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Total hip replacement is one
of the most frequently performed, universally successful
surgical procedures ever developed in the history of
medicine. The procedure is not only expected to dramatically
improve the quality of one's life, but also to last an
extremely long time and hopefully for the duration of an
individual’s lifespan. To understand the complexity of the
emerging concerns of metal on metal as a bearing surface,
one must first understand what represents a total hip
replacement.
A total hip replacement, quite simply will
reconstruct the cup of the pelvis and the ball of the femur
with artificial components. In its basics, the components
replacing the cup and the ball must adhere to the underlying
bone of the pelvis and the upper femur. This may be
accomplished in a variety of methods, the most acceptable
being the use of surgical "cement" to anchor the components
in place and/or the use of implant coatings manufactured to
allow bone to grow directly to the components thus locking
them biologically to the underlying bone.
Breaking this down further, the cup can be replaced
with a one-piece component or modular components consisting
of an anchoring shell and then an insert placed into the
shell. On the femoral side, the components can consist of a
ball and stem, with the stem placed into the marrow of the
femur and anchored into place with or without cement. Most
often, the ball and the stem are modular and the ball can be
varied in size, material and mechanical properties. The ball
can further be replaced by placing a cap on top of the ball
of the femur that has been surgically altered to accept the
new cap, with or without cement, similar to the above
applications. The fixation of the shell of the cup and the
stem of the femur are predictable in their anticipated
adherence to the underlying bone. The use of surgical cement
as an anchoring substance has vastly published results
demonstrating the longevity of both the cup of the pelvis
and the femoral replacement, either cap or stem. Perhaps to
be over-simplistic, the adherence of the cup and the stem or
the cap has been universally successful with or without
cement and their immediate loosening or failure has more or
less been eradicated.
That being said, what are we concerned about as far
as the major cause of failure with hip arthroplasty? The
number one failure occurs as a result of the bearing surface
which represents the articulation of the ball with the cup.
The bearing surfaces currently available are metal on
polyethylene, ceramic on polyethylene, ceramic on ceramic,
ceramic on metal and metal on metal. On the cup side, the
bearing surface would be attached to the underlying shell
either in the factory or at the time of surgery or a
non-modular cemented polyethylene cup or metal cup. On the
femur side, whether it is metal or ceramic bearing, the ball
is tapped onto the stem of the femur or a non modular
situation where the cap is pressed directly onto the bone,
with or without cement.
Now that we understand the variety of components,
metal on polyethylene and ceramic on polyethylene are still
the most widely used bearing surfaces. Data regarding their
performance and functional outcomes is extensive. Both of
these bearing surfaces carry excellent long-term results,
with polyethylene wear as the number one cause of failure
and not the ceramic or metal. With new polyethylenes
available, there has been significantly reduced incidence of
failure and incorrect positioning of the cup most likely the
cause of failure which creates increased stress on the
polyethylene.
Now let's shift to the hard on hard (metal or
ceramic) bearing surfaces. These hard on hard bearing
surfaces do not have enough clinical data to make absolute
conclusions as to their predictable longevity at this time.
We know that ceramic on ceramic almost never shows any
significant wear. The problem with ceramic on ceramic is
that there is a reported incidence of noise being generated
from the bearing surface referred to as squeaking. This is
not an immediate problem by and large, but rather occurs
over time and is now reported in over 20% of cases in some
series. Ceramic on metal, unfortunately, does not have
extensive data and, therefore, no conclusions can be made
regarding its ultimate wear predictability.
Finally, we come to the emerging problem of metal
on metal as a bearing surface. Metal on metal is not a new
bearing surface as it has been around for over 50 years. The
reason why no solid conclusions can be made is simply
because of the variability in the manufacturing techniques
and materials of both the cup and the ball, the surgical
techniques to position the cup and the ball have changed
over time, and the number necessary to generate the ultimate
success or failure of these components is virtually
impossible to determine. So what is alarming? What is
alarming is this; we are now seeing an upward trend in
failure of metal on metal components that is occurring at
eight years and beyond. The first thing that is
well-documented is that metal on metal creates a metallic
debris situation of not only small particles that accumulate
regionally around the hip joint, but also systemically in
organs throughout the body. The increase in the particles is
reported as high as 75% and more above normal. Equally
alarming is the fact that metal ions are released into the
system and also are absorbed not only locally, but
potentially in all body organs. The problem with metal ion
debris is that chromium and cobalt are known carcinogens.
