Abstract
Introduction
PEEK-OPTIMA™ has been considered as an alternative to cobalt chrome in the femoral component of total knee replacements. Wear simulation studies of both the tibiofemoral and patellofemoral joints carried out to date have shown an equivalent wear rate of UHMWPE tibial and patella components against PEEK and cobalt chrome (CoCr) femoral components implanted under optimal alignment conditions. In this study, fundamental pin-on-plate studies have been carried out to investigate the wear of UHMWPE-on-PEEK under a wider range of contact pressure and cross-shear conditions.
Methods
The study was carried out in a 6 station multi-axial pin-on-plate reciprocating rig. UHMPWE pins (conventional, non- sterile) were articulated against PEEK-OPTIMA™ plates, initial Ra ∼0.02µm. The lubricant used was 25% bovine serum (17g/l) supplemented with 0.03% sodium azide. The contact pressure and cross-shear ratio conditions were selected to replicate those in total knee replacements and to be comparable to previously reported studies of UHMPWE-on-CoCr tested in the same pin-on-plate simulators. Contact pressures from 2.1 to 25.5MPa were created by changing the diameter of the contact face of the pin, the cross-shear ratios ranged from 0 (uniaxial motion) to 0.18. Wear of the UHMWPE pins was measured gravimetrically and the surface topography of the plates assessed using a contacting Form Talysurf. N=6 was carried out for each condition and statistical analysis carried out using ANOVA with significance taken at p<0.05.
Results
When compared to conventional materials (UHMWPE-on-CoCr), the wear factor of UHMPWE-on-PEEK was generally lower than that of moderately cross-linked UHMWPE-on-CoCr. With increasing contact pressure, there was a trend of decreasing wear factor and a significant difference (p=0.001) in the wear factor of the UHMPWE pins tested under different contact pressures. The wear of UHMWPE-on-PEEK followed a similar trend as that of UHMWPE-on-CoCr. Under uniaxial motion (cross-shear ratio = 0), the wear of UHMWPE was low, introducing multi-axial motion increased the wear of the UHMWPE. There was a significant difference (p<0.01) in the wear factor at different cross-shear ratios however, post hoc analysis showed only the test carried out under unidirectional motion to be significantly different from the other conditions tested.
At the conclusion of the studies, there was a polished region in the centre of the plate, however, there was no significant difference in the post-test surface roughness of the plate under any of the conditions tested.
Conclusion
The influence of contact pressure and cross-shear ratio on the wear of UHMWPE pins has shown a similar trend when articulating against either PEEK-OPTIMA™ or cobalt chrome plates. The wear factors determined in this study will provide inputs to future computational models which will allow the wear of this all-polymer knee replacement to be investigated under a wider range of clinically relevant conditions.
