Effects of patient and surgical alignment variables on kinematics in TKR simulation under force-control

Simulation of total knee replacement (TKR) is typically achieved by integrating sliding/rolling motions and loads between the implant's articulating surfaces during an activity cycle such as walking. Clinically, however, important variations in implant alignment and duty occur due to variability in patient anatomy/arthritic deformity, compounded by choices or errors in surgical installation. This study investigated the effects of the activity cycle severity, frontal plane alignment, relative femoral/tibial component rotational position, and the tightness of the posterior cruciate ligament (PCL). Seven different (four fixed-bearing and three mobile-bearing) cruciate-retaining TKRs with different inherent constraints were tested on a force-control knee simulator. As well as the ISO standard wave forms for walking, an Enhanced Duty Cycle was used. The resulting anterior-posterior displacements and axial rotations were increased with the Enhanced Duty Cycle. Changing the line of action of the compressive force in the frontal plane (varus-valgus over/under-correction) did not appreciably change the kinematics. Rotating the tibial component shifted the rotational curves in the same direction as the misalignment. The PCL tightness produced the most noticeable effect on kinematics; a tight PCL reduced both displacements and rotations, and a loose PCL did the opposite.