INFLUENCE OF LUBRICANT AND TEMPERATURE ON THE WEAR OF UHMWPE ARTICULATING AGAINST PEEK OPTIMA

Introduction

UHMWPE articulating against PEEK-OPTIMA® has the potential for use as a novel bearing couple in joint arthroplasty due to its potentially low wear rates and the bioinertness of its wear debris. The aim of this study was to investigate the role of protein in the lubricant on the wear of UHMWPE articulating against PEEK at both room and physiological temperature.

Methods

The wear of GUR1020 UHMWPE pins articulating against PEEK plates (R_a ∼0.06µm) was compared to highly polished cobalt chrome plates (R_a <0.01µm) in a 6-station multi-axial pin-on-plate rig using kinematics to replicate those in total knee arthroplasty. Tests were carried out at either ∼20°C or ∼36°C and wear was investigated under varying concentrations of bovine serum (0, 25 or 90%). Studies were carried out for 1 Million cycles with wear of the UHMWPE pins assessed gravimetrically using unloaded soak controls to compensate for moisture uptake.

Statistical analysis was carried out using ANOVA with significance taken at p<0.05.

Results

Figure 1 shows wear factors for the UHMWPE pins. In 0% serum, the wear of UHMWPE against both PEEK and cobalt chrome tested at room and physiological temperatures was very low.

When lubricated with 25% serum and tested at room temperature as per standard practice at Leeds, the wear of UHMWPE against cobalt chrome was comparable to previously published data (2.13×10⁻⁷mm³/Nm) and the wear of UHMWPE articulating against PEEK was 3.53×10⁻⁷mm³/Nm. At physiological temperatures, the wear of UHMWPE against cobalt chrome was similar to tests at room temperature but against PEEK, there was a significant (p=0.017) decrease in wear factor [1].

The wear of UHMWPE against cobalt chrome was significantly (p=0.003) higher when tested in 90% serum compared to 25% serum at room temperature but this effect was not seen in UHMWPE articulating against PEEK (p=0.38). Testing at elevated temperature in 90% serum significantly (p=0.007) decreased the wear of UHMWPE against cobalt chrome and a deposition, likely to be protein, was evident on the surface of the plates. However, against PEEK, there was no significant difference in wear under the different temperature conditions.

Discussion

In pin-on-plate tests of UHMWPE against smooth metal counterfaces, the low wear rate and polymer deposition in tests lubricated with water has previously been observed. A similarly low wear rate has now been observed for UHMWPE against PEEK. Water does not produce adequate boundary lubrication.

At 25% and 90% serum and varying temperatures the wear behaviour of UHMWPE against PEEK is not the same as UHMWPE against cobalt chrome. There may be a number of factors contributing to these differences, including surface topography, protein precipitation, protein deposition, differing wear mechanisms and lubrication regimes. For example, UHMWPE-PEEK is a higher friction bearing couple than UHMWPE-CoCr and when tested at elevated temperatures, frictional heating at the bearing surfaces may accelerate protein precipitation and adsorption of protein onto the articulating surfaces which may affect wear.

This study shows that testing under different lubricant and environmental conditions can have a significant effect on wear.