Abstract
In designing artificial joints the main criteria are to reduce the wear-rate of the material and the reaction of the body to the wear particles produced. These can be achieved by using harder materials (metals, ceramics) or by reducing the chances of producing a wear particle by the choice of material and design. This paper will look at combinations of PEEK-OPTIMA against different counterfaces with the aim of reducing the wear-rate of artificial hips and knees.
Pin-on-Plate Studies Twenty-six different sets of experiments combining PEEK-OPTIMA in different formulations and against different counterfaces were conducted to evaluate the lowest wear combination.
The lowest wear-rate combination was CFR-PEEK PAN against low carbon CoCrMo alloy (K=0.144×10-6 mm3/Nm) which is only about 1/8th of the wear of UHMWPE against stainless-steel (1.1×10-6 mm3/Nm). Gamma radiation sterilisation did not seem to affect the PEEK wear-rate.
Hip Simulator Studies A 25 million cycle wear study has been conducted on the Durham Hip Simulator using 54 mm diameter alumina heads against CFR-PEEK thin-walled acetabular cups (MITCH). Five joints were in active stations and one acted as a loaded control. Wear was measured gravimetrically.
Particles were analysed using a NanoSight LM10 instrument at 0.5, 10 and 25 million cycles. Also an Atomic Force Microscope (AFM) was used to look at particles above 2μm which is the limit of the NanoSight instrument.
The wear-rate was linear over the whole 25 million cycle test at 1.16 mm3/ million cycles (range 0.811–1.392 mm3/million cycles). As the test progressed, the number of particles reduced and the dominant particle size increased from about 40nm to circa 200 nm. The AFM showed some particles as large as 3μm to exist also. No fluid film lubrication was observed to be generated in these joints so the low wear-rate was due to the inherent low-wear properties of the material combination.
Knee Simulator Studies CFR-PEEK was moulded into the interpositional bearings for experimental lateral and medial unicompartmental knee designs and tested for 5 million cycles using 5 pairs of active joints and one pair of loaded controls in the Durham Knee Simulator. Wear was measured gravimetrically.
Whilst the CFR-PEEK components gave a total wear-rate (both medial and lateral) of 2.72 mm3/million cycles, UHMWPE inserts in a similar application1, gave 9.67mm3/million cycles. This represents a reduction of 72 per cent for the wear of the CFR-PEEK components.
Conclusions CFR-PEEK used in the correct combination and application can give a much reduced wear-rate compared with UHMWPE. It does not have the problem of metal ion release and has been shown by others not to exhibit cytotoxicity.2
Correspondence should be addressed to ISTA Secretariat, PO Box 6564, Auburn, CA 95604, USA. Tel: 1-916-454-9884, Fax: 1-916-454-9882, Email: ista@pacbell.net
Acknowledgements: Invibio and Stryker funded the work described herein.
References
1 Laurent et al (2003) Wear255, 1101–1106. Google Scholar
2 Howling, et al (2003) Biomedical Materials Research Part B: Applied Biomaterials, 67B: 758–764. Google Scholar
