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Targeted Muscle Reinnervation: The Future Is Now
By Miki Fairley Targeted muscle reinnervation surgery is visionary for
the futureand practical for today.
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Kathy Stubblefield, OTR/L, watches patient Claudia Mitchell as she conducts functional testing with the six-motor neural-controlled prosthesis. Photograph courtesy of the Rehabilitation Institute of Chicago. |
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The astonishing explosion of futuristic devices and technology
being developed by universities and private companies, fueled by
funds from the Defense Advanced Research Projects Agency (DARPA),
hinge, for the moment, on this surgical technique pioneered by Todd
Kuiken, MD, PhD, notes an article in Wired magazine online
(August 7, 2007). Kuiken is the director of the Neural Engineering
Center for Artificial Limbs (NECAL) at the Rehabilitation Institute
of Chicago (RIC), Illinois.
The stories of shoulder-disarticulation amputees Jesse Sullivan
and Claudia Mitchell, who underwent the surgery and were fitted
with high-tech "thought-controlled" myoelectric prostheses at RIC,
made headlines around the world. However, it took a surgical
breakthrough to make possible the most effective use of this
prosthetic technology.
At the time of this article, five more persons at RIC and six at
Harborview Medical Center (HMC) in Seattle, Washington, have
undergone targeted muscle reinnervation (TMR) and have been fitted
with prostheses. Unheralded by the media, they have been going
about their daily lives enjoying improved prosthetic function and
quality of life. The surgical technique provides the complementary
functional ability for an amputee to more effectively use new
technologies as they are developed, while at the same time it
improves function for amputees using currently available
myoelectric technology.
The possibility of using nerve transfers to facilitate more
natural brain-thought prosthetic use has excited Kuiken since his
graduate student days 20 years ago, when a brief reference in a
paper he found while researching his thesis in biomedical
engineering caught his imagination.
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Todd Kuiken, MD, PhD |
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Today, TMR surgery is spreading beyond RIC, and that's what
Kuiken wants to see. Besides HMC, the procedure has been performed
at the University of Vienna Hospital in Austria, and will be
undertaken at Brooke Army Medical Center (BAMC), San Antonio,
Texas, and Walter Reed Army Medical Center (WRAMC) in Washington
DC, Kuiken notes.
"We've demonstrated enough efficacy now that it can be used for
patients without a research protocol," he says. "We're open to help
more people learn how to do it so it can be performed more widely.
We would like the prosthetic community to know that if a
prosthetist has a patient he or she thinks can benefit from the
surgery, we can work with other surgeons and prosthetists so that
it isn't necessary for all the patients to come to Chicago for
surgery and prosthetic fitting."
The first surgeon to perform the surgery outside Chicago was
Douglas Smith, MD, professor of orthopedic surgery at the
University of Washington, Seattle. Smith, a long-time friend and
colleague of Kuiken, was excited by Kuiken's vision. For about
three years, he worked with Kuiken and surgeon Gregory Dumanian,
MD, who performed the surgeries in Chicago, to prepare to use the
new technique in Seattle.
Smith notes how remarkable it is that this brilliant surgical
breakthrough came not from a surgeon, but from a physical medicine
and rehabilitation physician. Nerve transfers in other applications
have been done for decades, Smith explains, but Kuiken's vision
encompasses a new, unique, specialized use of the procedure.
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Douglas Smith, MD |
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Above-elbow amputees can greatly benefit from TMR surgery, Smith
says. However, he points out that that below-elbow amputees do not
really need this surgery since they still have small parts of the
muscles left in their forearm which fire appropriately when the
brain signals their hand to open or close. These signals can thus
enable the prosthetic device to operate with normal brain
thoughts.
The situation is much different for above-elbow amputees.
"Traditionally, about 80 percent of below-elbow amputees wear a
prosthesis every day, but only about 20 percent of above-elbow
amputees do so," Smith says. "They simply don't find them useful
enough. We hope to bring above-elbow amputees closer to the same
prosthetic functional ability as below-elbow users."
In above-elbow amputees, the muscles that normally operate the
forearm, wrist, and hand are gone, so the nerves that transmit the
brain's natural thought to open or close a hand simply reach a dead
end. "Transhumeral amputees have had to perform some strenuous
mental calisthenics to use muscles such as the biceps and triceps—
which are designed to move your shoulders and elbows—in order to
use prostheses," Smith explains. And of course, in shoulder-level
amputations, even these muscles are gone. "Patients' frustration
with not having a normal brain-thought reaction with the prosthesis
has always been a much larger area of concern than we had
realized," he adds.
With TMR, amputees now have the use of four signals—elbow up,
elbow down, hand open, and hand close—rather than the two
signals, elbow-up, elbow-down, available previously. Now hand
movements can be done quickly and intuitively with normal brain
thoughts. "Currently with a myoelectric prosthesis, you
sequentially control your elbow, then your wrist, then your hand
with the EMG [electromyographic] signals," Kuiken explains. "You
can also use a shoulder harness to control one function, but for
the most part, it's all sequential control. But with this
technique, we are allowing people to control their hand and elbow
at the same time, in a natural way."
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Illustration courtesy of Seattle Post-Intelligencer. Source: P-I reporting, DAVID BADDERS / SEATTLE P-I |
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Other Advantages
Successful TMR means that amputees now have a physiologic
conduit to the brain for controlling a prosthetic arm and hand,
using their body's own nerves and muscles. "Once it works, it will
keep on working," Smith says. Unlike an implanted chip or other
device, there is nothing to break down or wear out or which the
body might reject as foreign.
