Wear of Polyethylene Cups in Total Hip Arthroplasty. A Study of Specimens Retrieved Post Mortem*

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Abstract
The wear of polyethylene components of total hip replacements has previously been studied radiographically and by analysis of retrieved components. The extent of wear, however, has largely been determined from components retrieved at reoperation and has therefore been based on a subset of patients in whom the prosthesis is more likely to show excessive wear. The study of cups retrieved post mortem provides a better indication of the extent of wear of well functioning components. Twenty-six polyethylene liners were retrieved at autopsy from nineteen patients, after a mean of ninety-one months (range, thirty-three to 206 months) in situ. Each component had articulated with a thirty-two-millimeter-diameter femoral head. The components were evaluated visually for evidence of polyethylene wear such as pitting, scratching, and burnishing. Additionally; a shadowgraph technique was used to examine molds of the inner surface of the liner to determine the direction and extent of wear. The mean extent of linear wear of the retrieved liners was 0.45 millimeter (range, 0.17 to 1.07 millimeters), and the mean rate of wear was 0.07 millimeter (range, 0.02 to 0.18 millimeter) per year. The mean volumetric wear was 245.3 cubic millimeters (range, 13.0 to 779.1 cubic millimeters), and the mean rate of volumetric wear was 39.8 cubic millimeters (range, 1.0 to 131.3 cubic millimeters) per year. The mean rate of wear for the twenty-six liners was 45 to 69 per cent less than the rates reported in the literature for polyethylene liners retrieved at reoperation. Examination of the articulating surface did not reveal gross evidence of surface failure such as delamination or fatigue cracks. In general, the surfaces were merely burnished and scratched. The rates of wear of press-fit, metal-backed liners were significantly higher than those of all-polyethylene cemented components (p < 0.05). Additionally, statistical analysis revealed no correlation between wear and the patient's age, weight, or gender; the duration of implantation; or the thickness of the polyethylene. These results indicate that wear of the polyethylene of well functioning hip replacements is not as excessive as reported previously.