Dr. Amstutz
Dr. Gross
Dr. Su
Dr. Schmalzried
Dr. Amstutz
Metal-metal bearings have advantages for
active patients because their wear resistance is much greater
than that of polyethylene, they do not fracture and they perform
better with larger diameter balls. It has been known for three
decades that the serum and urine levels of cobalt and chromium
(the main elements in a metal-metal hip bearing) are increased
in patients with a metal-metal bearing. The health impact of
such ion levels has not been established and continues to be
debated. Trace element analysis requires very sensitive
equipment since the levels are in parts per billion (ppb).
Absent some environmental exposure, serum and urine levels of
cobalt and chromium are effectively zero in people with no
metallic implants. In patients with a well-functioning total
joint with a metal-on- polyethylene bearing, the serum levels
are generally less than 1 ppb. In patients with a
well-functioning prosthesis with a metal-metal bearing, the ion
levels are generally between 1 and 5 ppb. These levels are very
low and are 1/10th to 100 times less than posted industrial
toxic levels. If there is any increase in overall health risk to
patients with a metal-metal bearing prosthesis, it is very low.
The JRI continues to closely monitor patients with metal-metal
bearing prostheses to better define the benefits and risks of
this technology.
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Dr. Gross
There is one potential problem with
metal-on-metal bearings. Normal wear results in release of metal
particles into the body. One problem with any artificial bearing
surface is the wear regenerated by normal daily wear and tear.
Large volumes of plastic generated by metal on plastic
(traditional) replacements result in large amounts of bone
destruction, (osteolysis) around implants and has been the major
cause for failure in young patients. Both ceramic-on-ceramic and
metal-on-metal devices generate about 99% less wear debris than
traditional bearings and the debris generated seems to cause
less irritation to the bone than plastic debris does. There has
been some speculation of the potential for metal debris to cause
cancer. However, with careful studies to date, no links have
been demonstrated.
My opinion is that the advantage of metal on metal bearings
strongly outweighs the potential risks. Now we can return
patients to normal function with almost no restrictions and
expect their implants to last more than 10 years at very high
activity levels. Most patients will never require another
operation on their hip. If ceramic-on-ceramic or metal-on-crosslinked
polyethylene bearings are used, wear is also not a problem, but
significant restrictions remain. No running or jumping can be
allowed due to the risk of fracture of the implant: no crossing
of the legs or extreme bending can be allowed due to the risk of
dislocation. I am surprised that any person would choose
anything other than a large metal-on-metal bearing! |
Dr. Su
Metal ion dispersal
The current generation of surface replacements are
metal-on-metal bearings. That means both the ball and the socket
are made entirely of metal. Although this cuts down dramatically
on the wear and tear of the components, it has been shown to
cause metal ions to be dispersed through the body. Cobalt and
chromium ions are measurable in the blood stream, but have not
been shown to cause cancer or any other disease in humans.
Although the metal ions are measurable, no one knows what a safe
level is. Generally, people with functioning kidneys are able to
excrete the ions in their urine. |
Dr. SchmalzriedThe level of
metal ions depends most on the position of the implants. It
they are put in right, the level is low. The ion levels are no
higher for a well-positioned resurfacing than they are for a MoM
total hip. |
Additional articles about metal ions and hip resurfacing.
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