"The potential for the future is very exciting," says Kuiken.
"When the advanced arms and hands come out, they can be controlled
in a much better fashion."
In fact, some of the first DARPA prototype prostheses have been
delivered to RIC for trials, and initial results "are promising,"
Kuiken says.
Both Smith and Kuiken note that another advantage of the surgery
is that it often makes training amputees to use their new
myoelectric prostheses easier and faster.
"It's different because there are more signals, but in some ways
it's easier because it's more intuitive. You think 'open hand,' and
it happens; you think 'close hand,' and it happens," Kuiken says.
"You don't have to train the patient how to sequence through the
biceps and triceps to do all this."
An unexpected, pleasant byproduct of the nerve-transfer surgery
has been some sensory feel and feedback, similar to what happens
with a natural hand. "To our surprise, we are getting at least a
little crude sensory feeling," says Smith.
What the Surgery Involves
The human body is so generously endowed with sufficient nerves
and muscles that it is possible to remove an original nerve from a
portion of muscle that normally does something else and replace it
with a nerve that transmits brain signals for arm and hand
movements. The reinnervated muscle can now perform the function of
the amputated muscle. For instance, one of the two biceps muscles
and one of the three triceps muscles can be used for nerve
transfer, while the others are left with their original innervation
to continue their usual functions.
For transhumeral amputees, the musculocutanous nerve, which
controls shoulder and elbow movement, is removed from the medial
side of the biceps, which paralyzes that muscle but is left
remaining on the lateral biceps. The median nerve, which controls
forearm, wrist, and hand movement, is then transferred to the motor
point where musculocutaneous nerve originally entered the muscle.
If the transfer is successful, within two to six months the median
nerve will grow into the muscle, arborize, and find end points.
"It's not like hooking up an electrical wire, which works
immediately; it takes time for the nerves to arborize and grow into
the muscle," Smith explains. "Once the muscle is reinnervated and
starts to get signals, then patients need to build up the strength
of those signals by exercising that muscle. If they can generate a
signal that can be read by sensors, then they can use that signal
for prosthetic rehabilitation."
After a successful median nerve transfer, when the person thinks
about flexing his hand, making a fist, or closing his hand, the
rewired biceps will fire. However, the biceps that was not rewired
will not fire, but will still continue in its original purpose of
firing when the brain thinks about moving the elbow.
A similar procedure is followed with a nerve transfer on the
triceps. One of the triceps is paralyzed by removing the original
nerve, and then the distal part of the radial nerve, which controls
hand-open and wrist-up movements, is transferred.
For shoulder disarticulation amputees, the procedure utilizes
chest muscles. For instance, for one shoulder amputee, "Instead of
rewiring the medial biceps and lateral triceps, we rewired the
pectoralis major, the pectoralis minor, and the serratus anterior,"
Smith explains. "When rewiring chest muscles, you temporarily
paralyze them [by removing an original nerve] and then do nerve
transfers for the musculocutaneous, which controls elbow up; the
median, which is hand close; the ulnar, which, by spreading your
fingers, you can get hand open; and the radial, which does both
elbow down and hand open."
One major difference between RIC patients and Smith's patients
at Harborview, which is the only Level 1 trauma center for a
five-state region, is that all of Smith's patients have been
amputees due to trauma. For most of them, the nerve-transfer
surgery was performed along with the primary amputation surgery,
while the RIC patients had previously undergone amputation surgery
six to 18 months prior to nerve-transfer surgery.
The amount of time the surgery requires and the length of the
patients' hospital stay is considerably shorter than one might
expect.
For the trauma patients, nerve-transfer surgery added no
additional length to their hospital stay. For those having the
surgery at a later date, arm-level surgery required a day; the
shoulder-level patients needed a one- to three-day stay. "The
shoulder-level surgery is more complex and involves the brachial
plexus and the axillary and subclavian vessels," Smith says. The
length of stay has been about the same for the RIC patients,
according to Kuiken. Arm-level nerve-transfer surgery takes about
two hours, Smith says; the more complex shoulder-level surgery
requires four to six hours.
Amputees can also go about their regular life activities and
wear their usual prostheses while they are waiting for the
transferred nerves to reinnervate the targeted muscles.
More Advances to Come
As astonishing and promising as these advances are, much remains
to be done, according to Kuiken and Smith.
For the next generation of prosthetic technology, several
aspects need to advance and then come together. Under DARPA's
umbrella, new sensor technology capable of reading more complex
muscle signals, new actuators that relax and contract more like
muscles rather than spin like motors, and new energy sources to
overcome the limitations of batteries, are being researched and
developed, as well as new prosthetic designs to provide more
degrees of freedom.
Will future advances make using a prosthetic arm and hand as
fast and intuitive as using a natural arm and hand? "The human arm
is an incredible machine," Kuiken answers. "For instance, it has
70,000 sensors and tens of thousands of motor fibers. I'd be
reluctant to say that a prosthesis would ever match that, but we
are making leaps and bounds in producing better, easier to use, and
more natural arm systems."
And thanks to Kuiken's vision and the efforts of many others,
surgical advances and prosthetic technology are combining to come
ever closer to this ideal.
Miki Fairley is a freelance contributing editor and writer
for The O&P EDGE based in southwest Colorado. She can
be contacted via e-mail at miki.fairley@gmail.com 
Table Of Contents - December 2007
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