Abstract
To improve the longevity of total hip replacements (THR), it is necessary to prevent wear of the ultra-high molecular weight polyethylene (UHMWPE) bearing, as wear debris can cause osteolysis and aseptic loosening. Highly cross-linked UHMWPE reduces wear, sometimes stabilized with vitamin E to preserve its mechanical properties and prevent oxidative degeneration. An extra novel solution has been grafting the surface of UHMWPE with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC). This treatment uses a hydrophilic (wettable) phospholipid polymer to improve lubrication and reduce friction and wear of the bearing material.
We set out to test the wear and friction of ceramic-on-polyethylene (COP) THRs that had the PMPC surface treatment, or left untreated for control. Four groups of UHMWPE bearings were tested against identical 40mm ceramic heads (zirconia-toughened alumina). The UHMWPE bearings were highly cross-linked with/without vitamin E (HXL Vit. E: 125 kGy radiation dose / HXL: 75 kGy). In each group, half underwent the PMPC treatment (n = 3 for all four groups).
Testing was conducted on an AMTI hip simulator for 10 million walking cycles of ISO-14242-1, at 1 Hz, with diluted bovine serum (30 g/L protein concentration) as lubricant, at 37ºC, and with fluid absorption errors corrected with active soak controls. Using a previously published method, frictional torques and a frictional factor around three orthogonal axes about the femoral head were measured/computed, by data processing of the measurements of a 6-DOF load cell on each station of the hip simulator. Such friction measurements and stops for specimen weighing were carried out at regular intervals throughout the wear test.
The HXL liners without and with the PMPC treatment wore at 5.86±0.402 mg/Mc and 1.70±1.36 mg/Mc, respectively (p=0.013) (Fig. 1). The HXL Vit. E liners without and with the PMPC treatment wore at 2.14±0.269 mg/Mc and 0.736±0.750 mg/Mc, respectively (p=0.035). The wear rates of the untreated HXL and HXL Vit. E liners were significantly different (p=0.0002) but no difference in wear rate was found between the two PMPC treated groups (p=0.179), although, as mentioned above, the PMPC treatment very significantly reduced wear in each case. The ceramic femoral heads showed little wear (weight loss) themselves.
In general, the THRs showed decreasing friction over the 10 Mc, with the PMPC types showing a slight increase in friction towards the end of the test (Fig. 2). PMPC HXL liners showed the lowest friction factor (0.022±0.001) which was significantly lower (p<0.001) than the friction of the untreated liners (0.028±0.002) (Fig. 3). The PMPC HXL Vit. E liners showed lower friction factors than the untreated HXL Vit. E liners (0.034±0.002, 0.036±0.004, respectively), although this difference was not significant (p=0.116). Overall, the liners with the PMPC treatment displayed statistically significantly lower friction factors (p=0.003) than those untreated. The coincidence of some reduction of surface friction with larger wear reduction obviously suggests some but not necessarily full causality.
PMPC successfully reduced both the friction and the wear in these COP THRs during this extended 10 Mc test. This likely would translate to improved implant longevity in patients.
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