To The Editor:
In the article by Iida et al., Metallosis Due to Impingement Between the Socket and the Femoral Neck in a Metal-on-Metal Bearing Total Hip Prosthesis. A Case Report (81-A: 400-403, March 1999), the authors concluded that the presence of metallosis should be suspected even in asymptomatic patients who have a prosthesis with a Metasul bearing, and we are concerned regarding the use of titanium in the manufacture of this implant. We find this conclusion to be unjustified and unsupported by the facts presented in the study. The impingement damage described in their case report represents a malfunction of the implant that was most likely related to the position of the components4. The impingement resulted in titanium wear debris and secondary metallosis, which has been widely reported2,10-12. Impingement could have been avoided with the use of a larger femoral-head size since there is no significant increase in wear with a metal-on-metal bearing.
This case report implies that the risk of metallosis somehow increases with metal-on-metal bearings, presumably because of a concern about increased bearing wear. However, in the brief description of the retrieved components, the authors stated that there was no apparent damage to the metal cup (it would have been informative if they had clearly stated whether there was apparent damage to the femoral ball) and that no cobalt-chromium particles were detected in the tissues. Furthermore, the authors cited several retrieval studies in which similar socket-stem impingement was noted in the absence of marked bearing wear or damage. Metallosis has been reported in association with McKee-Farrar total hip replacements that had a loose acetabular component6. All of those implants had an equatorial bearing, which is now recognized as one of the main causes of the poor performance of this implant8.
The authors observed evidence of proximal stress-shielding on the radiographs made before the revision and suspected osteolysis in the region of the calcar and the greater trochanter. It would be interesting to know whether these questionable osteolytic lesions were present on the preoperative or immediate postoperative radiographs and whether tissue obtained from those sites corroborated the radiographic findings. Osteolysis was not clearly demonstrated on the radiograph made before the revision (Fig. 1 on page 401 of the article). How was osteolysis distinguished from stress-shielding? There was no histological verification of particle-induced osteolysis, and there was only a brief description of the pseudocapsule, in which numerous multinucleated giant cells and metallic particles were noted. Since giant cells are common in the presence of polymethylmethacrylate particles (which are likely to have been generated by the loose cemented acetabular component) and polyethylene particles (which are seen in tissues around Metasul bearings3), it would have been useful for the authors to have noted whether such particles were present in the tissues. Tissues from the osteolytic lesion should have been examined carefully for the presence of polymethylmethacrylate since polymethylmethacrylate-induced osteolysis associated with metal-on-metal implants has been reported12. Finally, while we agree that metallosis should be prevented and that a revision should be performed when there is evidence of excessive titanium-component wear, we believe that suspecting the presence of metallosis in all patients with a Metasul implant is unsupported.
Paul E. Beaulé, M.D., F.R.C.S.(C) Patricia Campbell, Ph.D. Harlan C. Amstutz, M.D.
Corresponding author: Paul E. Beaulé, M.D., F.R.C.S.(C), Joint Replacement Institute, 2400 South Flower Street, Los Angeles, California 90007
Dr. Iida, Dr. Kaneda, Dr. Takada, Dr. Uchida, Dr. Kawanabe, and Dr. Nakamura reply:
We appreciate the comments of Beaulé et al. It is true that an optimum range of motion, with minimal prosthetic impingement, may be achieved with a large head size and a medium neck length4. However, when choosing the optimal prosthesis for a particular patient, a surgeon is often restricted by various anatomical constraints4. In other words, it is difficult to avoid prosthetic impingement in every patient. Therefore, we cannot agree with their assertion that impingement could have been avoided with the use of a larger femoral-head size.
Although metallosis induced by titanium wear debris has been widely reported, the cause of the metallosis in our patient was different from that described in the studies cited by Beaulé et al. We did not implicate increased bearing wear as a cause of metallosis in our report. The wear of a metal-on-metal bearing was not our concern.
The main point of our article was to describe titanium-alloy metallosis due to impingement. We did not precisely describe the histological findings around the calcar region because osteolysis around that region was not extensive and because osteolysis related to metallic debris has been widely reported1,5,7,9. More precise analysis, including immunohistochemical studies, would be necessary to determine which types of particles - that is, metal, polyethylene, or polymethylmethacrylate - are responsible for osteolysis in any particular patient.
We simply wished to point out that careful follow-up is necessary for patients who have a Metasul prosthesis because the clinically important complication that we reported may develop insidiously.
Hirokazu Iida, M.D., Ph.D. Eishi Kaneda, M.D. Hideaki Takada, M.D. Kanji Uchida, M.D. Keiichi Kawanabe, M.D., Ph.D. Takashi Nakamura, M.D., Ph.D.
Corresponding author: Hirokazu Iida, M.D., Ph.D., Department of Orthopaedic Surgery, Faculty of Medicine Kyoto University, 54 Kawara-cho, Shogoin, Sakyo-ku, Kyoto 606, Japan
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