The Total Knee Replacement (TKR) has been used as the effective treatment for osteoarthritis of the knee. The load of the knee joint is generally applied at the heel strike as the impact loading. In the elderly who had muscle weakness or weakening eyesight, it can be anticipated that more excessive loads are often added to the knees when they stumble or trip over. And the varus / valgus alignments of the femur and tibia differs among patients. However, most finite element analyses considering the effect of the alignments have rarely been performed.

In this study, the mounting angle of the tibia component in the TKR knee was changed, and the effect of the change on the load transfer was assess using finite element analyses. Based on the CT images, the three-dimensional finite element models of the natural knee joint and TKR knee joint were created [Fig. 1]. Each model was constructed from hexahedoral elements with the isotropic material. The numbers of nodes and elements were 10,666 and 8,677 respectively. Under normal alignment, 5 degrees of varus, and 5 degrees of valgus knee, the static analyses at an applied load of 1000N and impact analyses at an applied load of 50 kg were performed. LS-DYNA ver760 software was used for the analyses.

The finite element analyses results showed that under the static loading, no stress shielding was observed in the tibial cancellous bone of the intact knee or TKR knee, and the maximum compressive stress was 1.5 MPa. While under the impact loading, the compressive stress generated inside of the cancellous bone was three times higher in the TKR knee joint than that in the intact knee, and the load transfer time was reduced. This result reveals that the cancellous bone have load bearing function especially in the impact condition.

When the impact load was applied to the varus and valgus TKR knee, the stress shielding was observed in the tibial cancellous bone, especially in the varus condition. In a case where the tibia component was mounted by tilting it at −5 to 5 degrees depending on the varus/valgus of the knee, the stress shielding was alleviated; the distribution of load was almost the same as that of the TKR knee joint model under the normal alignment [Fig.2]. The effect of a slight difference in the alignment on the stress distribution is expected to be a contributor to determine artificial knee joint shape, loading condition, and other design factors in developing revision arthroplasty or custom-made implant.

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.

The contact condition in the human knee joint must play important roles especially in dynamic loading situations where the loads transfer in the knee. In this study, the impact stress propagations through the inside of the knee joint were simulated using the three-dimensional finite element analysis (FEA). And the differences in the stress distribution were investigated between the intact knee and the total replacement condition.

The finite element (FE) models of an intact human knee joint and a total replaced knee joint were constructed with high shape fidelity. The intact model included the cortical bone, cancellous bone, articular cartilage, bone marrow, and meniscus. And the total replacement knee FE model, which is consisted of the artificial femoral and tibial components were also prepared to compare the impact propagations with the intact model (Figure 1). Impact load were applied to the proximal femur of the FE models under the same conditions as those of the weight-drop experiments with the knee joint specimens.

The FEA results showed that the impact stress propagated to the tibia through the knee joint for several milliseconds. The values and the time dependent change of the compressive strain on the cortical surface had good agreement with the experimental results. The compressive stress mainly propageted at the medial side, with 1.0 MPa at 1.2 milliseconds.

Especially, the impact stress propagated not only in the cortical surface area which has hard material property but also in the soft cancellous bone region inside the knee joint. The mass density of the cancellous bone has similar to that of the cortical bone, and thus the role of the load bearing in the cancellous area must be much increasing under the impact condition.

In the total replacement model, concentration of the impact compressive stress was observed with 2.8 MPa at the tibial region, while not under the normal intact conditions (Figure 2). Since the total replacement model is formed of different materials and the impact propagations were inhibited by the interfacial condition, such as sliding or debonding, it is considered that the contact condition between such materials have a great effect on the stress propagation.