Targeted Muscle Reinnervation: The Future Is Now
Targeted muscle reinnervation surgery is visionary for the future and practical for today.
|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.|
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.
|Todd Kuiken, MD, PhD|
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.
|Douglas Smith, MD|
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."
|Illustration courtesy of Seattle Post-Intelligencer. Source: P-I reporting, DAVID BADDERS / SEATTLE P-I|
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 firstname.lastname@example.org