KINEMATICS OF THE KNEE AND TOTAL KNEE PROSTHESIS DESIGN

Abstract

For many years, it has been taught that the human knee is a ‘hinge’ joint and that the motion of the knee is controlled by a ‘four-bar link’. This classic view of the motions of the knee suggests that there is a prescribed path for the knee as it proceeds from extension to flexion and flexion to extension. This prescribed motion includes ‘rollback’, a term used for the progressive posterior displacement of the femur on the tibia as the knee moves from extension to flexion,

Most of the total knee prostheses available today have been designed to permit the movements that are required by this model of knee motion. The design features necessary to permit this motion are a lack of constraint between the tibial and femoral components, and a ‘J’ curve of the posterior part of the femoral component such that the radius of curvature is smaller on the posterior portion of the component than on the distal part.

Studies of the anatomy of the knee date back to the 1800s, before radiological studies were possible. Radiological evidence does not support the four-bar link and rollback theories or indicate that a ‘J’ curve is necessary. Rather, radiographs suggest that the knee is more of a ball-in-socket joint on the medial side with little or no rollback in normal function. Three-dimensional studies of the moving human knee both in vitro and in vivo also demonstrated that the knee joint moves as a ball-in-socket joint on the medial side, and that the lateral side displaces posteriorly or anteriorly as necessary to accommodate the rotational position of the tibia relative to the femur.

These kinematic findings have led to the design of a pros-thesis that mimics the normal knee. The femoral prosthesis has a single radius of curvature to each condyle both in the sagittal and coronal planes. The mating tibial component has an exactly conforming geometry on the medial side leading to ball-in-socket type of kinematics. The lateral side of the tibial component allows anterior or posterior displacement of the femur, mining the normal changes that take place with internal and external rotation.

Initial clinical results total knee arthroplasty procedures performed with this prosthesis are just passing the three-year follow-up interval. There have been no reports of catastrophic problems, and surgeons have been pleased with the stability, the rapidity with which function is regained, and the excellent range of motion following arthroplasty. Patients who have a more traditional total knee arthroplasty in one knee and the medial pivot prosthesis in the other prefer the medial pivot because of the feeling of stability.