Abstract
There are several concerns about the current simulators for TKA. First, the knee is flexed in a “passive way” under the condition of applying constant muscular tension forces. Second, the effects of hip joint motion are not taken into account. Thirdly, the external load for example, upper body weight is not applied in a natural way. Finally, few simulators are capable of knee flexion greater than about 100°.
To this end, we have developed a novel knee simulator system that reproduces the active and natural knee motion to evaluate kinematics and joint forces of TKA. Our simulator system has the following advantages and innovative features. First, it is driven directly by muscles' tension forces, and the knee is capable of active flexion. Secondly, a hip joint is incorporated into it and the lower limb motion is achieved in a synergistic way between the hip and knee joints. Thirdly, it is capable of complete deep knee flexion up to 180°.
Figure 1 shows the structure of the system. Both the hip and knee joints are moved by the tension forces of four wires that simulate the functions of the mono-articular muscles ((1), (3)) and the bi-articular muscles ((2), (4)) by means of a multiple pulley system (Fig 2). The femoral and tibial components of TKA are secured in the distal end of the upper link (thigh) and the proximal end of the lower link (shank) respectively. The ankle assembly has three sets of rotary bearings whose axes intersect at a fixed point, the center of the ankle, allowing spherical movement of the tibia about the ankle center. Springs were stretched around the ankle center to substitute the muscles around the ankle. Weights I and II are counterweights so as to duplicate the weights of the human upper body, thigh and shank respectively. The wires are pulled to produce the hip and knee motions. The linear bearings running along vertical rods also prevent the system from collapsing.
In the experiment, a custom-designed posterior stabilized type TKA was attached to the simulator system for evaluation. The system was operated so as to reproduce the sit-to-stand features in a quasi-static manner in order to study the kinematics of TKA. Beyond 130°, the knee proceeded to flex passively because of upper body weight. Conspicuous internal/external rotation or valgus/varus motion of the tibia relative to the femur was not observed as the knee flexed. When our simulator system was driven in a quasi-static manner, it was able to measure the kinematics of TKA however, when the system was driven in a dynamic manner, it oscillated because the springs around the ankle were not stiff enough to hold the inverted pendulum-like system upright and the ratios of the tension force exerted by the four wires simulating muscles could not be determined appropriately.
