Introduction: Hip simulator and clinical retrieval studies have shown that metal-on-metal (MOM) hip implants commonly have biphasic wear. An initial high wear or running-in phase is generally followed by a low wear or steady-state phase. A number of hypotheses have been put forward to explain this biphasic phenomenon, including polishing of the metallic bearing surfaces and increasing conformity between the two articulating surfaces. The purpose of the present study was to compare the wear and lubrication of MOM hip implants between the running-in and steady-state periods.

Materials and Methods: A standard 28mm Metasul^TM MOM bearing (Centerpulse Orthopedics, a Zimmer Company, Winterthur, Switzerland) was investigated. The wear testing was carried out using a 6-station AMTI hip simulator in the presence of 33% bovine serum and 67% Ringer solution (PH 7.2). The bearing surfaces of both the femoral and acetabular components were measured using a coordinate measurement machine at different stages of wear testing. The dimensional changes of the bearing surfaces due to wear were directly incorporated into the elastohydrodynamic lubrication analysis using an in-house developed code.

Results: The initial running-in period occurred during the first 1 million cycles, and little wear was observed subsequently up to 5 millions cycles. The maximum total wear depth was measured to be around 13 μm at 1 million cycles. The predicted average lubricant film thickness between the two articulating surfaces was increased from 0.024μm at the beginning, to 0.09μm at the end of the first 1 million cycles. For a given composite surface roughness of 0.03μm often quoted for the metallic bearing surfaces, such an increase in the lubricant film thickness represents a transition from a mixed to a fluid film lubrication regime.

Discussion: A large improvement in lubrication was predicted as a direct result of the running-in wear of the bearing surfaces. This was mainly due to the increased conformity between the two articulating surfaces and the decreased diametrical clearance. It was particularly noted that the improvement in lubrication after 1 million cycles was so significant that continuous fluid film lubrication was possible, leading to extremely low wear for up to 5 million cycles, and only material fatigue and start-up and stopping for wear measurements could cause a further increase in wear.

It is possible in theory to optimise the geometry of the metallic bearing surfaces, based on the worn components, to minimise the running-in wear. However, such an improvement in lubrication cannot be readily achieved because of difficulties in surgical techniques and position of the components.

Introduction: Laboratory simulator and clinical retrieval studies of metal-on-metal (MOM) total hip replacements have shown that the metallic alloy, the femoral head radius, the clearance between the acetabular cup and femoral head and the cup thickness can influence the contact mechanics, the lubrication and the wear of the articulation. MOM hip resurfacing procedures have received significant attention recently. The purpose of the present study was to compare the contact mechanics between a MOM hip resurfacing implant and a MOM total hip replacement under identical conditions.

Materials and Methods: A 50mm diameter DUROM^TM MOM hip resurfacing prosthesis and a 28mm diameter Metasul^TM MOM bearing system (Centerpulse Orthopedics, a Zimmer Company, Winterthur, Switzerland) were investigated. All implants were manufactured from wrought-forged high carbon cobalt chromium alloy (Pro-tasul 21WF^TM). The diameters of the DUROM^TM femoral head and acetabular cup were 50mm and 50.145mm respectively, and the corresponding wall thickness of the acetabular component was around 4mm. The diameters of the Metasul^TM femoral head and acetabular cup were 28mm and 28.12mm. Three-dimensional finite element models were created to simulate the contact between the bearing surfaces of both the femoral head and the acetabular cup fixed to a three dimensional anatomically positioned pelvic and femoral bone consisting of both cortical (with 1mm thickness) and cancellous regions. The load applied to both models was 3200N.

Results: The maximum contact pressure at the bearing surfaces was found to be around 22MPa for the DUROM^TM and the contact area between the femoral and acetabular components was predicted to be 237mm². For the Metasul^TM bearing under identical conditions, the maximum contact pressure and the contact area predicted were approximately 47MPa and 74mm² respectively.

Discussion: A large reduction in the contact pressure, which should improve overall tribological performances, was noted for the DUROM^TM hip resurfacing prosthesis, as compared with the Metasul^TM bearing. The main reasons for this reduction were the large diameter of the articulation and the small acetabular cup thickness of the DUROM^TM system. In contrast, the Metasul^TM bearing has a smaller head diameter, and relies on a polyethylene backing underneath the metallic cup inlay to reduce the contact pressure at the articulating surfaces.

Aims: The purpose of the present study was to investigate the contact mechanics at the articulating surfaces in metal-on-metal hip implants. Methods: A 28mm diameter Metasul (from Sulzer Orthopedics Ltd.) was analysed in the present study. Both the femoral head and the acetabular cup were manufactured from matching cobalt chromium alloy. The cobalt chromium alloy acetabular inlay was thermo-mechanically bonded to an ultra high molecular weight polyethylene (UHMWPE) backing, which was in turn inserted into a titanium shell with a snap-þt for cementless þxation. The radial clearance between the femoral head and the acetabular cup was 60μm. Finite element method (ABAQUS 6.2) was used to model the contact at the articulating surfaces between the femoral head and the acetabular cup, under a load of 3.2kN. Results: The average contact pressure at the bearing surfaces was found to be about 45MPa. This was considerably lower than 63MPa if the UHMWPE backing was replaced by cobalt chromium alloy. It was also interesting to compare the present result with the use of a larger femoral head or a reduced clearance. In order to match the average contact pressure of 45MPa, it was found to be necessary to increase the femoral head radius to 18mm for a given radial clearance of 60μm or to decrease the radial clearance to 35μm for a þxed femoral head radius of 14mm. Conclusions: The use of an UHMWPE backing underneath a cobalt chromium alloy cup signiþcantly reduces the contact stresses experienced at the articulating surfaces in metal-in-metal hip implants.