To The Editor:
I read with great interest "Total Hip Arthroplasty with
Use of the Metasul Metal-on-Metal Articulation" (82-A:
789-798, June 2000), by Dorr et al. The authors are to be commended
for their very thorough report on a new hip prosthesis and for underscoring
the problems associated with the introduction of a new joint prosthesis
into clinical practice. The report, however, still leaves some important
questions unanswered.
The paper describes the results with a new cup (the Weber cup) that
has a metallic articulating surface. This cup was paired with an "old" cementless
femoral prosthesis with the proprietary name Anatomic Porous Replacement-II
(APR-II).
There are two designs of the APR femoral prosthesis (APR-I and
APR-II), which are characterized by the different forms and coatings
of their stems. These two designs were originally used with proprietary
polyethylene cups and demonstrated different clinical results in
five reports on the APR prosthesis, all coauthored by Dr. Dorr,
that are registered in the Medline database. This information is,
however, not revealed in the present paper. Will the authors explain
the omission?
Use of the APR-I stem in conjunction with a polyethylene cup was
associated with loosening, mainly of the femoral stem, which had
a rate of revision of 16%1.
Thus, the results for this APR-I prosthesis were much worse than those
reported for other "standard" polyethylene-on-metal
hip prostheses2.
On the other hand, use of the APR-II stem in conjunction with a
polyethylene cup was associated with a rate of femoral loosening
of only 0.8% and was not associated with acetabular loosening
or osteolysis3. These results,
which the authors do not mention, were better than the results reported
in the present paper, in which the combination of the APR-II stem
and the Weber cup was associated with a mechanical failure rate
of 2%. Will the authors explain why they did not continue
with the APR-II polyethylene-on-metal prosthesis and why they changed to
the Weber metallic cup?
When the authors say that they changed to a metal-on-metal hip joint
implant because of the severe wear observed previously on their "modular
metal-on-polyethylene articulations," they do not tell
us that this severe polyethylene wear occurred in association with
an APR-I stem prosthesis and not with the APR-II stem prosthesis
used in the present study. Will the authors please explain this
omission?
There are four possible combinations of the two APR stems (I and
II) with the two cups (polyethylene and metallic surface). The authors
actually investigated the clinical efficacy of three of these four
combinations and reported their results in two previous papers1,3. One could conclude that the favorable
results described in the present paper are associated with the use
of the APR-II stem and not with the use of a cup with a metallic
articulating surface. Will the authors please comment on this alternative
conclusion?
From the patient’s point of view, every failure matters.
After fifty-six operations with the Weber cup in the present series,
there were a total of three revisions and one deep infection, for
a failure rate of 7.1% after a follow-up of only 5.2 years.
It must be acknowledged that the number of operations is very small,
the confidence intervals are large, and the validity of the results
is, thus, correspondingly low. Still, the total rate of failure
in the present series is substantially higher than the
failure rate associated with conventional hip prostheses2. Given this perspective, would the
authors divulge whether they still use the metal-on-metal prosthesis
in their patients?
In the Abstract of the paper, the authors say that "the
Metasul articulation appears to be particularly indicated for more
active patients," although the data in their paper do not
provide any evidence for that recommendation. The reduced wear rates associated
with metal-on-metal systems observed in the laboratory might have
been the basis for this recommendation. Will the authors please
explain their recommendation?
When Dorr and colleagues began to use the metal-on-metal prosthesis
in 1991, this prosthesis was an entirely new design (in Weber’s
words, "Zurück in die Zukunft" or "back
to the future"). Thus, will the authors please state whether
the use of the prosthesis was approved by an ethics committee, whether they
obtained an Investigational Device Exemption from the Food and Drug
Administration (FDA), and what information was given to the patients
about this new prosthesis before the operation?
