| 22-Dec-2011
NIH researchers uncover clues related to metal-on-metal
hip implants
Lubricant in devices found to be graphite, not proteins
A new study, bringing together an interdisciplinary team of
physicians and engineers from the United States and Germany,
made a surprising finding about implants used in hip
replacement surgery: Graphite carbon is a key element in the
lubricating layer that forms on metal-on-metal hip implants.
The lubricant has more in common with the lubrication of a
combustion engine than that of a natural joint. The study
was funded by the National Institute of Arthritis and
Musculoskeletal and Skin Diseases (NIAMS), part of the
National Institutes of Health.
Made possible by an American Recovery and Reinvestment Act
grant and to be reported online in the Dec. 23 issue of
Science, “This finding opens new avenues of investigation to
help scientists understand how joint implants function, and
to develop strategies to make them function better,” said
NIAMS Director Stephen I. Katz, M.D., Ph.D. “The results of
such research could have important implications for several
hundred thousand Americans who undergo hip replacement each
year—as well as those who could benefit from the procedure,
but have been advised by their doctors to delay surgery
until they are older.”
Touted as one of the greatest advances in arthritis
treatment in history, hip replacement involves removing the
damaged hip and replacing it with a prosthesis to mimic the
natural ball-in-socket joint.
“For most people, the procedure brings relief from pain and
a return to normal function for the life of the prosthesis,
typically more than 10 years,” said Joshua J. Jacobs, M.D.,
lead investigator and chair of the Department of Orthopaedic
Surgery at Rush University Medical Center in Chicago. But
for younger, more active people, the prostheses’ limited
longevity often means postponing surgery—often for a number
of years, or having the surgery and facing the prospect of a
more difficult repeat surgery at some point when the
prosthesis fails. For that reason, scientists have sought
ways to improve the materials used.
One such way has been to design components with only
metal-bearing surfaces (so called metal-on-metal implants)
rather than a combination of metal- and polyethylene-bearing
surfaces that were used almost exclusively prior to the
1990s, and tended to break down over time. But
metal-on-metal implants, too, have issues.
“We know there are metal-on-metal systems that have not
performed well,” said Jacobs. “Problematic devices have
tended to release more metal debris through wear and
corrosion than devices that have performed well. This debris
can cause a local tissue response involving the bone,
ligaments, tendons and muscles around the hip.”
To better understand what happens in the artificial
joints—and consequently what might be improved upon—the
scientist turned to metal joint components that had been
removed in revision surgeries and a science called tribology,
which focuses on the phenomenon of friction, lubrication and
wear.
Earlier research by team members Alfons Fischer, Ph.D.,
professor of materials science and engineering at the
University of Duisburg-Essen, Germany, and Markus Wimmer,
Ph.D., associate professor of orthopaedic surgery also at
Rush, revealed that a lubricating layer forms on metallic
joints as a result of friction.
“There is good reason to believe that those layers form a
barrier to wear and corrosion on the surfaces of these
implants, so it certainly would behoove us to understand the
nature of these tribological reaction layers – what they are
made of, how they form, etc. – so that we may be able to use
this information to design metal-on-metal bearings going
forward that are far less susceptible to corrosion and
wear,” said Wimmer.
While researchers knew little about the layer, they assumed
that it was from proteins in the body that entered the joint
and somehow adhered to the surface of the implant. As such,
it would be, similar to lubrication in natural joints.
Instead, the scientists found that the layer actually
consists, at least in part—and perhaps in large part – of
graphitic carbon, a solid lubricant with industrial
applications. “This was quite a surprise, but the moment we
realized what had happened, many things suddenly started to
make sense,” said Laurence Marks, Ph.D., professor of
materials science and engineering at Northwestern
University, whose team led the experimental effort. “Knowing
that the structure is graphitic carbon really opens up the
possibility that we may be able to manipulate the system in
such a way as to produce graphitic surfaces. We now have a
target for how we can improve the performance of these
devices,” said Fischer. Marks is equally optimistic.
“Nowadays we can design new alloys to go in racing cars, so
we should be able to do this for implants that go into human
beings.”
The next phase, Jacobs said, is to relate that finding with
clinical outcomes -by examining the surfaces of
retrieved devices and correlating the observations with the
reason for removal. Marks also hopes to learn how cells are
affected if the graphite flakes off.
“As good as hip replacements are for people in their 60s and
70s, for people who are younger, and more active, there are
still question marks,” said Jacobs. “We are making a lot of
demands on the materials we are using if we want them to
last 30 or 40 years.” |
