PIN-ON-PLATE STUDIES ASSESSING THE WEAR PERFORMANCE OF CFR-PEEK AGAINST COCRMO

Abstract

In the majority of cases, failure of conventional metal-on-ultra-high molecular weight polyethylene (UHMWPE) artificial joints is due to wear particle induced osteolysis. Therefore, new materials have been introduced in an attempt to produce bearing surfaces that create lower, more biologically compatible wear. Polyetheretherketone (PEEK-OPTIMA) has been successfully used in a number of implant applications due to its combination of mechanical strength and biocompatibility.

Multi-directional pin-on-plate wear tests were performed on carbon fibre reinforced PEEK-OPTIMA (CFR-PEEK) against CoCrMo. PAN-based CFR-PEEK was tested against both low carbon and high carbon CoCrMo and Pitch-based CFR-PEEK was tested against high carbon CoCrMo only. The multi-directional motion of the pin-on-plate machine replicated the crossing of the wear paths that would be expected in vivo. For each test, four pin and plate samples were tested for two million cycles at a cycle frequency of 1 Hz under a 40 N load (which resulted in a contact stress of about 2 MPa). The lubricant used was bovine serum diluted with distilled water to a protein content of 15 gl-1. This was maintained at 37 °C. The wear was determined gravimetrically. Soak control specimens were used to account for any weight changes due to lubricant absorption.

The average steady state wear for the CFR-PEEK pins was found to be 0.144, 0.176 and 0.123 × 10-6 mm 3N-1m-1 for the CFR-PEEK PAN pins against low carbon CoCrMo, CFR-PEEK PAN pins against high carbon CoCrMo and CFR-PEEK Pitch-based pins against high carbon CoCrMo. A comparison of the results from the low and high carbon plates articulating against the PAN-based pins shows that the high carbon CoCrMo produced slightly higher wear than the low carbon CoCrMo. The protruding carbides on the high carbon CoCrMo plates may have caused this increase in wear. The lowest wearing material combination in this study was CFR-PEEK Pitch against high carbon CoCrMo. Published papers on the wear of UHMWPE against stainless steel [1] have shown higher wear factors (1.1 × 10-6 mm3N-1m-1).

Pitch and PAN-based CFR-PEEK against CoCrMo (low carbon or high carbon) provided low wear rates. On average, the Pitch-based material against high carbon CoCrMo provided the lowest wear in these tests. All the material combinations gave lower wear than found for UHMWPE-on-stainless steel tested under similar conditions. This gives confidence in the likelihood of this material combination performing well in orthopaedic applications.

The authors wish to thank INVIBIO Ltd for funding this research.