Jan Victor

The High-Performance Knee

Although total knee arthroplasty is effective in relieving knee pain and improving ambulatory function, the kinematics after knee replacement are quite different from those of the normal knee. During flexion of the normal knee, the lateral femoral condyle moves posteriorly, causing femoral external rotation. After knee replacement “paradoxical” motion typically occurs. The femur is displaced posteriorly during extension and moves anteriorly as the knee flexes. The cam-and-post mechanism of a posterior cruciate-substituting knee design limits anterior femoral translation so that the kinematics are less abnormal than with a posterior cruciate-retaining design. Additional modifications in the conformity of each tibial plateau and the cam-and-post geometry can further guide roll-back of the lateral more than the medial femoral condyle, causing femoral external rotation during knee flexion. Guided motion designs that reproduce more normal knee kinematics may result in improved range of motion and knee function following total knee arthroplasty.

An in-vitro study of human knee kinematics: natural vs. replaced joint

Cadaver specimens mounted on a knee simulator combined with motion tracking devices have been used for the kinematics study of the human knee. The results of these studies have shown that the classical four bar linkage model of the knee is not adequate. This obviously has repercussions for the design of knee implants. They should at least permit normal kinematic behavior to prevent abnormal loads on the soft tissues and on other joints of the leg. Studies where implant kinematics can be compared to kinematics of the natural knee of the same specimen are very rare, though. In this project, the kinematics of six fresh frozen human cadaver knees in the natural state and after replacement of the knee joint with a bi-cruciate stabilizing implant were evaluated using a knee simulator and stereophotogrammetry. Before testing, frames with reflective markers were fixed to the femur and tibia. Then, a CT scan was made and the images were processed to identify anatomical landmarks and their position with respect to the reflective markers. Thus, an anatomically relevant coordinate system could be defined for each bone. During a squat, the motion of the markers was continuously measured. Several load conditions were considered for each specimen. The data obtained enabled us to calculate the motion of the femur and tibia with respect to each other and present that in clinically relevant terms. After testing, a second CT scan was made to check the position of the implant with respect to the original articular surfaces and also for a control of the position of the marker frames. The results show that kinematics of the natural knee joint are more variable than expected. The knee replacement, on the other hand, produces a consistent kinematics pattern in all the specimens.