Introduction
Ultra high molecular weight polyethylene (UHMWPE) remains a widely used bearing in total joint replacement. Wear induced osteolysis remains a concern with such bearings and first generation highly crosslinked polyethylene (HXLPE) materials were introduced to address this issue [1]. However, there are concerns regarding bulk material property degradation of HXLPE [2] and oxidation after implantation which may be as a result of lipid absorption or cyclic loading [3].
ECiMa is a cold-irradiated, mechanically annealed a-tocopherol (vitaminE) blended UHMWPE and is a next generation HXLPE developed to maintain mechanical properties, minimise wear and to improve the oxidation resistance in the long-term.
The aim of this study was to compare the in-vitro wear rate and mechanical properties of three different UHMWPE acetabular liners; conventional UHMWPE, HXLPE and ECiMa.
Methods
Fifteen acetabular liners (Corin, UK) underwent a hip simulation (servo-hydraulic, type C6/2-07). Three conventional UHMWPE liners(GUR 1050, Ø32 mm, gamma sterilised in nitrogen to 30kGy) were tested to 3 million cycles (mc). Six HXLPE liners (GUR 1020, Ø40 mm, cross-linked to an irradiation dose of 75kGy and EtO sterilised) and six ECiMa liners (GUR 1020 blended with 0.1wt% vitamin E, Ø40 mm, cross-linked to an irradiation dose of 120kGy, mechanically deformed and annealed, and EtO sterilised) were tested to 5mc with all liners articulating against CoCrMo alloy femoral heads.
The wear simulator used a double-peak force curve [4], with a maximum force of 3.0 kN. The test lubricant used was calf serum with a protein content of 30 g/l supplemented with 1% (v/v) patricin as an antibacterial agent. The simulators operated at a frequency of 1 Hz and were stopped after the first 0.5 mc and every 1.0 mc thereafter; volumetric wear rate was determined gravimetrically. Moisture uptake was monitored using loaded soak controls; one for UHMWPE and two for HXLPE and ECiMa.
Three of the ECiMa liners were cut in half post wear testing. One half of each was subject to accelerated ageing (ASTM F2003-02, 5atm of pure oxygen at 70ºC for 14 days), while the other half was tested as received. Slices, 200 µm thick, were taken from each liner half. Hexane extraction was performed on both the aged and unaged samples prior to oxidation analysis to remove any lipid contaminants from the wear testing. Oxidation analysis was performed using a Fourier Transform Infra-red technique (ASTM F2102-01) throughout the thickness of each liner half. Average oxidation indices for each sample were determined.
To provide comparative data, samples of untested ECiMa material (n=3) were aged in accordance with ASTM F2003-02.
Dog bone specimens, 3.2 mm thick (ASTM D638 type V) were machined from ECiMa material. Half of the samples were aged in accordance with ASTM F2003-02. Uniaxial tension testing was carried out as per ASTM D638 to characterise the mechanical properties of the aged and unaged material. Specimens were machined in three different directions; parallel to the direction of consolidation, parallel tothe direction of deformation and orthogonal to both of these directions.
Ultimate tensile strength (UTS), yield strength and percent elongation values were measured. These values were compared to mechanical data available for the other material types. A further set of dog bone specimens (1.8 mm thick, ASTM D638 type IV) were machined from HXLPE. Half of the samples were aged in accordance with ASTM F2003-02. All samples were tested to characterise the mechanical properties. These values have also been compared to mechanical data available for conventional UHMWPE.
Figure 1: Comparison of wear rate (mm3/mc) for the 3 different materials (5mc data for HXLPE and ECiMa)
Results and discussion
There was a 95% and a 83% reduction in the wear rate for the ECiMa liners compared to the conventional UHMWPE and HXLPE liners respectively (Figure 1).
All of the oxidation values for the wear tested ECiMa liners, before and after ageing, and the aged, untested ECiMa samples were negative and below the level of detection. This indicates a high level of throughthickness oxidation resistance even after aggressive ageing and cyclic loading.
Table 1: Comparison of Mechanical Properties Note: ECiMa data presented is the average of the three directions tested
The data indicates an increase in UTS, yield strength and percent elongation of 12%, 10% and 8% respectively, for unaged ECiMa compared to HXLPE. Importantly, the mechanical properties were not substantially degraded when compared to conventional UHMWPE and superior to the HXLPE samples.
Conclusion
The reduced wear rate during in-vitro hip simulation of ECiMa compared to conventional UHMWPE, coupled with improved mechanical properties and long-term oxidation resistance in comparison to HXLPE, indicates ECiMa as a promising next generation UHMWPE bearing material.
References
- Jacobs et al. JBJS, 2007, 86 (12): 2779-2786
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- Rowell et al. ORS 2010: Paper #358
- Paul J.P. IMechE Proc. H, 1966, 181 (3J): 8-15
- Dumbleton et al. CORR, 2006, 453:265-271
