In real life, immediately after joint replacement, the artificial joint is actually bathed in blood\ud (and clotted blood) instead of synovial fluid. Blood contains large molecules and cells of size\ud ~ 5 to 20 2m suspended in plasma and considered to be a non-Newtonian (pseudoplastic)\ud fluid with density of 1060 Kg/m3 and viscosity ~ 0.01 Pas at shear rates of 3000 s-1 (as\ud obtained in this work). The effect of these properties on friction and lubrication is not fully\ud understood and, so far to our knowledge, hardly any studies have been carried out regarding\ud friction of metal-on-metal bearings with various clearances in the presence of lubricants such\ud as blood or a fluid containing macromolecules such as hyaluronic acid (HA) which is a major\ud component of synovial fluid increasing its viscosity and lubricating properties. In this work,\ud therefore, we have investigated the frictional behaviour of a group of Smith and Nephew\ud Birmingham Hip Resurfacing implants with a nominal diameter of 50mm and diametral\ud clearances in the range ~ 80 2m to 300 2m, in the presence of blood (clotted and whole\ud blood), a combination of bovine serum (BS) with hyaluronic acid (HA) and carboxymethyl\ud cellulose (CMC, as gelling agent) adjusted to a range of viscosities (~0.001-0.2 Pas), and\ud bovine serum with CMC adjusted to a similar range of viscosities.\ud These results suggested that reduced clearance bearings have the potential to generate high\ud friction especially in the presence of blood which is indeed the in vivo lubricant in the early\ud weeks after implantation. Friction factors in higher clearance bearings were found to be lower\ud than those of the lower clearance bearings using blood as the lubricant. Similar trends, i.e.\ud increase in friction factor with reduction in diametral clearance, were found to be also the\ud case using a combination of BS+CMC or BS+HA+CMC as lubricants having viscosities in\ud the range 0.1-0.2 and 0.03-0.14 Pas, respectively. On the other hand, all the lubricants with\ud lower viscosities in the range 0.001-0.0013 and 0.001-0.013 Pas for both BS+CMC and BS+HA+CMC, respectively, showed the opposite effect, i.e. caused an increase in friction\ud factor with increase in diametral clearance.\ud Another six large diameter (50mm nominal) BHR deflected prostheses with various\ud clearances (~ 50-2802m after cup deflection) were friction tested in vitro in the presence of\ud blood and clotted blood to study the effect of cup deflection on friction. It was found that the\ud biological lubricants caused higher friction factors at the lower diametral clearances for blood\ud and clotted blood as clearance decreased from 2802m to 502m (after deflection).\ud The result of this investigation has suggested strongly that the optimum clearance for the 50\ud mm diameter MOM BHR implants to be ¿1502m and <2352m when blood lubricant used, so\ud as to avoid high frictions (i.e. avoid friction factors >0.2) and be able to accommodate a\ud mixed lubrication mode and hence lower the risk of micro- or even macro-motion specially\ud immediately after hip implantation. These suggested optimum clearances will also allow for\ud low friction (i.e. friction factors of <0.2-0.07) and reasonable lubrication (dominantly mixed\ud regime) for the likely cup deflection occurring as a result of press-fit fixation.Smith & Nephew Orthopaedics Ltd
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