The effect of design variables of condylar total knees on the joint forcesin step climbing based on a computer model

The ability to climb a steep step or rise from a low chair after total knee replacement may be enhanced if the required force in the quadriceps muscle is reduced. This can potentially be achieved if the total knee produces a large lever arm measured from the femoral-tibial contact point to the patel lar ligament. A reduced quadriceps force would also reduce the patello-femo ral force and the femoral-tibial contact force. The contact point location is likely to be a function of the geometry of the femoral and tibial compon ents in the sagittal plane, including the relative distal and posterior rad ii of the femoral profile, the location of the bottom-of-the-dish of the ti bial surface, the radius of the tibial surface, and the presence or absence of the posterior cruciate ligament. A three-dimensional model of the knee was developed including the quadriceps and various ligaments. In the study, the motion was confined to flexion extension and displacement in the sagit tal plane. The quadriceps was assumed to be the only muscle acting. A stand ard software package (Pro/Mechanica) was used for the analysis. For a femor al component with a smaller distal radius, there was 12% reduction in the q uadriceps muscle force and up to 11% reduction in the patello-femoral force from about 100 up to 60 degrees flexion. However, apart from that, there m ere less than 10% differences in all the forces as a function of all of the design variables studied. This was attributed to the relatively small chan ges in the lever arm of the patella tendon, since the tendon moves in an an terior-posterior direction along with the femur. An additional factor expla ining the results was the change in the anterior-posterior contact point as controlled by the forces in the patella tendon and in the soft tissues. Th e results imply that for a standard condylar replacement knee, the muscle a nd contact forces are not greatly affected by the geometrical design variab les. (C) 2001 Elsevier Science Ltd. All rights reserved.