Abstract
Introduction
Current clinical practice in total knee arthroplasty (TKA) is largely based on metal on polyethylene bearing couples. A potential adverse effect of the stiff metal femoral component is stress shielding, leading to loss of bone stock, periprosthetic bone fractures and eventually aseptic loosening of the component. The use of a polymer femoral component may address this problem. However, a more flexible material may also have consequences for the fixation of the femoral component. Concerns are raised about its expected potential to introduce local stress peaks on the interface.
The objective of this study was to analyze the effect of using a polyether-etherketone (PEEK-Optima®) femoral component on the cement-implant interface. We analyzed the interface stress distribution occurring during normal gait, and compared this to results of a standard CoCr component.
Materials and methods
An FEA model was created, consisting of a femoral component cemented onto a femur, and a polyethylene tibial component. A standard loading regime was applied mimicking an adapted gait cycle, according to ISO14243-1. The implant-cement interface was modelled as a zero-thickness layer connecting the implant to the cement layer. Femoral flexion/extension was prescribed for the femur in a displacement controlled manner, while the joint loads were applied to pivoting nodes attached to the tibial construct, consistent with the ISO standard. Implant-cement interface properties were adopted from a previous study on CoCr interface debonding[1].
Results
The highest stresses were found during the heel strike phase of the walking cycle (Figure 1). Both for the PEEK-Optima® (A) and CoCr implant (B), the highest stresses were found near the chamfers of the posterior condyles, which is the location where tibiofemoral contact occurred. Also around the pegs, small stress intensities were found. Surprisingly, the CoCr implant produced higher peak Von Mises stresses than the PEEK-Optima® implant.
Figure 1. Von Mises stress distribution at the implant-cement interface in case of a PEEK-Optima® (A) and a CoCr (B) femoral component.
Discussion
In contrast with our initial assumption, the current results show that the cement-implant interface stresses with a PEEK-Optima® component were lower and more focal than with a CoCr component. However, the significance of this difference is yet unknown, as additional data on the strength of the implant-cement interface strength of PEEK-Optima® components is needed for the prediction of implant loosening. We furthermore intend to expand the current simulations with more demanding tasks, such as stair climbing and rising from a chair, as such high flexion tasks may be more detrimental to the implant-cement interface.
In conclusion, this study warrants further investigation of the use of PEEK-Optima® as a replacement for CoCr in femoral TKA components.
