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Questions and Answers about the Cormet by Corin LTD marketed by Stryker in the
US
1. What is the difference between the BHR and the Cormet physically? Is the
metal alloy different, is the acetabular cup different and is the cap and stem
different?
The BHR and the Cormet Hip Resurfacing (CHS) device are both devices used for
hip resurfacing as opposed to devices that are used for total hip replacement.
Hip resurfacing with these newer devices (the BHR & CHS are FDA approved
products in the US, the ReCap (Biomet), Conserve Plus (Wright Medical), Durom (Sulzer/Zimmer)
are not FDA approved products in the US) is a rebirth of a relatively old
concept in adult reconstructive surgery in the US. Early efforts at hip
arthroplasty in the 30’s sought to “cap” the femoral head – and were considered
interpositional arthroplasties – devices to put artificial material between 2
painful bone surfaces at the hip (femoral head and acetabulum). In the 1970’s,
this concept was reborn as cemented “resurfacing total hip arthroplasty” as a
means of avoiding placing a cemented femoral component into the proximal femur.
These designs involved the use of a cemented all-polyethylene (actually – ultra
high molecular weight polyethylene – or UHMWPE) acetabular component which was
cemented into place. These were designed to avoid the devastating effects of
loosened cemented femoral components, but unfortunately resulted in devastating
problems on the acetabular side. Many femoral components survived for many years
with this approach – but most devices failed within 10-15 years following
implantation. Dr. Harlan Amstutz of Los Angeles was one of the pioneers of this
operation.
As scientists continued to study successful and unsuccessful results of hip
replacements, sometimes devices surviving up to 20+ years, metal-on-metal (MOM)
bearings (first introduced as a bearing surface in Europe in the 1960’s) emerged
as a very reasonable alternative bearing for total hip replacement. Dr. Amstutz
(US), Dr. Wagner (Germany) and Mr. McMinn (UK) were pioneers in revisiting the
resurfacing operation in the early 1990’s. The early McMinn experience with MOM
resurfacing was actually with devices manufactured for him by Corin in the early
1990’s. During this experience he had a series of hip implantations that were
unsuccessful and he believed it was directly attributable to manufacturing
processes used in making these devices. He parted ways with Corin in
approximately 1995 when he founded Midland Medical Technologies (the
manufacturer of the Birmingham device). He became a champion of that device, the
as-cast technology engendered in that device, and the technique and training
involved in implanting that device. Corin changed its manufacturing process and
developed the Cormet Hip Resurfacing device, which is the device presently
marketed in the US through a partnership with Stryker.
The reason to make this lengthy explanation is that it is important for you and
for patients to realize that the long term success of an operation is due to the
interaction of three components: the device, the surgery to implant the device,
and the patient’s use of the device. In addition, there are many similarities
between these two resurfacing devices because the initial experiences of the MOM
resurfacing operation were shared (McMinn worked with Corin).
The differences between the devices themselves are going to be subtle ones – as
both have to have the basic features of a MOM resurfacing device – a femoral
“cup” or “ball”, and a single-piece acetabular component. Both devices feature a
bearing surface of high-carbon content Chrome-Cobalt. Both components feature
geometries and clearances that allow for lubrication of the joint (synovial
fluid) in such a way as to maximize range of allowable motion with extremely low
wear rates. The BHR does this with “as-cast” Chrome-Cobalt components with
porous outer surfaces on the acetabulum to allow bone-growth fixation. The CHS
employs a double-heat treated surface (to achieve hardness, to allow
predictability of manufacture to achieve sphericity and clearance), and uses a
titanium plasma-sprayed outer surface to achieve bone attachment of the
acetabulum. Both have Hydroxylapatite (HA) coatings to assist bone attachment.
In the US, both femoral components are to be used with cement (non-cemented
devices are available outside of the US). There are slight differences in
designs as they try to optimize predictability of fixation and stress transfer.
The BHR supporters have chosen to make the “double-heat-treated” technology a
point of contention – based in large part up Mr. McMinn’s personal experience
with a device that behaved poorly. This is a bit of an injustice to a technology
that is widely used throughout the orthopaedic industry and is, in fact, the
most commonly used technology for the manufacture of MOM components (and all
other resurfacing devices). Scientifically, the issues that are important to the
metallurgy are clearance, hardness, lubrication, sphericity, and a number of
issues relating to the predictability of manufacture. Both technologies achieve
this, and patients should feel comfortable that they are receiving very highly
successful and high performing (and high performance) devices when their hip
operation is done with these materials.
2. Is the instrumentation used during surgery for the Cormet better than for
the BHR in your opinion?
As I mentioned above, during my tenure as Chairman of the Biomedical Engineering
committee of the American Academy of Orthopaedic Surgeons, we consistently
confirmed that the long-term success of a joint replacement operation was
dependent of the use of well designed and manufactured devices, implanted
properly, and used properly by patients. The resurfacing operation is a
technically demanding operation. It is a form of total hip replacement, but it
involves preservation of the upper portions of the femoral head and neck. As
such, instruments to aid in exposure of the hip, and to properly position
components in their respective bones are important. In my hands, I find the
Cormet instruments easier to use to achieve the goals of proper exposure,
positioning and implantation of the components. It is an easier set of
instruments to use, and I believe, allows for more predictable instruction of
surgeons who wish to learn this resurfacing operation.
