TO THE EDITOR:
The observations made by Devane et al. in "Measurement of Polyethylene Wear in Acetabular Components Inserted with and without Cement. A Randomized Trial" (79-A: 682—689, May 1997) are provocative. The object of the study was to evaluate and compare polyethylene wear in acetabular components inserted with and without cement as part of a total hip replacement. In order to generate meaningful data, an investigation should compare prosthetic components that are similar except for the mode of fixation. However, in this particular study, it is inferred that the acetabular components inserted with cement were similar to those inserted without it; this is erroneous. In fact, the acetabular components inserted without cement have since been removed from the market because of obvious design flaws, including titanium bearing surfaces; modularity with a poor hexagonal capture mechanism; and thin, non-conforming polyethylene. These findings have been described previously1. It is presumptuous to assume that fixation of acetabular components without cement is tenuous if the conclusions are based on components with major design flaws.
Thomas H. Mallory, M.D.: Joint Implant Surgeons, 720 East Broad Street, Columbus, Ohio 43215
Mr. Devane, Dr. Robinson, Dr. Bourne, Dr. Rorabeck, Dr. Nayak, and Mr. Horne reply:
Dr. Mallory correctly points out that the acetabular component inserted without cement in our study was a first-generation component with several major design flaws and has since been withdrawn from the market. However, this so-called wear machine was compared with a cemented component that also has subsequently been proved to have major design flaws. We studied a metal-backed socket inserted with cement, a system that is no longer recommended by most surgeons because of concerns regarding the radiographic appearance at the metal-cement interface. However, with regard to polyethylene wear, the prosthesis designed to be inserted without cement proved greatly inferior to the prosthesis designed to be inserted with cement, despite the fact that both groups had what are now accepted as major design flaws.
We found osteolysis to be strongly associated with polyethylene wear only in the hips in which the prosthesis had been inserted without cement. This led us to conclude that a good early clinical result with this prosthesis did not necessarily lead to a good long-term result. This particular cup was designed by very able surgeons in consultation with engineers, yet it was found to have major design flaws. It is important that the theoretical correction of these flaws in the so-called second-generation cups designed to be inserted without cement be assessed carefully by measurement of polyethylene wear in clinical studies. The advantages of metal-backed acetabular components compared with acetabular components inserted with cement remain unproved.
Peter A. Devane, M.B., Ch.B.; J. Geoffrey Horne, M.B., Ch.B.: Department of Surgery, Wellington School of Medicine, P.O. Box 7343, Wellington South, New Zealand
Eric J. Robinson, M.D.; Robert B. Bourne, M.D.; Cecil H. Rorabeck, M.D.; Naresh N. Nayak, M.D.: Department of Orthopaedic Surgery, University Hospital, 339 Windermere Road, London, Ontario N6A 5A5, Canada