We have developed a new type of knee prosthesis which is capable to make 180° knee flexion, and have designated it as Complete flexion knee (CFK). Since the kinematics and kinetics of knee prosthesis vary depending not only on its articulating surface shapes but also on the stiffness of soft tissues, its performance should be assessed under various kinds of lower limb activities.

The objective of this study is to perform simulation analysis of various lower limb activities to evaluate the performance of CFK using the 2D and the 3D mathematical models. Kinematic analyses using X-ray picture or stress analyses using FEM are extensive however, kinematic analyses can not introduce stresses and FEM can not introduce kinematics. Mathematical model analyses can introduce vital information about kinematics and kinetics at the same time.

First, we carried out an in-vitro experiment using cadaver knee under the condition of passive knee flexion-extension. After that, we performed a simulation using the same parameter variables as the in-vitro experiment in order to assess the validity of our 2D and 3D models by comparing the results about the joint contact forces and kinematics with those from the experiment.

In the in-vitro experiment, the femoral bone of a cadaver knee was fixed on a jig. In order to secure the tibiofemoral contact, each muscle was pulled with constant force respectively. Then the tibia was carried through from 40° to 140° of knee flexion. The contact forces between the femur and the tibia were measured by a load sensor. During the process, fluoroscopic images were taken, and then 3D positions/orientations of the tibia relative to the femur were introduced from the images using the pattern matching method.

Our 2D and 3D models of total knee arthroplastic joint included the tibio-femoral and patello-femoral compartments, incorporating major muscles, patella tendon and primary ligaments. The patella tendon and primary ligaments were represented with non-linear springs, whose mechanical properties were determined from the literature. In our 2D model, “thigh and calf” contact was taken into account at deep knee flexion.

Using our 3D model, the simulation was performed up to 100° of knee flexion. After that we had to alternate the model from the 3D to the 2D because the patella stacked into the femoral intercondylar, the thigh-calf contact occurred and the 3D model did not introduce the converged solution.

Over all, both the experimental and simulation results were in good agreement with each other. The results from the simulation showed that the contact points were located unusually anteriorly. The post-cam contact occurred at 44° of knee flexion, indicating that the tibia was strongly pulled to the posterior. As for the contact resultant force, large differences between simulation and experiment were found. This may be because the soft tissues of the cadaver were not intact, while we determined their properties from the literature in the simulation.