Wear of Polyethylene Acetabular Components in Total Hip Arthroplasty: An Analysis of One Hundred and Twenty-Eight Components Retrieved at Autopsy or Revision Operations.

By Murali Jasty, M.D., Devon D. Goetz, M.D., Charles R. Bragdon, B.S., Kyla R. Lee, B.S., Amy E. Hanson, B.A., John R. Elder, and William H. Harris, M.D., Boston, Massachusetts

Investigation performed at the Orthopaedic Biomechanics Laboratory and the Hip and Implant Unit, Department of Orthopaedic Surgery, Massachusetts General Hospital and Harvard Medical School, Boston

Abstract: We evaluated the rates of volumetric wear and the patterns of wear of 128 acetabular components retrieved during an autopsy or a revision operation between one and twenty-one years after total hip arthroplasty. Twenty-two all-polyethylene components were retrieved at autopsy from hips that had been functioning well at the time of death (Group A). The remaining 106 components -- eighty-four all-polyethylene components (Group B) and twenty-two metal-backed components (Group C) -- were retrieved during revision operations. All 128 components had been inserted with cement.

The mean rate of volumetric wear, determined directly with a fluid-displacement method, was thirty-five cubic millimeters per year (range, eight to 116 cubic millimeters per year) for Group A, sixty-two cubic millimeters per year (range, eight to 256 cubic millimeters per year) for Group B, and ninety-four cubic millimeters per year (range, twelve to 284 cubic millimeters per year) for Group C.

Multivariate regression analysis showed a significant relationship (p < 0.05) between the size of the femoral head and the calculated mean annual rate of volumetric wear. The rate of volumetric wear was highest in association with thirty-two-millimeter femoral heads and lowest in association with twenty-two millimeter heads; according to linear regression analysis, this represented a 7.5 per cent increase (Group A) or a 10 per cent increase (Group B) in the rate of wear for every one-millimeter increase in the size of the head. Linear regression analysis also showed a significant relationship between the duration that the implant had been in situ and the rate of wear (p < 0.05), with the rate being highest initially after the operation and decreasing with an increasing duration in situ. With the numbers available, the patient’s age and gender and the side of the arthroplasty did not have a significant relationship to the annual rate of volumetric wear. Increased thickness of the polyethylene was related to a decreased rate of wear (p < 0.05) in the group of metal-backed components, which had a 25 per cent increase in thickness, but not in the other groups. The estimated median annual rates of wear, after adjustment of confounding variables to a hypothetical constant set of median values for the parameters (duration in situ, 132 months; diameter of the femoral head, twenty-six millimeters; and thickness of the polyethylene, eight millimeters), were significantly different among the three groups of components (p < 0.05).

Histological evaluation of the work surfaces showed the predominant mechanisms of wear to be abrasion and adhesion rather than fatigue-cracking or delamination. The highly worn areas were polished to a glassy finish on gross examination, but scanning electron microscopy showed numerous multidirectional scratches along with fine, drawn-out fibrils with a diameter of one micrometer or less oriented parallel to each other. These fibrils are the most likely source of submicrometer wear particles. Thus, wear appeared to occur mostly at the surface of the components and to be due to large-strain plastic deformation and orientation of the surface layers into fibrils that subsequently ruptured during multidirectional motion.

Effects of Sterilization Methods on the Performance of Total Hip Replacements

By Stephen Li, Ph.D., Senior Scientist, Bridgette D. Furman, Program Manager, Hospital for Special Surgery, Department of Biomechanics and Biomaterials, New York, New York

Summary: Acetabular cups sterilized by gamma irradiation in a very low oxygen environment provides a 26% reduction in hip simulator wear rates from components gamma sterilized in air and a 57% reduction from components ethylene oxide sterilized. Non-gamma irradiation methods do not cause cross linking and have been demonstrated to have higher wear.

Wear of High-Dose Gamma Irradiated Polyethylene in Total Joint Replacement - Long Term Radiological Evaluation

By Oonishi, H, Saito M, Kadoya Y Osaka-Minami National Hospital, Kawachinagano City, Osaka 586 Japan

Trans Orthop Res Soc, 97-17, 1998

Osteolysis induced by submicron ultra-high molecular weight polyethylene (UHMWPE:PE) particles from acetabular component has been recognized as the major cause of long term failure in total hip replacements (THR's). We previously reported an in vitro study which demonstrated an improved wear resistance of high-dose (100 MRad) gamma irradiated PE. We clinically used acetabular components made of this irradiated PE between 1971 and 1978. In the paper, the long-term wear characteristics of this irradiated socket was evaluated.

Results: Both irradiated and non-irradiated cups showed initial wear rates of 0.15 and 0.39 mm/year respectively. After two years, the wear rate decreased to steady state and the wear rate of 26 irradiated cups was measured to be 0.06 1 0.005 mm/year which was significantly smaller than that of non-irradiated sockets (0.29 1 0.015 mm/year).

Gamma irradiation increases the crosslinking of PE polymer chain thus potentially increases the wear resistance. This study demonstrated the significantly improved wear characteristics of high-dose gamma irradiated PE in long term clinical follow up.

Prospective Clinical and Joint Simulator Studies of a New Total Hip Arthroplasty Using Alumina Ceramic Heads and Cross-Linked Polyethylene Cups

By B.M. Wroblewski, P.D. Siney, D. Dowson, S.N. Collins From the Wrightington Hospital for Joint Disease, Wigan, and the University of Leeds, England

The Journal of Bone and Joint Surgery, March 1996, Vol. 78-B, No. 2.

We report the findings from independent prospective clinical and laboratory-based joint-simulator studies of the performance of ceramic femoral head of 22.225 mm diameter in cross-linked polyethylene (XLP) acetabular cups. We found remarkable qualitative and quantitative agreement between the clinical and simulator results for the wear characteristics with time, and confirmed that ceramic femoral heads penetrate the XLP cups at only about half the rate of otherwise comparable metal heads.

In the clinical study, 19 hips in 17 patients were followed for an average of 77 months. In the hip-joint simulator a similar prosthesis was tested for 7.3 million cycles. Both clinical and simulator results showed relatively high rates of penetration over the first 18 months of 1.5 million cycles, followed by a very much lower wear thereafter. Once an initial bedding-in of 0.2 mm to 0.4 mm had taken place the subsequent rates of penetration were very small. The initial clinical wear during bedding-in averaged 0.29 mm/year; subsequent progression was an order of magnitude lower at about 0.022 mm/year, lower than the 0.07 mm/year in metal-to-UHMWP Charnley LFA's.

Our results show the excellent tribological features of alumina-ceramic-to XLP implants, and also confirm the value of well-designed joint simulators for the evaluation of total joint replacements.