Kinematics of medial unicondylar knee arthroplasty: an in vivo investigation.

This study investigated the weight-bearing flexion of 7 knees with previous medial unicondylar knee arthroplasty (UKA), using a dual fluoroscopic imaging system. The data indicated that the patients had consistent posterior translation of the medial femoral condyle and consistent internal rotation of the tibia with flexion. The articular contact location remained just anterior and medial to the center of the polyethylene insert throughout the range of motion. Femoral condylar lift-off was found in 2 patients at a maximum flexion angle of 130.7 +/- 10.2 degrees. The other 5 patients had a maximum flexion angle of 113.9 +/- 7.2 degrees. The small range of tibiofemoral contact excursion in both anteroposterior and mediolateral directions indicated a medial pivot contact pattern during flexion, corresponding to the observed internal tibial rotation. This may help reduce tibial loosening, thus increasing implant longevity. The observed coincidence between condylar lift-off and higher flexion angles may reveal the causative factors that enhance knee flexion after UKA.