Again, because of the extreme difficulty in amassing numbers
to come up with logical conclusions, there is no proof that
metal ion debris will cause increased cancer. However, it is
universally accepted that this is a concern. So since this
is not a proven entity, we will ignore cancer for now,
What we can't ignore
is the fact that these components are, in fact, debonding
from the bone at an alarming rate. The debonding is not on
the femoral side by and large, but rather on the cup side.
The largest orthopedic companies in the world have spent
considerable monies marketing these products and have been
forced to withdraw them from the orthopaedic marketplace. In
particular, Zimmer has already announced major recalls and
even though there is no class action suit, this most likely
is soon to happen, DePuy (Johnson & Johnson) has announced a
recall of one of its metal shells at the end of this past
year because of failure and Smith & Nephew's metal product,
even though not recalled and extremely popular outside the
United States, is now being closely reevaluated. The number
one surgeon in England and the biggest proponent of this
Birmingham Hip System have discussed with
America's number one metal on metal
expert and proponent the fact that major concerns are now
arising in
England
regarding not only loosening, but some type of immunologic
process that has disastrous consequences.
Over 15 years ago, I was asked to be one of the
investigators for metal on metal components and declined. I
have significant concerns regarding the articular surface
and the longevity of various products.
I have personally re-operated on patients with
failed metal on metal components with loose cups, and at the
time of surgery noted a milky-like substance that was so
alarming that I felt we were dealing with an infection.
After discussions with several extremely knowledgeable
metal on metal proponents, it appears that this situation
is, in fact, not new and has lead the number one proponent
of metal on metal as a bearing surface in the United States
to recommend that larger metal balls in combination with
metal cups no longer be used. In addition, the same
physician has recommended that if a metal on metal bearing
surface is to be used; a small ball should be used with a
stem and not a cap placed on the femur, commonly referred to
as a resurfacing procedure. As you may have guessed, this
topic is nothing short of explosive in the orthopedic
community.
Throughout the
country, and particularly in Southern
Florida, metal on metal and resurfacing
procedures have been widely touted as the procedure of
choice for young people. The reason for this is the
preservation of more bone on the femur side, so that if and
when another operation is needed, more bone will be
available to reconstruct the femur. This, of course, implies
that by using this device and using this explanation,
someone is anticipating failure of these components. Another
curious fact is that the resurfacing procedures are not
recommended in women of childbearing ages. If this procedure
is so good that it is recommended for young people, why are
young females in childbearing years being advised not to use
the components? It appears that there is, indeed, a concern
that the debris generated from these components may have
serious consequences on a fetus.
So why have I never used metal on metal bearing
surfaces, and in particular, the resurfacing arthroplasty?
The answer never sounded right. It still does not sound
right. There is something wrong with this situation that
from this point on is only going to get worse. Please
remember, not everybody with a metal on metal bearing
surface is going to have a failure, some of metal on metal's
biggest proponents have said emphatically that this is not
an issue if the components are placed properly, but rather,
it is more of a technically related issue, which simply
implies that they are not being manufactured correctly or
are not being placed in the proper positions. While the
manufacturing issue is completely out of my realm, the
technical issues are pretty straightforward to me. No one,
no matter how good they think they are, can be perfect every
single time. If these components and this bearing surface
represent an unforgiving operation, then again I quite
simply am not going to do this, and will not subject any
patient to something that I do not think is appropriate.
After all is said and done, I have the distinct
ability to look back over 20 years in my private practice in
South Florida and can honestly tell you that the failure
rate of our total hips are less than 1%.
Given the odds, with
the success rate in the area of 98-99%, why on earth am I
going to change to a bearing surface with all the potential
disasters that I have listed above?
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