In vivo kinematics of high-flex mobile-bearing total knee arthroplasty, with a new post-cam design, in deep knee bending motion.

PURPOSE: The objective of this study was to evaluate the in vivo knee kinematics to assess the available functional motion of the characteristic mobile-bearing prosthesis design and to examine whether the artificial joint would work in vivo according to its design concept.

METHODS: We studied 14 knees (11 patients) implanted with the Vanguard RP Hi-Flex prosthesis. This prosthesis has a highly original form of post-cam called a PS saddle design with high compatibility, and with a rotating plate mobile-bearing mechanism. The cylinder-type post-cam is designed to enable contact in early flexion ranges, and to prevent paradoxical anterior femoral component movement. Each patient performed weight-bearing deep knee bending under fluoroscopic surveillance. Motion between each component including the polyethylene insert was analyzed using the 2D/3D registration technique.

RESULTS: The mean range of motion was 122.0°. The mean femoral component rotation for the tibial tray was 5.0°. No paradoxical anterior movement of the nearest point was confirmed between the femoral component and the tibial tray in the early flexion ranges. Initial contact of the post-cam was confirmed at a knee flexion angle of 33.8°. Subsequently, the wide contact of the post-cam was maintained until flexion reached 120° in all knees, but disengagement of the post-cam was observed in two knees when flexion was ≥130°.

CONCLUSIONS: The results of this study demonstrated that the prosthesis design generally works in vivo as intended by its design concept. The present kinematic data may provide useful information for improvement of high-flex type prostheses.