Abstract
Introduction
Conventional implant designs in total knee arthroplasty (TKA) are based on metal on UHMWPE bearing couples. Although this procedure is quite successful, early loosening is still a matter of concern. One of the causes for early failure is stress shielding, leading to loss of bone stock, periprosthetic bone fractures and eventually aseptic loosening of the component. The introduction of a polyetheretherketone (PEEK) on UHMWPE bearing couple could address this problem. With mechanical properties more similar to distal (cortical) bone it could allow stresses to be distributed more naturally in the distal femur. A potential adverse effect, however, is that the femoral component and the underlying cement mantle may be at risk of fracturing. Therefore, we analyzed the effect of a PEEK-Optima® femoral component on stress shielding and the integrity of the component and cement mantle, compared to a conventional Cobalt-Chromium (CoCr) alloy implant.
Methods
We created a Finite Element (FE) model of a reconstructed knee in gait, based on the ISO-14243-1 standard. The model consisted of an existing cemented cruciate retaining TKA design implanted on a distal femur, and a tibial load applicator, which together with the bone cement layer and the tibial implant is referred to as the tibial construct. The knee flexion angle was controlled by the femoral construct, consisting of the femoral implant, the bone cement and the distal femur. The tibial construct was loaded with an axial force, anterior-posterior (AP) force and a rotational torque, representing the ground reaction force, soft tissue constraints and internal/external rotation of the tibia, respectively. The integrity of the femoral component and cement mantle were expressed as a percentage of their yield stress. Stress shielding in the periprosthetic femur was evaluated by the strain energy (density) in the bone and compared to a model replicating an intact knee joint.
Results
Considering implant durability, the CoCr and PEEK-Optima® femoral components performed equally well, with peak stresses reaching only 12–18 percent of their respective yield stresses (Figure 1(A)). The bone cement experienced higher loads in the reconstruction with the PEEK-Optima® implant, but the principal stresses were within a safe range, with a maximum of 20 percent of the ultimate compressive load (Figure 1(B)). As anticipated, the more compliant polymer implant resulted in a strain energy magnitude and distribution similar to that of an intact knee (Figure 2,3), which could prevent the loss of bone stock on the longer term.
Discussion
Our simulations indicate that the femoral implant and cement mantle are not at risk of failure during gait. Moreover, the hypothesis that stress shielding can be reduced by a polymer implant is corroborated by this model. ISO loads can be considered an underestimation and so we intend to expand the model with more comprehensive loading regimes, based on musculoskeletal simulations of gait as well as more arduous physical activities. We plan to include activities like squatting or stair ascending as they are likely to be more detrimental to the implant performance.
