Wear and Friction of UHMWPE-on-PEEK OPTIMA™

Abstract

PEEK-OPTIMA™ is being considered as an alternative bearing material to cobalt chrome in the femoral component of total knee replacement to provide a metal-free implant. The aim of this study was to investigate the influence of lubricant temperature (standard rig running and elevated temperature (~36 °C)) on the wear of a UHMWPE-on-PEEK OPTIMA™ bearing couple using different lubricant protein concentrations (0%, 2%, 5%, 25% and 90% bovine serum) in a simple geometry pin-on-plate configuration. Friction was also investigated under a single temperature condition for different lubricant protein concentrations. The studies were repeated for UHMWPE-on-cobalt chrome in order to compare relationships with temperature (wear only) and lubricant protein concentration (wear and friction). In low lubricant protein concentrations (≤ 5%) there was no influence of temperature on the wear factors of UHMWPE-on-PEEK. With 25% bovine serum, the wear factor of UHMWPE-on-PEEK reduced by half at elevated temperature. When tested in high protein concentration (90% serum), there was no influence of temperature on the wear factor of UHMWPE-on-PEEK. These temperature dependencies were not the same for UHMWPE-on-cobalt chrome. For both material combinations, there was a trend of decreasing friction with increasing protein concentration once protein was present in the lubricant. This study has shown the importance of the selection of appropriate test conditions when investigating the wear and friction of different materials, in order to minimise test artefacts such as polymer transfer, and protein precipitation and deposition.

Third Body Wear of UHMWPE-on-PEEK-OPTIMA™

“…When compared to control tests carried out against smooth plates, the damage simulation protocol used in this study did not generate scratches with a lip height of sufficient magnitude to influence the wear of UHMWPE against either damaged PEEK or CoCr plates. The wear factor of the UHMWPE pins in the control study was similar to that previously reported under comparable conditions [8]. It is interesting to note that the variability in the data was consistently larger in the UHMWPE-on-PEEK bearing couple compared to UHMWPE-on-CoCr and that following third body damage simulation, the variability in the data was further increased for both material combinations.…”

“…The plate was mounted in a lubricant-containing bath, which reciprocated over a length of 20 mm at 1Hz, and as the bath reciprocated, the UHMWPE pin rotated (±20 • ) via a rack and pinion mechanism. a constant axial load of 160 N was applied through the pin (8 mm flat contact face) to give an average contact pressure of 3.18 MPa consistent with previous wear simulation of the UHMWPE-on-PEEK bearing couple [8]. All wear simulation studies were carried out using 25% bovine serum supplemented with 0.03% sodium azide solution as a lubricant and were carried out under rig running (room) temperature conditions as previously described and optimised for the UHMWPE-on-PEEK bearing couple [8].…”

“…a constant axial load of 160 N was applied through the pin (8 mm flat contact face) to give an average contact pressure of 3.18 MPa consistent with previous wear simulation of the UHMWPE-on-PEEK bearing couple [8]. All wear simulation studies were carried out using 25% bovine serum supplemented with 0.03% sodium azide solution as a lubricant and were carried out under rig running (room) temperature conditions as previously described and optimised for the UHMWPE-on-PEEK bearing couple [8]. Prior to the start of the study, the UHMWPE pins were soaked for a minimum of 2 weeks to maximise water uptake prior to cleaning and weighing using an XP26 digital microbalance (Mettler Toledo Inc., OH, USA) with a resolution of 1 µg.…”

“…Firstly, this was a fundamental study carried out in a pin-on-plate configuration using simplified geometries of components and whilst the input kinematics replicated the average cross-shear and contact pressure in a total knee replacement [29]. The loading and motion profiles were simplified and consistent with previous studies [8], and it is not known whether the results would be replicated in a clinical setting. The components used were not sterilised or cross-linked; however, sterilisation by ethylene oxide, which is commonly used for TKRs, has been shown not to influence mechanical properties or induce crosslinking in polyethylene and, therefore, does not influence wear [30].…”

“…Several in vitro pin-on-plate and whole joint simulation studies have been carried out investigating the wear performance of the PEEK-on-ultra-high molecular weight polyethylene (UHMWPE) bearing couple and have demonstrated an equivalent rate of wear for this bearing couple compared to CoCr-on-UHMWPE for a well-positioned implant tested in a clean environment under standard conditions [3,7,8]. In addition, an in vivo large animal study has also investigated the potential to use PEEK in an all-polymer TKR [9,10].…”