Artificial joints employing ultra-high molecular weight polyethylene (UHMWPE) are widely used to treat joint diseases and trauma. Wear of the polymer bearing surface largely limits the use of these joints in younger and more active patients. Previous studies have shown the wear factor used in Archard's law for the conventional polyethylene to be highly dependent on contact pressure and this has produced variability in experimental data and has constrained the reliability and applicability of previous computational predictions. A new wear law is proposed, based on wear volume being dependent on, and proportional to, the product of the sliding distance and contact area. The dimensionless proportional constant, wear coefficient, which was independent of contact pressure, was determined from a multi-directional pin on plate study. This was used in computational predictions of the wear of the conventional UHMWPE hip joints. The wear of the polyethylene cup was independently experimentally determined in physiological full hip joint simulator studies. The predicted wear rate from the new computational model was generally increased, with an improved agreement with the experimental measurement compared with the previous computational model. It was shown that wear in the UHMWPE hip joints increased as head size and contact area increased. This resulted in a much larger increase in the wear rate as the head size increased, compared with the previous computational model, and is consistent with clinical observations. This new understanding of the wear mechanism in artificial joints using the UHMWPE bearing surfaces, and the improved ability to predict wear independently and to address previously described discrepancies offer new opportunities to optimize design parameters.
The finite element method was employed in this study to compare the contact mechanics at the bearing surfaces between a metal-on-metal hip resurfacing prosthesis and a total hip replacement with a similar bearing combination. The hip resurfacing prosthesis was implanted and modelled in a full three-dimensional pelvic and femoral bone. A significant reduction in the predicted contact pressure by over 53 per cent as well as a corresponding increase in the contact area by approximately 220 per cent was found in the hip resurfacing prosthesis, in comparison to the total hip replacement. The reduced contact pressure and increased contact area in the hip resurfacing system were due to the combination of the larger bearing size and increased elasticity from the metallic cup and the underlying bone support. The hip resurfacing prosthesis may therefore offer a significant improvement in the tribology at the metallic bearing surfaces, thus offering a potential advantage in terms of long-term clinical success over current total hip replacements with reported survivorships over 20 years.
The effect of geometry change of the bearing surfaces owing to wear on the elastohydrodynamic lubrication (EHL) of metal-on-metal (MOM) hip bearings has been investigated theoretically in the present study. A particular MOM Metasul bearing (Zimmer GmbH) was considered, and was tested in a hip simulator using diluted bovine serum. The geometry of the worn bearing surface was measured using a coordinate measuring machine (CMM) and was modelled theoretically on the assumption of spherical geometries determined from the maximum linear wear depth and the angle of the worn region. Both the CMM measurement and the theoretical calculation were directly incorporated into the elastohydrodynamic lubrication analysis. It was found that the geometry of the original machined bearing surfaces, particularly of the femoral head with its out-of-roundness, could lead to a large reduction in the predicted lubricant film thickness and an increase in pressure. However, these non-spherical deviations can be expected to be smoothed out quickly during the initial running-in period. For a given worn bearing surface, the predicted lubricant film thickness and pressure distribution, based on CMM measurement, were found to be in good overall agreement with those obtained with the theoretical model based on the maximum linear wear depth and the angle of the worn region. The gradual increase in linear wear during the running-in period resulted in an improvement in the conformity and consequently an increase in the predicted lubricant film thickness and a decrease in the pressure. For the Metasul bearing tested in an AMTI hip simulator, a maximum total linear wear depth of approximately 13 microm was measured after 1 million cycles and remained unchanged up to 5 million cycles. This resulted in a threefold increase in the predicted average lubricant film thickness. Consequently, it was possible for the Metasul bearing to achieve a fluid film lubrication regime during this period, and this was consistent with the minimal wear observed between 1 and 5 million cycles. However, under adverse in vivo conditions associated with start-up and stopping and depleted lubrication, wear of the bearing surfaces can still occur. An increase in the wear depth beyond a certain limit was shown to lead to the constriction of the lubricant film around the edge of the contact conjunction and consequently to a decrease in the lubricant film thickness. Continuous cycles of a running-in wear period followed by a steady state wear period may be inevitable in MOM hip implants. This highlights the importance of minimizing the wear in these devices during the initial running-in period, particularly from design and manufacturing points of view.
Effective lubrication performance of metal-on-metal hip implants only requires optimum conformity within the main loaded area, while it is advantageous to increase the clearance in the equatorial region. Such a varying clearance can be achieved by using nonspherical bearing surfaces for either acetabular or femoral components. An elastohydrodynamic lubrication model of a novel metal-on-metal hip prosthesis using a nonspherical femoral bearing surface against a spherical cup was solved under ISO standard specified dynamic loading and motion conditions. A full numerical methodology of considering the geometric variation in the rotating nonspherical head in elastohydrodynamic lubrication solution was presented, which is applicable to all non-spherical head designs. The lubrication performance of a hip prosthesis using a specific nonspherical femoral head, Alpharabola, was analyzed and compared with those of spherical bearing surfaces and nonspherical Alpharabola cup investigated in previous studies. The sensitivity of the lubrication performance to the anteversion angle of the Alpharabola head was also investigated. Results showed that the nonspherical head introduced a large squeeze film action and also led to a large variation in clearance within the loaded area. With the same equatorial clearance, the lubrication performance of the metal-on-metal hip prosthesis using an Alpharabola head was better than that of the conventional spherical bearings while worse than that of the metal-on-metal hip prosthesis using an Alpharabola cup. The reduction in the lubrication performance caused by the initial anteversion angle of the nonspherical head was small, compared with the improvement resulted from the nonspherical geometry.
Various complex structures are employed in different metal-on-metal (MOM) hip prostheses for different purposes. For the elastohydrodynamic lubrication (EHL) study of these prostheses, the elastic deformation calculation is not a trivial task due to the effect of complex structures. The finite-element method (FEM) is an attractive approach for solving these problems because of its flexibility in handling complex geometries. Moreover, the availability of high-level ready-to-use finite-element commercial software may facilitate the modelling of complex structures and thereby reduce the time spent on implementing the analysis. In this study, a numerical method was developed to solve the EHL problems of MOM hip prostheses with complex structures. In this method, the elastic deformation was calculated from the product of the flexibility matrix of the lubrication nodes and the nodal force. The flexibility matrix was obtained by inverting the stiffness matrix, which was obtained through finite-element analysis using commercial software. The nodal force was obtained by transferring the hydrodynamic pressure according to the isoparametric element definition. This method was validated for a typical 28 mm diameter MOM total hip replacement. The effects of the structures of the femoral head, the wall thickness of the cup, as well as the bone quality of patients on lubrication performance of MOM hip resurfacing prostheses were subsequently investigated.
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