The controversy regarding adverse wear in metal-metal bearings

Thomas P. Gross, MD 3/5/2010

I have used over 3000 metal bearings in
primary total hip and hip resurfacing as well as revision
surgery. I have revised 2 for adverse wear 7 years after
implantation. I know that most other high volume hip
resurfacing surgeons have a similar experience. The
revisions were straightforward and the patient enjoyed the
same rapid and complete recovery as if she had a primary hip
replacement.

Currently less than 5% of my practice
involves revision surgery. However, I have revised over 100
metal plastic replacements for excess wear. Furthermore
significant wear related damage to the tissues is seen in
virtually all metal plastic hip replacement or knee
replacement revised for other causes.

A surgical group that has seen a
surprisingly large number of wear‐related failures of metal
bearing implants has coined the term “pseudotumor” when an
inflammatory soft tissue mass is seen around the hip of a
metal bearing implant. However, this inflammatory soft
tissue reaction to metal wear debris is not much different
than the inflammatory reaction that we have seen with
plastic wear debris for many years.

All artificial bearing implants give off
wear particles. The question is, which type of wear debris
is best tolerated by the body? During the last 20 years of
joint replacement polyethylene osteolysis (bone destruction
caused by plastic wear debris) has been a major problem. But
anyone who has revised total joints is also aware that
polyethelene debris also is always associated with large
amounts of soft tissue reaction around the joint.
Polyethelene has been improved, and metal bearings have been
developed. Both give off much less wear debris than the old
polyethelene implants. The question is which results in less
wear related damage? At this point we do not yet have the
answer. Adverse wear reaction is a serious problem, but
fortunately it is very rare.

Lets put this into perspective. The most
common reason resulting in revision of total hip
replacements in the US is hip instability (recurrent
dislocation). 20% of all hip revisions are done for this
reason. This is far more common than
adverse wear reaction. Hip instability is a very disabling
condition that occurs in 3‐5 % of hip replacements. The rate
of instability for large head metal bearings is less than
1/2 %. Larger bearings are the solution for this problem. Large
head metal bearings (resurfacing and total hip) are
currently the only ones that allow reconstructing the hip in
a biomechanically normal fashion to avoid instability.
Proponents of plastic and ceramic bearings realize this and
have made their bearings thinner recently to allow larger
heads to be inserted (32‐36mm). This has made them more
stable, but 32‐36mm does not yet approximate normal femoral
head sizes in the average female (48mm) and average male
(52mm) patients. These larger head (32‐36mm) implants for
plastic and ceramic bearings have only been in use for a few
years and it is not yet clear if these bearings will break
at a higher rate because they are thinner. I would not
recommend impact sports on thin plastic and ceramic
bearings. Anatomic sizing that matches the patient’s own size
is only possible with large metal head designs. These are
stable and can tolerate repetitive full impact without
breaking. Wear rates are not significantly increased by
running.

In the last few years we have learned that
these rare cases of adverse wear in
metal bearings are related to three factors: steep
acetabular inclination greater than 55 degrees, small
component sizes, certain component designs with an extremely
shallow arc of coverage. At this point it is still only a
very tiny percentage of patients with cup inclination angles
above 55 degrees that have had wear problems. If a patient
with an inclination angle above 55 degrees develops symptoms
years after surgery, I would first check metal levels and an
MRI. If the levels were high or a soft tissue mass developed
I would recommend revision. So far this has happened twice
in my practice.

More important, however, is prevention of
this adverse wear complication. Since this information about
cup inclination has become available several years ago we
developed and tested a protocol  for measuring the
inclination by XR during the operation. The paper reporting
this technique will be published in CORR this year. Using
this technique in every case, I now have had no
cups implanted with inclination greater than 55 degrees since
10/2007. We expect that this technique will completely
eliminate this rare cause of failure in metal bearing hip
implants: adverse wear reaction.