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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.

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.

Changes to the bearing surfaces of eighteen uncemented total hip replacements retrieved at revision surgery were assessed by three-dimensional binocular microscopy, Rank Taylor Hobson talysurf measurements, scanning electron microscopy, and X-ray dispersive analysis. Abrasions on the non-articular surface of the polyethylene cups were present. Bone particles were found in tracks in the bearing surfaces of both the titanium femoral heads and the polyethylene cups and were responsible for wear of these surfaces. Although the wear of the femoral heads appeared substantial to naked eye examination, the surface finish of these surfaces remained within the British ISO standards for titanium alloy when assessed by the methods used above. This study concluded that direct contact between polyethylene and bone should be avoided in total hip arthroplasty and that ‘third body’ wear from bone particles occurred in these uncemented prostheses. Both components of this type of implant should be replaced at revision surgery and titanium should be avoided as a bearing surface in hip arthroplasty. Present methods of assessing the surface finish of titanium should be re-evaluated and more reliable ones considered.