Effect of Joint Loadings, Ligament Tensions, and Total Knee Designs on Kinematics During Flexion.

BACKGROUND: Reproducing anatomic kinematics during flexion in a total knee arthroplasty (TKA) requires a stable medial side and progressive posterior displacement of the lateral femoral condyle. Under surgical conditions, however, the forces across the knee are relatively small, and it is not easy to predict functional kinematics where loads are much higher. The aims of this study were to determine the relative effects of the ligament tensions and the TKA itself on the kinematics that occur under low and high load conditions during a full range of flexion.

METHODS: A crouching machine was developed with simulated femur, tibia, muscles, and collateral ligaments. There were four non-cruciate total knee arthroplasties that were designed: ultracongruent (UC), medially congruent (MC), medial pivot (MP), and replica intercondylar (REP), with guiding surfaces. There were two loading conditions: low load to simulate surgical conditions and high load for functional conditions. There were three collateral ligament tensions: normal, 0.5x normal, and 1.5x normal. The motion of the femur on the tibia was measured from 12 to 130° flexion. The transverse femoral axis projected on the tibia demonstrated displacements of the lateral and medial femoral condyles, as well as axial rotation. Representative pilot statistical tests were carried out for the MC and REP designs, with P = 0.1.

RESULTS: Under low load, all designs showed small medial displacements, but with some lateral displacements and axial rotations. Under high load, the UC, MC, and MP showed almost parallel motion with small anterior-posterior (AP) displacements and rotations. The REP showed stable medial and more pronounced lateral displacements for both low and high loads. For all four designs, the behaviors were similar for the three ligament tension conditions. There was a statistically significant difference between rotations and lateral displacements of the MC and REP under low and high load.

CONCLUSIONS: The kinematics under low-load surgical conditions generally did not match the motion under functional conditions, except for the REP design, which produced similar motion for surgical low loads and functional high loads. For the REP design, the motion was controlled mainly by the contoured shapes of the bearing surfaces.