Effects of total knee replacement design on femoral-tibial contact conditions.

Ten fresh knee specimens with prosthetic components inserted were tested in a loading rig. Compressive and shear force were applied to the femur with the tibia held fixed. The location of the femoral-tibial contact points was measured. The contact reaction forces, the shear forces, and the rocking moments transmitted to the tibial component were calculated. The variations in the test conditions were: high and low compressive force, flexion angles of 0 degree, 45 degrees, and 90 degrees, three curvatures of tibial plastic inserts, and the posterior cruciate retained or resected. When the posterior cruciate was retained, the contact points were close to the center of the component; for cruciate resection, the contacts were close to the anterior of the component. The shear forces and rocking moments were higher for cruciate resection, but the contact reaction forces were lower. There is a wide variety of knee prosthesis designs, but the amount of inherent stability between the femoral and tibial surfaces, and whether the posterior cruciate ligament is retained or sacrificed, are two of the most important design variables. This study shows that cruciate resection increases the shear forces and the rocking moments to the tibial components and that additional fixation means may be necessary to compensate. On the other hand, cruciate retention with low conformity gives higher contact forces, which may lead to more wear in the long term. Cruciate sacrificing designs with intercondylar guiding surfaces are a separate category of design and were not considered in this study.