Kinematics of the knee after prosthetic replacements

A method was developed for describing the flexion-extension motion of the knee joint in a sagittal plane, by employing ciné radiography and then plotting the loci of three points on the femur relative to the tibia. Normal knees displayed similar patterns, the curves were smooth, and flexion-extension curves superimposed. A standard knee outline with standard curves was defined, with which to compare the abnormal and prosthetic knees. Arthritic knee patterns differed significantly from the normals. The curves were irregular and the flexion-extension curves were different. Instability, joint surface irregularity and muscle spasm were thought to be the main causes. The curves for the MacIntosh tibial plateaus were abnormal in a similar way to the arthritics. This may have been due to residual instability and to the replacement of only one bearing surface. The condylar replacement prostheses (the uni-condylar, duo-condylar and Geometric) tended to produce close-to-normal knee patterns, probably due to restoration of a relatively normal geometry, and to replacement of both bearing surfaces. The knees with fixed hinge prostheses (Walldius, Shiers and Guépar) showed abnormalities of motion and of the relative postion of the femur on the tibia. These features would be undesirable for the correct functioning of soft tissues around the knee. The stabilo-condylar hinge, which did not require much bone resection, gave a motion comparable to the condylar replacements.