One of the main concerns about the currently available simulators is that the TKA is driven in a “passive way” for assessment. For the simulators for the wear assessment, the tibio-femoral relative motion is automatically made by using the knee kinematics and loading profile of a normal gait. As for the simulators for the kinematics and kinetics assessment of TKA, also the predicted loading profiles introduced from the theoretical model are applied as the input data to drive the simulator. It should be noted that the human joints are driven by the muscles' forces and external loads, and their kinematics and kinetics are the “outcome”. This being so, the knee simulator should be driven by the muscles' forces and upon these conditions the TKA performance is to be assessed. Some other concerns about the current simulations are as follows. The effects of hip joint motion are not taken into account. The upper body weight is applied along a vertical rod in such a way as a crank-slider. Furthermore, few simulators are capable of knee flexion greater than about 110°.

Considering the above, we have developed a novel knee simulator which makes it possible to reproduce the active and natural knee motion to assess kinematics and kinetics of TKA. In the experiment, the custom-designed PS type TKA was attached and the simulator was operated so as to reproduce the sit-to-stand features, thereby introducing the tibio-femoral loading profiles during the motion.

Figure 1 illustrates the external appearances of the simulator and a close view of the knee joint compartment. Since our simulator is composed of a multiple inverted pendulum, the knee part bears the upper body weight in a physiological way. The holder bracket is set to prevent the simulator from collapsing for security. The dimension and weight of each link were set as close as those of each segment of a normal male subject. Our simulator is driven by the wire pull mechanism which substitutes the human musculo-skeletal system of lower limb. Figure 2 shows close views of tibial tray with load cells. In Fig.2a, cell FR, FC and FL are to measure the tangential components of tibio-femoral contact force, i.e., the Anterio-Posterior force (AP force). The rest five cells are to measure the normal components of tibio-femoral contact force (normal force). As shown in Fig.2c, the tibial insert of TKA is mounted on the lid of the tibial tray box.

In the experiment, a PS type TKA whose maximum flexion angle of 150° was attached to the simulator for evaluation. The simulator was operated so as to reproduce the sit-to-stand features and the data concerning about the AP force, F_t, and the normal force, F_n were recorded.

Figure 3 shows the variations of knee flexion angles and knee contact forces respectively as a function of normalized time. Our knee simulator may have a potential for substituting the in vivo measurement.

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.

We have developed a novel knee simulator that reproduces the active knee motion to evaluate kinematics and joint reaction forces of TKA.

There have been developed many kinds of knee simulators; Most of them are to predict TKA component wear and the others are to evaluate the kinematics and/or kinetics of TKA. The most simulators have been operated using the data of the loading and kinematics profile of the knee obtained from normal gait. Here a problem is that such variables as joint force and kinematics are the outcome caused by the application of muscles' and external forces. If so, a simulator should be operated by the muscles' and external forces so as to duplicate the in vivo condition. Other disadvantages for the current knee simulators are; a knee joint motion is made passively, the effects of the hip joint motion are not taken into account, and the maximum flexion angle is usually limited at about 100°.

Considering the above, we have developed a knee simulator with the following advantages and innovative features. First, the simulator is driven by the muscles' forces and an active knee motion is made with bearing the upper body weight. As a result, the knee shows a 3D kinematics and generates the tibio-femoral contact forces. Under this condition, the TKA performance is to be assessed. Secondly, a hip joint mechanism is also incorporated into the simulator. The lower limb motion is achieved by the synergistic function between the hip and knee joints. Under this condition, a natural knee motion is to be reproduced. Thirdly, the simulator can make complete deep knee flexion up to 180°. Thus not only the conventional TKA but also a new TKA for high flexion can be attached to it for the evaluation.

Figure 1 shows the structure of the simulator, in which both the hip and knee joints are moved in a synergistic fashion by the pull forces of four wires. The four wires are pulled by the four servomotors respectively and reproduce the functions of the mono-articular muscles ((1), (3)) and the bi-articular muscles ((2), (4)) through the multiple pulley system. It should be noted that weight A and B are not heavy enough for the inverted double pendulum to stand up straight. They are applied as counter weights so that each segment duplicate the each segmental weight of the human lower limb. Figure 2 shows a sequential representation of stand to sit features: (a) at standing, (b) at high flexion, and (c) at deep flexion. At a state of 130° knee flexion between (b) and (c), hamstrings wire (4) becomes shortest and then exhibits an eccentric contraction, thereby attaining deep flexion.

Our knee simulator can be a useful tool for the evaluation of TKA performance and may potentially substitute the in vivo experiments.