One third of the world population have a life style to sit sedentary on a floor. Thus far the patients who had undergone TKA surgery loose deep flexion of the knee and various designs of artificial knee joint capable of deep knee flexion have been proposed. Among them, Bi-surface knee prosthesis (Kyocera Inc., Japan) is of special interest because of its unique design with a ball-and-socket joint. Although some patients attained a sedentary sitting with this prosthesis, the X-ray studies revealed that the femoral condyles and tibial insert tended to separate at about 150° of knee flexion, indicating a risk of subluxation when standing up.

Thus we have developed CFK (Complete Flexion Knee, Japan Medical Material Co., Japan) by further improving Bi-surface knee to enable the patient to make knee flexion as much as 180°. Our CFK has a ball-and-socket joint and whose socket part is jutted to form a tibial post. Since the ball and the cam become into a single sphere and the ball-socket and post-cam joints form a spherical bearing, CFK can provide high stability and mobility at the same time.

Besides its kinematic performance, CFK has to be assessed with its strength and durability. Since the durability of an artificial knee joint is attributed to wear of the polyethylene insert, it is essential to focus on determining the stress on it. Although the FEM analyses have been most extensive for stress analysis, whose results greatly depend upon the way how to create the meshes. The stress values introduced from the FEM are the Von Misses stresses; while wear is mainly attributed to the shear stresses. For these reasons, we employed a photoelasticity for determining the magnitude and distributions of stresses on the insert.

The models of Bi-surface, CFK and a conventional posterior stabilizer knee, Scorpio NRG (Stryker Co., USA) were used for the experiments. Epoxy resins (Araldite AER 250, 2400, Ciba Geigy Co., Japan) were selected to fabricate the tibial insert models. Special equipment was used to apply 2 kg force on the model by setting knee flexion angle at 0°, 30°, 60°, 90° and 120° respectively. After that the stressed model was sliced along the anterio-posterior direction and photoelastic fringes in each slice were observed. The results demonstrated that while knee angle was smaller than 90°, shear stress on the lateral slice became higher in the order, NRG, CFK, and Bi-surface, indicating NRG has high conformity in the condylar surface. After knee angle bacame larger than 90°, shear stress on the mid-posterior slice became higher in the order, CFK, Bi-surface and NRG. We may conclude that CFK has optimal configuration at deep knee flexion not only for kinematic but also for load bearing viewpoints.