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. Beaulçª M.D., F.R.C.S.(C)
Patricia Campbell, Ph.D.
Harlan C. Amstutz, M.D.
Corresponding author: Paul E. Beaulçª 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 Beaulç?¥t 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 Beaulç?¥t
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