Dorr and colleagues began to use the new prosthesis one year
after the enactment of the Safe Medical Devices Act of 1990. This
act requires the manufacturer to report known device failures to the
FDA. Thus, will the authors please provide any knowledge they have
about the number of failed Weber cups and APR-II prostheses registered
in the FDA database as failed devices? Furthermore, will the authors
please specify whether their statement in the present paper, "we
are not aware of any reports of disassembly of the metal surface
from the polyethylene," is based on information from the
FDA database?
Dorr et al. cited only one other paper that describes the clinical results
associated with the Weber prosthesis5,
and that study included the results of only 110 operations. However,
according to the manufacturer, over 100,000 Weber prostheses have
been sold6. The discrepancy between
the huge numbers of prostheses sold and the extremely scarce information
on this prosthesis in scientific journals contradicts the assertion
by Hurwitz et al.7 that physicians
obtain information primarily from medical journals. The production
of high-quality scientific information in serious orthopaedic journals obviously
cannot keep pace with the galloping development in the artificial
joints market. The editors of orthopaedic journals should ponder
this situation. As a remedy for this information lag, I suggest
the foundation of an information center, similar to the Cochrane
Collaboration, that will produce quick, simple, and easy access
to information about the ongoing developments in the field of joint
replacement.
L.D. Dorr, Z. Wan, D.B. Longjohn, B. Dubois, and R. Murken reply:
The letter from Dr. Surin raises some questions regarding the data
in our article, which was a report on the outcome of the use of
the Metasul metal-on-metal articulation in total hip arthroplasty.
In our series of patients, we observed no loose femoral components
and only one loose cup, which did allow us to further identify the
effects of particulate formation on bone. Acetabular osteolysis
was not observed in any of the hips, and calcar resorption was observed in
two hips. Dr. Surin is concerned with the femoral component that
we used and our failure to be more descriptive in regard to the
history of its use. However, since no femoral component was loose,
the femoral component is of little importance to the overall results
of the series. Although Dr. Surin describes the APR-II as an "old" femoral
component, we still use the APR-II with circumferential coating.
We published an updated report on the results with the APR-II in
September 20008.
A critical review of our previously published articles, as referenced
by Dr. Surin, would have answered most of the questions raised.
Dr. Surin’s comparison of prior APR studies with the current
Metasul series is not a correct comparison because prior studies
reported results using different techniques. The importance of reporting
the data on failure and success for future improvements with total
hip arthroplasty must be recognized and understood. The APR-II study
that was quoted3 emphasized the
problems of using a modular sleeve on the stem and the benefit of
circumferential coating. The wear rate was ominous.
Metasul was used not because of failure of the APR-II stem but because
the problem reported with the APR-II series3 was
that the linear wear of the polyethylene averaged 0.19 mm3 per year. This wear rate was no improvement
from that reported with the APR-I series (average linear wear rate,
0.20 mm3per year)1. We knew that wear of this magnitude
would cause osteolysis and failure of fixation sooner or later9. We needed a more reliable articulation
surface for the durability of the entire total hip replacement.
Dr. Surin’s concern with the reoperation rate for dislocations was
addressed in our first publication10.
The occurrence of increased complications is not unusual with use
of new technology and is a reason for limiting use of the technology
while complications are identified. In fact, the Weber cup series
was done as a limited series of operations prior to the study of
the Metasul articulation undertaken with an FDA Investigational
Device Exemption, and it was subsequently approved for community
use in August 1999.
Our report targeted the results experienced by the authors with the
Weber Metasul cup. It was not a report on the efficacy of the Safe
Medical Devices Act or on the overall national failures of an implant.
Except for reports on this subject by specialty societies, this
type of study is not commonly included in United States medical
journals.
Finally, the concern of Dr. Surin regarding the discrepancies
in information on implant usage was not a subject of this report, and
we will not comment on it. We do think that Hurwitz et al.7 are correct in their respect for medical
journals (which include Internet publications of journals) as a
primary source of information for physicians. The case of the Metasul
is a good example of physicians awaiting published results to use
a device because the percentage of its use in the United States
is still small compared with that of polyethylene. Many orthopaedic
surgeons in the United States are awaiting further published validation
of this articulation surface.