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.
"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."
