In Vivo Kinematics of a Posterior Stabilized Total Knee Arthroplasty

Background Various postoperative evaluations using fluoroscopy have reported in vivo knee flexion kinematics under weight bearing conditions. This method has been used to investigate which design features are more important for restoring normal knee function. The objective of this study is to evaluate the kinematics of a Posterior-Stabilized TKA in weight bearing deep knee flexion using 2D/3D registration technique. Patients and methods We investigated the in vivo knee kinematics of 9 knees (9 patients) implanted with a Posterior Stabilized TKA (Triathlon PS, Stlyker Orthopedics, Mahwah, NJ). Under fluoroscopic surveillance, each patient did a deep knee flexion under weight-bearing condition. Femorotibial motion including tibial polyethylene insert were analyzed using 2D/3D registration technique, which uses computer-assisted design (CAD) models to reproduce the spatial position of the femoral, tibial components from single-view fluoroscopic images. We evaluated the knee flexion angle, femoral axial rotation, antero-posterior translation of contact points, and post-cam engagement were evaluated. Results The mean maximum flexion angle was 121.0±9.5°. The amount of femoral axial rotation was 7.5±1.5°. The femorotibial contact point moved posteriora££4.9±4.5mm on medial compartment, 10.0±3.3mm on lateral compartment with knee flexion. The mean knee flexion angle at initial post-cam engagement was 47.5±17.2°. The kinematic pattern was medial pivot. Discussion The contact point constantly moved backward especially on the lateral side. At early flexion, both the medial and lateral contact point moved posteriorly, which might be caused by a change in sagittal radius at 10° flexion. The post-cam engagement occurred at midflexion, that might prevent the paradoxical anterior translation of the femur with respect to tibia during knee flexion.