Abstract
Introduction
Vitamin-E has been introduced into highly-crosslinked polyethylene liners to reduce the oxidation potential of the material while maintaining low wear rates. However, little has been reported on adverse testing of the material with one test on diffused vitamin-E polyethylene [1] and no adverse tests of vitamin-E blended polyethylene reported. Adverse testing of crosslinked polyethylene has focused on the use of large diameters, the incorporation of third body particles, roughening of the counterface or severe activity [2–4]. This investigation considers the wear of vitamin-E blended highly-crosslinked polyethylene under standard and adverse conditions articulating against uncoated and chromium nitride (CrN) coated metal heads.
Methods
Seven metal heads were tested against prototype ϕ52 mm 0.1 wt% vitamin-E blended highly-crosslinked polyethylene liners (Corin, UK). Three heads remained as cast double heat treated metal (MoP) while four, of similar metallurgy, were coated with CrN via electron beam physical vapour deposition (CrNoP) (Tecvac, UK) and polished to a similar surface finish. Tests were conducted for 5 million cycles (mc) under conditions described in ISO 14242–3: 2009. Alumina particles (mean size 2.4 μm) at concentrations of 0.15 mg/mL were added to the lubricant for 1 mc to consider the effect of severe head damage. Testing continued for a further 1 mc without the presence of the particles and then 3 jogging intervals (14,400 cycles each) were conducted at slow, medium and fast speeds [3]. Wear volume was determined gravimetrically for the heads and liners and fluid collected throughout the testing was analysed for cobalt concentration using graphite furnace atomic absorption spectroscopy.
Results
Wear rates of the liners were similar under standard conditions for both combinations (Figure 1). The introduction of alumina particles created a 17 fold increase in the wear of the MoP liners and increased head wear and cobalt release rates (Figure 2). Damage to the uncoated metal heads was observed as the average surface roughness Ra, of the uncoated heads was rougher (0.018 μm) than the coated heads (0.007 μm). The CrNoP bearings showed a small increase in liner and head wear but not cobalt release. The removal of the alumina particles saw the CrNoP bearings recover in liner and head wear while wear of the liners and cobalt release remained elevated in MoP bearings. Jogging did not significantly increase the wear of the MoP and CrNoP liners.
Discussion
Under standard testing the use of large diameter MoP appears low wearing, but adverse conditions can increase the polyethylene wear and cobalt release. During 3rd body testing higher cobalt levels than those reported in adverse metal-on-metal tests [5] were observed, although this test was extreme and not clinically relevant. CrN coating the heads showed improved wear resistance and reduced cobalt release during all forms of adverse testing.
