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On the True Wear Rate of Ultra High-Molecular-Weight Polyethylene in the Total Hip Prosthesis^*

Robert M. Rose, SC.D., H. J. Nusbaum, PH.D., H. Schneider, S.B., M. Ries, S.B., I. Paul, PH.D., A. Crugnola, PH.D.^¶, S.R. Simon, M.D.^# and E. L. Radin, M.D.^#

From the Department of Materials Science and Engineering and the Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge

Six total hip prostheses of the Charnley-Mueller design from six different manufacturers were tested by total joint simulation for the equivalent of one year of use. Two Charnley prostheses were similarly tested for the equivalent of ten years of use. Dimensional changes of the acetabular components were similar to those in previous clinical and laboratory reports of wear, ranging from 0.035 to 0.1 millimeter per year. The true wear rates, as determined by recovery of the wear debris, ranged from 0.3 to 10.2 milligrams of debris per year. Such wear accounted for only small fractions (between 1 and 30 per cent) of the dimensional changes. Thus, most of the changes previously ascribed to wear are in fact due to creep or plastic flow. The smallest wear rate (0.3 milligram per year) was exhibited by the prosthesis in which the polymer had the highest molecular weight at the articular surface; the highest wear rate (10.2 milligrams per year) was exhibited by the prosthesis in which the polymer had the lowest molecular weight at the articular surface. The true wear rates of the Charnley prostheses for the equivalent of the tenth year of use were 0.76 and 1.1 milligrams of debris. Therefore, it is likely that no marked acceleration of wear rate occurs with time, and that an acetabular component that exhibits a low wear rate initially should continue to do so over the first decade of use. The faster-wearing prostheses also tended to release much larger particles of debris. If these results extrapolate to the long term, then the only significant problem associated with wear should be the tissue reaction to the debris, as the dimensional changes due to wear will not be large enough to impair mechanical function. A better understanding of the relationship between wear and distribution of molecular weight should lead to a solution of this problem as well.

CLINICAL RELEVANCE: Some total hip prostheses may release relatively large (ten milligrams per year) quantities of relatively coarse polyethylene wear debris into the surrounding tissues, whereas others have negligible rates of true wear in the absence of fragments of acrylic cement or other external abrasives. The difference in wear behavior is related to different molecular structures due to processing of the polyethylene, and should be controllable. Clinical roentgenographic evaluations of wear are mostly indicators of dimensional changes due to creep. Even the highest wear rates are only one-third of the total dimensional change. Thus, the chief clinical question is that of the biological effects of the debris and not of mechanical problems due to dimensional changes.

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