TO THE EDITOR:
I read with great interest "Wear of Polyethylene Acetabular Components in Total Hip Arthroplasty. An Analysis of One Hundred and Twenty-eight Components Retrieved at Autopsy or Revision Operations" (79-A: 349—358, March 1997), by Jasty et al. I offer two alternative explanations for observations recorded during the study—namely, the negative correlation between the rate of wear and the duration in situ and the rise in the rate of volumetric wear as the radius of the head increased.
Jasty et al. indicated that the negative association of the rate of debris production with the duration in situ is incompatible with the long-term increased rates of wear linked to the degradation of ultra-high molecular weight polyethylene. In fact, this observation can result entirely from the sampling involved in this type of retrieval study and, thus, such an inference cannot be drawn. The wear rates decrease as the duration in situ increases simply because the sockets that become loose as a result of a high rate of wear are revised earlier than those with a low rate of wear. Indeed, it is possible to observe an overall negative association between the rate of wear and the duration in situ while individual sockets have an increased rate of wear with time. The overall correlation is dependent on the relative sizes of the different contributory factors. The only method of determining the long-term variations in the rates of wear is sequential measurement of the penetration into the socket on radiographs. However, such methods of measuring wear incorporate a creep component and can be relatively inaccurate unless more sophisticated three-dimensional techniques are used. Furthermore, other factors, such as changes in the surface topography of the femoral head or the activity level of the patient, may contribute to any long-term increase in the rate of volumetric wear.
Second, the explanation that the increase in the rate of wear as a function of the radius of the head is due to variations in contact area is not supported either experimentally or theoretically. The volumetric wear of an ultra-high molecular weight polyethylene surface sliding against a hard counterface is found to be approximately proportional to the sliding distance and the load across the contact area. It is independent of the contact stress. The increase in the rate of volumetric wear observed by Jasty et al. can be explained by the increase in the sliding distance of the larger heads.
I congratulate Jasty et al. on their courage in reporting their data in full.
Richard M. Hall, Ph.D.: Division of Orthopaedic Surgery, St. James's University Hospital, Clinical Sciences Building, Level 5, Room 5.8, Beckett Street, Leeds LS9 7TF, England
Dr. Jasty, Dr. Goetz, Mr. Bragdon, Ms. Lee, Dr. Hanson, Mr. Elder, and Dr. Harris reply:
We thank Dr. Hall for his insightful comments. We agree that there may be other explanations for our observations. However, we believe that the explanations that we provided are justified for the following reasons.
The reduction in rates of wear as the duration in situ increases may be due to less wear in the longer-surviving specimens in the retrieval study. However, this observation was also made in recent radiographic studies that involved sophisticated methods, suggesting that wear occurs at a high rate initially but then decreases to a steady state after the wearing-in phenomenon, or creep, takes place2. We believe that this is the case.
We do not mean to imply, however, that long-term degradation of ultra-high molecular weight polyethylene will not lead to increased wear with time. We recently performed destructive testing on the specimens used in our study and found very little oxidative degradation. In a subsequent study, we examined a set of components with a high degree of oxidative degradation and found very high rates of wear1.
We also agree with Dr. Hall that wear is proportional to the sliding distance. We used the term surface area of contact to describe the total area of contact that is actually sliding. In other words, with a larger head, a larger area is undergoing wear, thus leading to a larger wear volume. The combination of larger contact area and larger travel distance through that contact area leads to greater wear with larger heads. We tried to convey the message that the rate of wear is less dependent on contact stress.
Murali Jasty, M.D.; Charles R. Bragdon, B.S.; Kyla R. Lee, B.S.; Amy E. Hanson, M.D.; John R. Elder; William H. Harris, M.D.: Department of Orthopaedic Surgery, Massachusetts General Hospital, Boston, Massachusetts 02114
Devon D. Goetz, M.D.: Des Moines Orthopaedic Surgeons, 6001 Westown Parkway, West Des Moines, Iowa 50266