Abstract
Introduction
Edge loading in acetabular hip implants is generally due to mal-orientation or low tissue tension. It is known that edge loading of metal-on-metal THA may lead to higher metal wear and ion release with corresponding adverse body reactions. The inclination angle of the acetabular cup has been positively correlated with the wear rate of explanted components 1. However, no data published is known about wear rates of edge loaded hard – soft hip bearings.
Methods
For the hip simulator study, seleXys cup inlays, size 28/EE, (Mathys Ltd Bettlach, Switzerland) were used. Standard PE parts and vitamys® inlays (highly cross-linked, vitamin E stabilised UHMWPE) were tested in the same run. PE inlays were machined out of sintered GUR 1020 slabs, packaged and gamma-sterilised in inert atmosphere at 30 kGy. The vitamys® material was made in-house by adding 0.1 wt.-% of vitamin E to GUR 1020 powder from Ticona GmbH, Kelsterbach/Germany. Cross-linking used 100 (±10) kGy gamma-irradiation and the final sterilisation was gas plasma. Cup inclination was varied: besides the protocol of ISO 14242-1 with an inclination angle corresponding to 45 ° in the medial-lateral plane, a steep cup position corresponding to 75 ° was tested, too. To our knowledge, this is the highest inclination angle ever tested in a hip simulator. The testing was conducted in a servo-hydraulic six-station hip simulator (Endolab, Thansau/Rosenheim, Germany) at a temperature of 37±1°C. Tests were run at the RMS Foundation (Bettlach / Switzerland) for five million cycles. The test fluid was based on bovine serum diluted to a protein concentration of 30 g/l and stabilised with sodium azide and EDTA. At lubricant change interval of 500,000 cycles, the inlays were measured gravimetrically with an accuracy of 0.01 mg.
Results
The wear rate of the standard UHMWPE inlays tested with an inclination of 75° was 16% lower than those of the inlays with 45 ° inclination. For the vitamys® inlays, wear rates were about the same for both inclination angles (cf. Figure 1). After the test, the 75 ° inlays were polished tribologically on the caudal wall of the inlays while on the pole the tool-marks were still present (Figure 2, vitamys®). The polished surface of the 45 ° inclination samples was lager and covered about 2/3 of the articulation surface (vitamys®, Figure 3) or almost the whole articulation (standard PE). Hence, the hard – soft bearings tested showed no significant effect of inclination angle on the wear rate. This is true for a position as steep as 75 °, just before subluxation would occur.
Conclusions
Based on the present hip simulator study, it seems that metal-on-polyethylene bearings are exempt of accelerated wear rate when subjected to edge loading conditions. Using the newest generation of HXLPE, stabilised with vitamin E, combines superior oxidation resistance 2, low wear and highest forgiveness for component mal-orientation.
