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Knee joint forces are determined either through mathematical modeling or by in vivo measurement using an instrumented knee prosthesis. In the model studies, significant differences exist among the results and the data for high knee flexion are few. The in vivo measurement data are available for small to moderate flexion but not for high flexion yet. We created a 2D mathematical model of the lower limb incorporating several new features such as a patello-femoral mechanism, a thigh-calf contact at high knee flexion and co-contracting muscles' force ratio, then used it to determine knee joint forces arising from high knee flexions in four kneeling conditions: rising with legs in parallel, with one foot forward, with or without arm use. With arms used, the maximum values of knee joint force decreased to about 60% of those with arms not used. When rising with one foot forward, if arms are not used, the forward leg sustains a force as large as that sustained when rising with legs parallel. By comparing our modeling methodology and results with those in the literature, we determined some of the causes of the differences in the results, thereby providing creditable data especially during high flexion of the knee.

The objective of this study was to set forth guidelines for designing such an artificial knee joint that can make deep flexion. To this respect, we performed 2D model simulation for deep squatting in which the conventional posterior stabilized knee prosthesis was fitted. In the simulation we introduced a newly defined slip ratio in order to describe an over slipping phenomenon; the slipping and the rolling directions were mutually reversing. Then, the results demonstrated that the slip ratio was over 1 at high and deep flexion, indicating that the femoral rollback did not occur even though the post and the cam contacted. The relation also indicated that the conventional post-cam mechanism would not work appropriately at high/deep flexion. To overcome the above, and to promote femoral rollback even at high/deep flexion, we proposed a new design of the post-cam mechanism, which we called as a ball-ditch mechanism.

The objective of this study is to analyze kinetics and kinematics of artificial knee joint at deep flexion, using a 2D geometric model. Our knee model was composed of the tibio-femoral and patello-femoral joints, including the patella tendon and the quadriceps muscles. Since the model was 2D, it neglected the rotation and the varus/valgus motion. In the model, such assumption was made that articulation surfaces were rigid and a point contact was made. Also it was assumed that the force and moment equilibrium conditions were always satisfied. The motion to be studied was decided as the motion from standing to deep squatting with heel-rising. From the simulation we found such results that the femur did not make a rollback and contact force at post-cam became pretty large in high and deep flexion, indicating the post-cam mechanism did not function well. Also patello-femoral contact forces reached larger than tibio-femoral contact forces. Our results demonstrated that how to design the post-cam and patello-femoral joint should be critical issues for developing a new prosthesis which can make deep flexion.

Many kinds of knee simulators have been developed. There are several concerns about the current simulators. First, the simulators for wear testing are driven using knee kinematics and loading profile data obtained from a normal gait. However, knee kinematics and knee joint forces are "the outcome" caused by muscular forces and external loads applied to the knee joint. As for the simulators for the kinetics and kinematics measurement of total knee arthroplasty (TKA), the knee is flexed in a "passive way" by applying constant muscular tension forces. This being so, simulators should be driven by muscular forces and external loads, i.e. in an "active way". 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 physiological way. Finally, few simulators are capable of knee flexion greater than about 100°. To this end, we have developed a novel knee simulator that has the following advantages and innovative features. First, it is driven directly by muscles' tension forces, thereby generating active flexion. Secondly, a hip joint is incorporated and the lower limb motion is achieved in a synergistic way between the hip and knee joints. Thirdly, it is capable to make a complete deep knee flexion.

The primary objective of this study is to distinguish between mobile bearing and fixed bearing posterior stabilized knee prostheses in the mechanics performance using the finite element simulation. Quantifying the relative mechanics attributes and survivorship between the mobile bearing and the fixed bearing prosthesis remains in investigation among researchers. In the present study, 3-dimensional computational model of a clinically used mobile bearing PS type knee prosthesis was utilized to develop a finite element and dynamic simulation model. Combination of displacement and force driven knee motion was adapted to simulate a flexion motion from 0° to 135° with neutral, 10°, and 20° internal tibial rotation to represent deep knee bending. Introduction of the secondary moving articulation in the mobile bearing knee prosthesis has been found to maintain relatively low shear stress during deep knee motion with tibial rotation.