Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract: It is well documented that hard bearing combinations show a running-in phenomenon in vitro and there is also some evidence of this from retrieval studies. In order to investigate this phenomenon, five Birmingham hip resurfacing devices were tested in a hip wear simulator. One of these (joint 1) was also tested in a friction simulator before, during, and after the wear test and surface analysis was conducted throughout portions of the testing. The wear showed the classical running in with the wear rate falling from 1.84 mm3 per 106 cycles for the first 106 cycles of testing to 0.24 mm3 per 106 cycles over the final 2×106 cycles of testing. The friction tests suggested boundary lubrication initially, but at 1×106 cycles a mixed lubrication regime was evident. By 2×106 cycles the classical Stribeck curve had formed, indicating a considerable contribution from the fluid film at higher viscosities. This continued to be evident at both 3×106 and 5×106 cycles. The surface study complements these findings.Keywords: metal-on-metal resurfacing, wear, running in, lubrication, simulator study INTRODUCTIONlarge-diameter metal-on-metal joints were not initially fully fluid film lubricated and friction tests on other types of metal-on-metal joint have revealed Early, small-diameter (less than 32 mm) metal-onmetal hip joints were prone to premature failure [1], lower friction factors post-wear than initially [8], pointing towards more favourable lubrication after although some examples are known to have been in place successfully for up to 20 years [2, 3]. This wear testing. It has also been noted that the average linear wear rate (microns per year) for retrieved suggests there is a favourable tribological condition in some cases, although not in the majority of cases metal-on-metal joints is lower for joints with a longer survivorship, indicating that this wearing-in phase is for the early designs of metal-on-metal joints. New-generation larger-diameter metal-on-metal hip also likely to occur in vivo [12]. All this suggests that the articulating surfaces run in during early stages of joints have been more successful in the midterm [4, 5] although longer-term clinical results are not yet the wear test, and that this improves the lubrication conditions and hence lowers the wear even further available.Hard bearing joints often show a wearing-in period as the tests progress. during simulator wear tests [6][7][8][9][10], where the initial wear rates are higher than the steady state wear...

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Is the wear factor in total joint replacements dependent on the nominal contact stress in ultra-high molecular weight polyethylene contacts?K Vassiliou* and A Unsworth Centre for Biomedical Engineering, University of Durham, UK Abstract: The exact dependence of wear factor on contact stress, load and apparent contact area is much disputed in the literature. This study attempts to solve this dispute. Pin-on-plate studies of ultra high molecular weight polyethylene against stainless steel were conducted under di erent combinations of load (33-250 N ), nominal stress (0.56-12.73 MPa) and face diameter, as well as two tests where both stress and load were kept constant, while the diameter was changed. For these tests the centre of the pin face was bored out to create four di erent average pin diameters with similar face areas. Diameter and load were found to have no signi cant e ect on the wear factor, while the wear factor decreased with increasing contact stress according to the relation K=2×10Õ 6sÕ 0.84.Keywords: ultra high molecular weight polyethylene wear, contact stress, load, pin-on-plate set-up NOTATION ness term has been incorporated into the wear factor. Archard assumed Hertzian contact theory for a at K wear factor (mm3/N m) non-deformable surface in contact with a nominally at deformable surface with spherical asperities evenly L load (N ) R a surface roughness parameter (ím) distributed in depth. UHMWPE ultra high molecular weight polyethylene V wear volume (mm3) 1.2 E ect of stress and load on wear factor x sliding distance (m) Although the Lancaster equation suggests that the wear volume is dependent only on the particular combination of materials, the load and the distance slid, the situation 1 INTRODUCTION may not be quite so simple. The literature is divided as to the exact e ect of load and stress on the wear rate of 1.1 Wear ultra high molecular weight polyethylene ( UHMWPE ). The simple model of wear, according to the Lancaster This is made more di cult by the di erent operating equation [1] states that V =KLx, where V is the wear conditions in each study, including di erences in apparvolume (mm3), L is the load (N ), x is the sliding disatus and lubricants. tance (m) and K is the wear factor (mm3/N m). This Barbour et al. [4], using pin-on-plate apparatus, indicates that the wear volume is dependent on the load indicated that the wear factor decreases with increasing and the sliding distance for any given con guration. The nominal contact stress. Wang et al. [5 ] s...

Total hip replacements offer relief to a great many patients every year around the world. With an expected service life of around 25 years on most devices, and with younger and younger patients undergoing this surgery, it is of great importance to understand the mechanisms of their function. Tribological testing of both conventional and hard bearing joint combinations have been conducted in many centres throughout the world, and, after being initially abandoned owing to premature failures, hard bearing combinations have been revisited as viable options for joint replacements. Improved design, manufacturing procedures, and material compositions have led to improved performance over first-generation designs in both metal-on-metal and ceramic-on-ceramic hip prostheses. This paper offers a review of the work conducted in an attempt to highlight the most important factors affecting joint performance and tribology of hard bearing combinations. The tribological performance of these joints is superior to that of conventional metal- or ceramic-on-polymer designs.