (DOF) joint simulator. Anatomical CS were established for each joint and functional CS were calculated based on joint kinematics during passive motion. Loading profiles were applied to the knee joints using different CS definitions. Resulting kinematics and kinetics were obtained to quantify the (1) reduction in intraknee kinematic response variation, (2) reduction in kinematic cross-talk, (3) reduction in interknee kinematic response variation, and (4) improvement in force control performance, when using functional CS compared to anatomical. Functional CS, compared to anatomical, (1) significantly reduced intraknee kinematic response variation across 12 combined loading conditions for nearly all DOF, (2) significantly reduced kinematic cross-talk during anterior-posterior (AP), varus-valgus (VV), and internal-external rotation (IRER) laxity testing across many DOF, (3) significantly reduced interknee kinematic response variation for all DOFs over a gait profile and combined loading conditions, and (4) significantly improved anterior-posterior and varus-valgus force/torque (FT) control performance during dynamic loading profiles. The advantage of using functional CS for in vitro testing has been demonstrated across all considered domains. Functional CS should be used when performing in vitro knee joint testing.