The interface condition between the prosthesis and the bone tissue must play important roles during dynamic loading transfer through the knee joint. In this study, the three- dimensional impact finite element (FE) simulations were performed to investigate the impact stress propagation.

The FE models of a totally replaced knee joint were constructed with the high shape fidelity. The models included the cortical and cancellous bone, articular cartilage, bone marrow, and the artificial femoral and tibial components. The artificial components were set to the femoral and tibial contact area. The FE meshes had 7251 nodal points and 5547 hexahedral elements (Figure 1).

The interfacial condition between the artificial component had two kind of contact situations, bonding situation and no-bonding ones. In the bonding situation, the interface between the artificial components and the cancellous bone had fully fixations. The no-bonding allowed the tie-breaking of each other although the interface had the high coefficient of friction. The three kind of the impact loading (1, 5, and 10kgW) were applied from the proximal femur to the distal side of tibia.

In the FE simulations, the impact stress propagated to the tibia through the TKR joint components during several milliseconds. On the interfacial surface at the cancellous side of the proximal tibia, the difference in the stress distribution was observed according to the contact situation of the TKR component (Figure 2). The fully fixation (tied to each other) model showed the high compressive stress on the interface. On the other hand, in the no-bonding model, the compressive stress distributed discontinuously and the high compressive stress was observed only in the hole area and edge of the tibial component during the impact loading. In previous research, the cancellous bone had important roles for the load transmission inside the joint especially under the impact loading condition. However, this study indicated that the stress shielding was caused by the imperfect bonding at the interface. More consideration of the interface situation between the bone and component is required to keep stability for impact loading.

In biomechanical finite element (FE) simulations, the mechanical nonlinear behaviors must be considered frequently and depend on several properties, such as structural, material, and contact situation. The hexahedral meshes were widely applied to the modeling with the mechanical nonlinearities and can decrease the computer resources and improve the accuracy of the simulations. However, it is quite difficult to construct the three-dimensional hexahedral meshes of complicated shapes such as human joints.

This study proposes the development of the semi-automatic meshing technique which consists of only hexahedral elements, thereby reducing the number of elements without spoiling the shape fidelities. In order to create the three-dimensional models of the tibial plateau and femoral condyle, the simply-shaped ‘seed’ models consisting of only hexahedral elements were prepared. The seed meshes were located into the surface of the target bone and expanded until they fitted the target surface. When the seed meshes expanded and intersected with the target surface, the contact condition was detected and the seed surface slide on the target one. These procedures are repeated until the seed meshes filled up inside the complicated target surface. Figure 1 shows the transformed and filled seed meshes inside the surface. The boundary between the cortical and cancellous bone was kept clearly. In the finite element meshes, there was no concentration of elements, and each hexahedral element had the good aspect ratio.

Figure 2 shows the impact FE simulation of the TKR joint model, which was constructed by hexahedral elements using this technique. The impact stress propagated cleary through the TKR joint. The number of elements were reduced by a sixth, compared with that of the tetrahedral ones. Because the number of nodes and elements of the model can be defined beforehand, it is easy to predict the scale of the final model. Therefore, this technique is very effective in creating the huge skeltal models which build the complicated biomaterial shapes by the hexahedral elements.