Acknowledgement This study was supported by DePuy Synthes Joint Reconstruction (Leeds, UK), the Centre of Excellence in Medical Engineering funded by WELMEC (Wellcome Trust and EPSRC, WT088908/z/09/z), the NIHR Leeds Musculoskeletal Biomedical Research Unit and the EPSRC Centre for Innovative Manufacturing in Medical Devices These experimental and computational studies have shown that the most important factor influencing the wear of TKRs was the position of the relative contact point at the femoral component and tibial insert interface.

A composite cushion acetabular cup for a total hip replacement has been designed and developed jointly by Leeds University and DePuy International. In order to assess the long-term performance of this novel design, two sets of simulator tests of more than 4 million cycles duration have been carried out with the cushion bearings using the Leeds PA hip joint simulator with bovine serum as the lubricant. The results of these simulator tests were compared to the results from a previously reported study that used 32 mm ultrahigh molecular weight polyethylene (UHMWPE) acetabular cups. Under a physiological walking cycle simulation, with continuous cyclic motion and loading, the composite cushion cups produced negligible wear compared to a volumetric wear rate of 32 mm3 per million cycles for the conventional UHMWPE acetabular cups. This study has demonstrated for the first time the beneficial effects of fluid film lubrication in reducing wear in composite cushion acetabular cups.

The results of hip simulator tests on a total of 16 total hip joints, all of them 22.25 mm Charnley designs, are presented. Wear at up to 6.75 million cycles was assessed by using a coordinate measuring machine. The results gave good agreement with clinical estimates of wear rate on the same design of joint replacement from a number of sources. Good agreement was also obtained when comparison was made with the published results from more sophisticated simulators. The major source of variation in the results was found to occur in the first million cycles where creep predominates. The results of this study support the use of this type of simplified simulator for estimating wear in a total hip prosthesis. The capability to test a significant number of joints simultaneously may make this mechanism preferable to more complex machines in many cases.

A number of total hip components explanted at revision with bearing surfaces in either cobalt-chromium-molybdenum alloy or titanium-6% aluminium-4% vanadium alloy were examined and compared to contemporaneously manufactured but unused items; particular attention was paid to the bearing surfaces which were examined visually, by low-power microscopy, scanning electron microscopy (SEM), confocal microscopy, white light interferometry, laser profilometry and conventional stylus profilometry. The cobalt alloy heads maintained their surface finish well over periods up to 12 years. The titanium implants became badly damaged over much shorter periods although even badly scratched heads continued to meet the current standards for titanium alloy heads. Analysis showed that the damage to the titanium alloy heads was not a random but a well-defined process of scarring of a consistent size created by abrasion with small particles of bone. These damaged heads had the potential to wear the matching UHMWPE components rapidly creating large amounts of polymer debris. The finding that measurement of these damaged heads is within current standards raises concerns as to whether current standards incorporate fully the requirements for clinical performance.

A pin-on-plate reciprocating machine has been used to assess the wear performance of ultrahigh molecular weight polyethylene when sliding against untreated, titanium nitride-coated, and ‘Hardcor’-treated stainless steel counterfaces. The initial surface roughness of the stainless steel counterfaces was smooth (Ra = 0.01–0.02 μm). Following the surface treatments the surfaces were significantly roughened. In the first set of wear tests run in water the wear rates of the UHMWPE on the roughened treated surfaces were significantly greater than on the smooth untreated stainless steel surface. Both sets of treated surfaces were then polished to the same smoothness as the initial untreated surface. In the second set of tests, run with bovine serum as a lubricant, there was no significant difference in the wear rates between the three counterfaces.

It is necessary to accurately measure the change in volume of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups during hip simulator tests in order to determine wear rates. This study investigates the accuracy of dimensional measurement of an acetabular cup socket using two different coordinate measuring machines. The repeatability of the measurement was found to be primarily dependent upon the accuracy of relocation of the datum before each set of measurements.