Abstract
Introduction
PEEK-OPTIMA™ has been considered as an alternative to cobalt chrome in the femoral component of total knee replacements. Whole joint 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. In this study, the influence of third body wear on UHMWPE-on-PEEK was investigated, tests on UHMWPE-on-CoCr were carried out in parallel to compare PEEK to a conventional femoral component material.
Methods
Wear simulation was carried out in simple geometry using a 6-station multi-directional pin-on-plate simulator. 5 scratches were created on each PEEK and CoCr plate perpendicular to the direction of the wear test using a diamond stylus to produce scratches with a geometry similar to that observed in retrieved CoCr femoral components. To investigate the influence of scratch lip height on wear, scratches of approximately 1, 2 and 4µm lip height were created. Wear simulation of GUR 1020 UHMWPE pins (conventional, non-sterile) against the plates was carried out for 1 million cycles (MC) using 17g/l bovine serum as a lubricant using kinematic conditions to replicate the average contact pressure and cross-shear in a total knee replacement. Wear of UHMWPE pins was measured gravimetrically and the surface topography of the plates assessed using a contacting Form Talysurf. Wear factors of the pins against the scratched plates were compared to unscratched controls (0µm lip height). Minimum n=3 for each condition and statistical analysis carried out using ANOVA with significance taken at p<0.05.
Results
For the control tests (0µm lip height), the wear factor of UHMWPE pins was similar (p=0.64) against PEEK and CoCr plates. Against CoCr, with an increasing lip height, an exponential increase in wear factor of UHMWPE pins was observed; for PEEK, with increasing lip height, the wear factor did not show an exponential increase. When articulated against the largest scratches, 4µm, the wear factor of UHMWPE was significantly higher against CoCr than PEEK (p=0.01). At the conclusion of the study, on the PEEK plates, a polishing effect of the pin against the plates was observed and in the area of the wear test, the lip height of the scratches was lower than pre-test values; for the CoCr plates, no change in lip height was measured after 1MC wear simulation.
Conclusion
The exponential relationship between scratch lip height in CoCr and wear of UHMWPE has previously been described. However, the trend in the wear of UHMWPE was different when articulating against scratched PEEK compared to CoCr, with a significantly higher wear factor of UHMWPE against CoCr than PEEK at a scratch lip height of 4µm. This study suggests that the third body wear behaviour of this all-polymer knee replacement will be different to that of conventional implant materials.
