Kinematics of the knee at high flexion angles: an in vitro investigation

Restoration of knee function after total knee, meniscus, or cruciate ligament surgery requires an understanding of knee behavior throughout the entire range of knee motion. However, little data are available regarding knee kinematics and kinetics at flexion angles greater than 120 degrees (high flexion). In this study, 13 cadaveric human knee specimens were tested using an in vitro robotic experimental setup. Tibial anteroposterior translation and internal-external rotation were measured along the passive path and under simulated muscle loading from full extension to 150 degrees of flexion. Anterior tibial translation was observed in the unloaded passive path throughout, with a peak of 31.2+/-13.2 mm at 150 degrees. Internal tibial rotation increased with flexion to 150 degrees on the passive path to a maximum of 11.1+/-6.7 degrees. The simulated muscle loads affected tibial translation and rotation between full extension and 120 degrees of knee flexion. Interestingly, at high flexion, the application of muscle loads had little effect on tibial translation and rotation when compared to values at 120 degrees. The kinematic behavior of the knee at 150 degrees was markedly different from that measured at other flexion angles. Muscle loads appear to play a minimal role in influencing tibial translation and rotation at maximal flexion. The results imply that the knee is highly constrained at high flexion, which could be due in part to compression of the posterior soft tissues (posterior capsule, menisci, muscle, fat, and skin) between the tibia and the femur.