A ten-station joint simulator was used to examine the wear properties of 18 total hip prostheses. The wear rates of polyethylene acetabular cups bearing against titanium alloy femoral components were compared to those with either Type 316 stainless steel or cobalt-chrome alloy controls. Three titanium alloy prostheses and three controls were tested from three different manufacturers. Wear was determined by weighing the acetabular cups, using soak-controls to correct for fluid absorption. One million cycles were run under physiological loading, with bovine serum lubrication. For two of the three sets of prostheses, the titanium alloy generated higher mean polyethylene wear rates than the controls. However, the difference was not statistically significant (p > 0.05). The titanium alloy components showed more extensive surface abrasion than either the stainless steel or cobalt-chrome components. This appeared to be an artifact of joint simulator testing, since such extensive surface abrasion was not observed on numerous prostheses removed from patients after several years of use. The third set of titanium alloy prostheses, which had been subjected to a special surface hardening process, showed virtually no surface abrasion, and the mean polyethylene wear rate was identical to that with the cobaltchrome controls. All of the polyethylene wear rates were in the range generally reported for total hip prostheses in clinical use.

The spectrum of effects leading to the bone loss observed around failing implants is explained by means of a wear-debris-activated, macrophagic osteolytic mechanism. This concept is presented as the universal failure mechanism for all arthroplasty components, irrespective of fixation mode. The early descriptions of this bone-destruction process can be traced back to various clinical reports in the early 1950s which described failure of polymeric hemiarthroplasty implants, such as nylon and polyethylene cup arthroplasties, and the early polymethyl methacrylate, short-stem, Judet implants. Thus, polymeric debris and macrophages appear to be particularly reactive agents in the bone lysis phenomenon seen around contemporary total hip designs, but any particulate material small enough to be phagocytosed may contribute to this mechanism (metallics or ceramics). In addition to wear at the articulating surfaces, micromotion at any interface also has the potential to produce wear particles, thereby adding to the osteolytic process. Thus, the three potential sources of wear debris in contemporary total joints are (1) metal/ultrahigh-molecular-weight polyethylene (UHMWPE) joint articulation, (2) implant/coating interfaces, and (3) implant/bone interfaces. Present and future designs of joint replacement components must emphasize material and design combinations that will reduce the incidence of debris formation. Such designs will extend the survival time of the implant and also preserve bone stock in the eventuality that revision becomes necessary.

Large stiff, small moderately stiff, and small flexible femoral total hip components cemented in the proximal femur have been investigated using a three-dimensional finite element model and validated using experimental strain gage techniques. A physiological load of three times body weight (2000 N), 18 deg from vertical was utilized to compute stresses in the bone, cement, and metal construct. For a well-fixed total hip replacement, the results show stem stresses to be between 5 and 20 percent of the reported fatigue limits for the metals analyzed. Cement stresses in the proximal-lateral region were negligible for all stems tested. In looking at the failure stress for cement, the proximal-medial cement stresses were seen to be only 4 to 6 percent of the reported compression failure criteria and the distal stresses only 5 to 10 percent of tensile failure criteria. The proximal-medial bone stress was 30 percent of normal for a titanium alloy small stem, 21 percent of normal for a moderately stiff small stem, and only 6 percent of normal for a large stiff stem. Distal bone stress on the medial cortex was 39 percent higher for the stiff stem than for the flexible stem and 20 percent higher laterally. In viewing these results in light of the stem design or femoral component failure criteria, the cement does not appear to be the weak link for the well-fixed total hip replacement, regardless of the stem design. If failure initiation does not begin within the cement in a well-fixed hip, physiological changes in the bone may be responsible for loosening at the cement-bone interface, and the cement may then be prejudiced. Some advantages of load transfer to bone are gained with the use of flexible stems, but the significance of these changes can only be confirmed or disputed with detailed clinical and radiographic follow-up.

In view of published comments that the wear resistance of Ti-6Al-4V alloy bearing against ultrahigh molecular weight polyethylene (UHMWPE) was not adequate for total joint replacements, the literature was reviewed and compared with ongoing hip simulator studies. Two types of laboratory tests were contrasted: those run under “clean” conditions and those run with intentional contamination by acrylic cement particles. Wear-screening devices as well as hip simulators were involved in both types of studies, and both produced a dichotomy of viewpoints on the suitability of the Ti-6Al-4V/UHMWPE combination. However, the two most recent studies with hip simulators indicated that the Ti-6Al-4V-UHMWPE combination performed acceptably well. The published data from a major series of Ti-6Al-4V hip replacements have established excellent clinical and radiographic results. In seven surgical revisitations, the Ti-6Al-4V/UHMWPE combinations showed no sign of adverse wear phenomena. Thus, the clinical observations are supportive of the results of the two most recent hip simulator studies. It may well be that the laboratory models produced a variety of severe test conditions that compromised the performance of the Ti-6Al-4V and resulted in contradictory data. The hip simulator studies are being continued to establish the limits of the laboratory models and their validity with respect to the clinical situation.