Animation 7: Model of the replaced knee in extension, 60° and 120° flexion, with a circular femoral component, a flat tibial component and an interposed fully conforming meniscal bearing

The mathematical model of passive flexion of the replaced knee described shown in Animation 7 explains the observations made on the unloaded prosthetic joint. At each flexion angle, the anteroposterior position of the femur on the tibia was determined from the condition of zero force in each ligament, a condition which required a roll-back of the model femur on the tibia of 7.5 mm over 90° of flexion. At each flexion angle, if the femur is placed anterior to this neutral position, the ACL (and MCL) are stretched and the PCL and LCL slackened, requiring the application of external force. If the femur is placed posterior to the neutral position, the PCL and LCL are stretched and the ACL and MCL are slackened. Thus, roll-back during passive flexion of the replaced joint is required to ensure that passive structures of the joint, articular surfaces as well as ligaments, are not loaded.