Objectives

The high revision rates of the DePuy Articular Surface Replacement (ASR) and the DePuy ASR XL (the total hip arthroplasty (THA) version) have led to questions over the viability of metal-on-metal (MoM) hip joints. Some designs of MoM hip joint do, however, have reasonable mid-term performance when implanted in appropriate patients. Investigations into the reasons for implant failure are important to offer help with the choice of implants and direction for future implant designs. One way to assess the performance of explanted hip prostheses is to measure the wear (in terms of material loss) on the joint surfaces.

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.

The introduction of unicondylar knee prostheses has allowed the preservation of the non-diseased compartment of the knee whilst replacing the diseased or damaged compartment. However, as is well known, there is concern that the body’s biological reaction to ultra-high molecular weight polyethylene (UHMWPE) wear particles leads to bone resorption and subsequent loosening and failure of the joint. Also, in some cases, delamination of the UHMWPE tibial bearing surface has been found to occur leading to failure of these conventional joints. Therefore new material combinations have been investigated within the laboratory.

The unicondylar knee that was tested consisted of CoCrMo tibial and femoral components between which a mobile Pitch-based carbon fibre reinforced polyetheretherketone (CFR-PEEK OPTIMA®) meniscal bearing was mounted. The joints were supplied by INVIBIO Ltd. Tribological tests were performed on these knees using the Durham six station knee wear simulator and the Durham friction simulator II. In both cases the loading and motion were similar to the standard walking cycle. On the six station wear machine five stations applied both the loading and motion and were the active stations and one applied loading only as it was used as the loaded soak control station. Approximately every 500,000 cycles, the wear of the CFR-PEEK meniscal bearings was assessed gravimetrically (using a Mettler Toledo AX 205 balance, accurate to 0.01 mg) and the loaded soak control was used to take account of any change in weight due to lubricant absorption. The joints were tested to five million cycles (equivalent to approximately five years in vivo) with diluted new-born calf serum as the lubricant which gave a protein content of 17 gl-1. At periods throughout the wear test the surface topography was measured on the Zygo NewView 100 non-contacting profilometer. Friction tests were performed at the beginning and the end of the wear test.

The average volumetric wear rate of the medial and lateral components was found to be 1.70 and 1.02 mm3/million cycles respectively (range 0.66 – 2.73 and 0.59 – 2.45 mm3/million cycles respectively). This is lower than the reported wear rate of metal-on-UHMWPE unicondylar knee joints (6.69 and 2.98 mm3/million cycles for the medial and lateral components respectively) [1]. The surface topographical analysis of the CFR-PEEK bearings showed a reduction in surface roughness and also a change to more negative skewness (i.e. more valleys than peaks) which may aid in lubrication. Before and after wear testing the joints were found to be operating in the boundary/mixed lubrication regime.

The Pitch-based CFR-PEEK unicondylar knee joints performed well in these wear tests. They gave lower volumetric wear rates than metal-on-UHMWPE uni-condylar knee prostheses. The friction tests showed that at physiological viscosities, these joints operate in the boundary/mixed lubrication regime. These results show that this novel joint couple may potentially be an alternative solution for the reduction of wear and osteolysis.

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

Carbon fibre reinforced polyetheretherketone (CFR-PEEK) has been introduced recently as an alternative material to be used in joint prostheses. During injection moulding of the CFR-PEEK the carbon fibres tend to become orientated in the direction of the plastic flow. The direction of these fibres may affect the wear produced by these materials.

Reciprocation only and reciprocation plus rotation (multi-directional) pin-on-plate wear tests were performed on PAN-based CFR-PEEK against itself. The plates were manufactured with the carbon fibres mainly orientated either longitudinally (in the direction of reciprocation motion) or mainly transversally (perpendicular to the direction of motion) to determine the effect of carbon fibre orientation on the wear of these materials. For each test, four pin and plate samples were tested (two reciprocation only and two reciprocation plus rotation) for three and a half 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 de-ionised water to a protein content of 17 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 samples that underwent reciprocation motion only was found to be 5.41 and 18.7 × 10-8 mm3N-1m-1 for the longitudinal carbon fibres and the transverse fibres respectively. For the multi-directional tests, the average steady-state wear was 5.88 and 19.9 × 10-7 mm3N-1m-1 for the longitudinal and transverse fibres respectively. It is clear from these results that for both reciprocation motion only and reciprocation plus rotation the wear was considerably lower with the fibres orientated in the longitudinal direction than the transverse direction. Also, these tests show that reciprocation only gives approximately an order of magnitude lower wear than multi-directional motion.

It can be concluded that the wear rate of CFR-PEEK is lower when the sliding motion occurs in the same direction as the carbon fibre orientation. Also, in these pin-on-plate tests, the wear produced using reciprocation motion only was an order of magnitude lower than that for the tests using multi-directional motion.

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