Schmalzried et al. are correct in stating that multiple factors affect the rate of polyethylene wear. The purpose of our first report3 was to provide early information to the orthopaedic community regarding accelerated wear of Hylamer liners. After at least two years of clinical follow-up, we observed a rate of revision of 3 per cent (five) of the first 143 liners that we had implanted. (Another implant was scheduled to be revised at the time of a later follow-up.) Three of the five patients were more than fifty-nine years old, none was obese, and two had inflammatory arthritis (presumably with a reduced level of activity). One patient had a femoral implant fixed without cement (presumably indicating an increased level of activity).
In our recent investigation6, we studied the entire group of patients in whom we had implanted Hylamer acetabular liners; our purpose was to try to answer some of the questions now raised by Schmalzried et al. While we could find no significant correlation between the rate of wear and the age, activity, or weight of the patient, some of our data could be interpreted as suggesting such a trend. If this were shown to be the case, we would not be surprised to find that patients who had a higher level of activity—that is, those who were younger, more active, and heavier—had more wear of a given material than their lighter and less active counterparts.
The question remains with regard to why a material that demonstrated satisfactory wear characteristics in vitro should be associated with accelerated wear in a sufficient number of patients to warrant these reports. The mean rate of wear of Hylamer in the laboratory is within the range of clinical wear rates reported for acetabular replacements implanted without cement1,4,5,8, yet a minority of patients who had these replacements without cement demonstrated accelerated wear. Schmalzried et al. suggested that accelerated wear occurs in patients who are more active. Such a hypothesis awaits proof that can be provided only by prospective studies.
Perhaps the most important question that the experience with Hylamer raises is why laboratory testing did not predict the clinical performance of this material. Unfortunately, it is not uncommon for a material to perform well in the laboratory but not match that performance in vivo. Charnley's experience with Teflon and the experience of Wright et al. with a previous so-called improved polyethylene (Poly II) are such examples. If the hip-simulator method of McKellop et al. demonstrated no difference or a reduction in the rate of wear of Hylamer compared with that of conventional polyethylene, but the clinical performance suggests otherwise, perhaps the question to be asked is whether the in vitro testing adequately replicated all of the forces imposed on polyethylene during activities of daily living. A recent study9 involving multiaxial loading of polyethylene suggested that the rate of wear may be much higher when the in vivo ranges of motion are replicated. In addition, the original wear tests on Hylamer were not performed on material that had been treated with gamma irradiation in air and that had been artificially aged. The results of testing under these conditions may be more revealing.
We are once again about to see the introduction of new and improved polyethylenes. They will have improved cross-linking, be sterilized in the absence of oxygen, and have been tested more rigorously than previous materials. Considering the experiences of the recent past, it would be prudent to carefully observe the patients in whom these implants are used and to prospectively record such factors as age, level of activity, weight, and type of counterface, so that we may better answer these important and vexing questions in the future.
R. Poss, M.D.
M. Spector, Ph.D.
Department of Orthopedic Surgery
Brigham and Women's Hospital
75 Francis Street
Boston, Massachusetts 02115
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this commentary. No funds were received in support of this commentary.