A new concept in guided motion total knee arthroplasty.

When the normal knee is flexed from 0 to maximum, the lateral femoral condyle displaces posteriorly, but the medial condyle remains almost in the same position. There is anteroposterior and rotational laxity about this neutral path. The hypothesis of this study was that this motion could be provided in a total knee arthroplasty by specially shaped bearing surfaces, in particular, where the lateral femoral condyle converged inward around the periphery from extension to full flexion. Tibial surfaces were generated with such femoral condyles moving through a neutral motion path. Displacing the femur in anteroposterior or rotation about the neutral position at a given angle of flexion intersected a volume of the tibial surface in both directions, showing that the femur was "at the bottom of the tibial dish." The volumes were comparable with those for conventional condylar replacement total knee arthroplasties. These data supported the general concept of converging femoral condyles, which may be present in the natural knee itself to guide motion.