3. Are you using Computer Aided Surgery for placement of the Cormet?
I have helped Stryker develop a program to use computer assisted surgery (CAS)
techniques to implant this device. By FDA rules, we are not allowed to teach new
surgeons to use the Cormet device using the CAS program from Stryker, as the
instruments used for the implantation trial have to be used in the instruction.
For those who frequently use CAS techniques these computer approaches will be
easy to learn and will be useful. There is some evidence from the UK (Professor
J Cobb) that computer aided placement of resurfacing devices will be more
predictable – and therefore contribute to the longevity of an implant. While I
use computer assisted surgery extensively in other areas (such as total knee
arthroplasty) I do not currently use it for resurfacing THA in my hospital
because it requires a computer hardware and software upgrade that I do not yet
have. (I have been promised it though!)
4. Why do you like the Cormet better than the BHR from a surgeon’s point of
view?
For the US surgeon, I prefer the Cormet device because I believe the Cormet
approach does a better job of helping the American surgeon provide the American
patient with a durable and predictable long-term successful result. As I have
mentioned above, the long-term success of such an operation depends on using a
well thought out, well designed and manufactured device, making certain that it
is implanted properly, and being certain that is placed in the appropriate
patient. We have been very fortunate in the US to have many different
technologies and well designed/manufactured devices available for addressing
end-stage hip disease. We have technologies that are now showing successes out
to 25 years. This has not been true for our colleagues around the world. Corin
submitted the Cormet device to the US for clinical trial in 2001, and committed
itself and its investigators to learning about the operation and the devices.
This is a very important step in the process of new technology introduction, as
surgeons and companies find it easy to promote operations and devices, but don’t
always provide the necessary steps to insure that the introduction of that
technology to a broader population can be done safely. The BHR has enjoyed
widespread success worldwide because of the significant educational efforts of
Mr. McMinn and his colleagues. They have responsibly introduced the resurfacing
operation, and educated surgeons and patients on its performance. I am concerned
that that has not been the case in the US, where FDA approval was granted based
upon Mr. McMinn’s significant success, but few American surgeons had come into
contact with the device and instruments. Corin, however, has performed a
clinical trial with the Cormet device, and with its partnership with Stryker,
has committed to the most thorough, consistent, and vigorous training and
education program in the US. As is Mr. McMinn with the BHR device, I am biased
about the product and support behind the Cormet device (I am a consultant
surgeon for Stryker and a part of their teaching program) because I believe that
it will allow American patients to receive an excellent device, implanted
properly in the appropriate patient, and with the best chance for long-term
success.
5. Is the Cormet still double heat treated? If so, can you provide me studies
and information that indicate that this is not a problem as McMinn seems to
indicate?
I can provide you with two avenues of investigation. Corin/Stryker has provided
me with the following information listed below. As a clinician, though, I
respect Mr. McMinn’s enormous efforts and energies to bring this technology
safely to the patient population it is appropriate for.,I do not, however,
find Mr. McMinn’s arguments convincing. He is confusing a technology (double
heat treatment) – with a specific device (the McMinn device made by Corin) –
that he used in 1995. That is neither the present Cormet device nor other
contemporary devices.
From Corin: The Cormet devices are ‘double-heat treated’. This is a
standard manufacturing process for many metal-on-metal orthopedic devices
manufactured today. The first treatment is called hot isostatic pressing and it
eliminates the porosity (tiny holes) that could be present in the mating
surfaces of an ‘as-cast’ component. The second treatment is called solution
annealing and this ensures a uniform distribution of small carbides allowing
some carbon to dissolve into the material creating a hardening effect. The
effects of this process versus ‘as-cast’ components has been tested on hip
simulators (hip simulator testing is considered the gold standard for testing
hip prostheses in the orthopedic industry).
Double heat-treated vs. as-cast under sever gait conditions. Proc. IMchE Vol.220
Part H: J. Engineering in Medicine
- Published and peer-reviewed study
- Similar wear characteristics reported for both devices
6. Are there more available sizes of the Cormet than the BHR?
I do know that Cormet now offers sizes in 2mm increments on the femoral side
(similar to the Wright C+) and has developed corresponding acetabular components
that now double the number of device options previously available. These have
been available and cleared through the FDA as of January 1, 2008. I believe that
the BHR was planning to do the same, but don’t know if that has yet happened. I
should tell you that, based on results of the Cormet experience in a US
population performed as part of the investigational study for the FDA, Corin/Stryker
do not offer the smallest size femoral component in the US (40mm femoral head
with 46mm acetabular component). They found, as Dr. Amstutz has reported, that
small sizes have been associated with a much higher failure rate than
anticipated and did not believe it safe for introduction in the US.
I hope this answers your questions in a way that is helpful.
I wish to applaud your efforts to bring an understanding of these technologies
to those patients who would like to learn more. If there are any additional ways
that I can be of help I would be happy to do so.
Dr. Bernard Stulberg
Cleveland Clinic
Center for Joint Reconstruction
1730 W 25th Street
Cleveland Ohio 44113
Ph: 216-363-3300
Fax: 216-736-7969
Email:
bstulberg.ccjr@sbcglobal.net
September 2008
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