Relative positions of the contacts on the cartilage surfaces of the knee joint

The goal of this project was to determine the centers of contacts (points of closest approach of the articular surfaces) for the tibio-femoral and patello-femoral joints throughout the flexion range, with a focus on high flexion where there is potential overlap between the contacts. The purpose was to determine the implications to the design of joint replacements and tissue engineered implants. Eight cadaveric knees were mounted in a rig with different loading combinations applied to the femur, including axial load, anterior/posterior shear, and internal/external torque. The range of flexion was 0° to 155°. Reference points on the bones measured during the experiments were used to later reconstruct an accurate 3D computer model of the multiple joint positions and determine the centers of contact between the opposing bearing surfaces. The tibio-femoral contact at 0° flexion was displaced 5 mm anterior to the notch (the end point of the articular cartilage on the mid-line of the femoral sulcus) on the medial side, while remaining level with the notch on the lateral side. The patella contacts on the femur extended 15 mm posterior to the intercondylar notch with a centerline between the lateral and medial paths being several millimeters lateral to the center of the femur. The centers of the patella contacts were close to the inner margin of the medial condyle and did not directly overlap with the centers of the tibial contacts. On the lateral side the patella contacts ended where the tibial contacts began. For applications to medial unicondylar knee replacement design it was shown that patellar impingement on the anterior of the femoral component would occur at 110° flexion. For TKR design, a continuous patella contact up to high flexion could be obtained by extending the trochlea 15 mm posterior to the intercondylar notch.