Abstract
INTRODUCTION
Wear induced osteolysis, material property degradation and oxidation remain a concern in cobalt chrome on polyethylene THR. ECIMA is a cold-irradiated, mechanically annealed, vitamin E blended HXLPE developed to maintain mechanical properties, minimise wear and improve long-term oxidation resistance. This study aimed to compare the in-vitro wear rate and mechanical properties of three different acetabular liners; UHMWPE, HXLPE and ECIMA.
METHODS
Twelve liners (Corin, UK) underwent a 3 million cycle (mc) hip simulation. Three UHMWPE (GUR1050, Ø32 mm, γ sterilised), three HXLPE (GUR1020, Ø40 mm, 75 kGy γ, EtO sterilised) and six ECIMA (0.1 wt% vitamin E GUR1020, Ø40 mm, 120 kGy γ, mechanically annealed, EtO sterilised) liners articulated against CoCrMo femoral heads (Corin, UK). Wear testing was performed in accordance with ISO 14242 parts 1 and 2, in calf serum, with a maximum force of 3.0 kN and at a frequency of 1 Hz. Volumetric wear rate was determined gravimetrically.
ASTM D638 type V specimens were machined from ECIMA material for uniaxial tension testing. Ultimate tensile strength (UTS), yield strength and elongation values were measured. These values were compared to mechanical data available for the other material types.
Following completion of the ECIMA wear testing, three of the tested liners were cut in half. One half of each was subject to accelerated ageing in accordance with ASTM F2003-02, while the other half was tested as received. Each liner half was cross-sectioned and a microtome was used to section 200μm thick slices from each cross-section. Oxidation analysis was performed using a Fourier Transform Infra-red technique in accordance with ASTM F2102-01 throughout the thickness of each liner half. Average oxidation indices for each sample were determined.
RESULTS
The reduction in wear rate for the ECIMA liners compared to the UHMWPE and HXLPE liners was 95 % and a 83 % respectively. There was an increase in UTS, yield strength and percent elongation of 45%, 16% and 32% respectively, for unaged ECIMA compared to HXLPE. Following ageing of the ECIMA samples, there was minimal change in all three mechanical properties. Importantly, the mechanical properties were not substantially degraded and were more comparable to conventional UHMWPE than HXLPE. Further to this, following an aggressive ageing protocol, the ECIMA material maintains the mechanical properties of the unaged condition.
All of the oxidation values for the wear tested ECIMA liners, before and after ageing, and the aged, untested ECIMA samples were negative, which shows oxidation levels below the level of detection throughout the thickness of the samples. This indicates a high level of through-thickness oxidation resistance for the ECIMA specimens even after being subject to an aggressive ageing protocol and cyclic loading.
DISCUSSION
These in-vitro wear results indicate that ECIMA is a very low wearing material with the potential to reduce wear related osteolysis in-vivo. Importantly, the mechanical properties were generally maintained unlike the degradation found in many modified polyethylene materials and were more comparable to UHMWPE than HXLPE. These properties make ECIMA a promising next generation bearing material.
