Surgery Improves Use of Artificial Arms

Study Shows Surgery Called TMR Allows Amputees to Have Better Control of Prostheses

Feb. 9, 2009 -- A new surgery may give people with an amputated arm better control of a new artificial limb.

The surgery, called targeted muscle reinnervation (TRM), allows amputees to control multiple parts of an artificial lim at the same time.

Current prostheses do not restore adequate function of the arm and hand. Most prostheses are body-powered; they capture remaining shoulder motion with a harness and transfer that movement through a cable to operate the hand, wrist, or elbow. This allows just one joint to be moved at a time.

Access to nerve-control information is lost during amputation. The TMR procedure transfers remaining arm nerves to chest or upper-arm muscles. Once nerve function has been restored through the surgery, these muscles provide correct electrical signals to control the elbow, wrist, and hand.

To test the effectiveness, researchers led by Todd Kuiken, MD, PhD, from the Rehabilitation Institute of Chicago, and colleagues tested five amputees who had had the surgery. A comparison group included five participants who had not had amputations. For the study, conducted between January 2007 and January 2008, participants performed various arm movements. Researchers tested their ability to control a virtual prosthetic arm.

While non-amputees were faster than TMR patients, the times were close. For instance, TMR patients were able to complete elbow and wrist movements in an average of 1.29 seconds, compared to 1.08 seconds for the non-amputee participants.

"These early trials demonstrate the feasibility of using TMR to control complex multifunction prostheses," the researchers write. "Additional research and development need to be conducted before field trials can be performed."

In an accompanying editorial, Gerald Loeb, MD, chief executive of SynTouch, a start-up company developing tactile sensor technology, said the speed and accuracy of the TMR patients' movements "represent substantial improvements over previous myoelectric systems."



Targeted Muscle Reinnervation Improves Use of Prosthetic Arm

February 8, 2007 Researchers are reporting another case where they have used targeted muscle reinnervation (TMR) a technique that involves transfer of residual nerves, in this case from an amputated arm into the chest of the patient to provide more intuitive use of a prosthetic arm as well as sensory feedback.

"The idea that we've been pursuing is the information that you need is still there in the residual nerves; the nerves that used to go down to the arm have all the motor commands and the sensation nerves that used to give feeling," lead author Todd A. Kuiken, MD, from the Neural Engineering Center for Artificial Limbs at the Rehabilitation Institute of Chicago, Illinois, told Medscape.

They transfer those nerves to spare muscle and skin in the patient's chest, using the muscle "as a biological amplifier of the nerve signals that we can then record from the surface and use to operate the prosthesis," he said. "In the same way, we use the skin as a transducer to the old hand sensation nerves, so now when you touch her chest in that area, she feels her hand."

Their report appears in the February 3 issue of the Lancet.

Retaining Sensory Pathways

The new technique uses residual nerves from the amputated limb and transfers them to alternative muscle groups that are no longer biomechanically functional since they are no longer attached to the missing arm, the researchers explain in their report. The target muscles are denervated, so that they can be reinnervated by residual nerves that served the arm before amputation.

The result is that the brain's motor commands for the amputated arm and the sensory feedback from the amputated hand can be salvaged and used in the operation of a prosthesis. Dr. Kuiken compared the concept to a broken telephone: the handset that replaces it may be new, but the conversation still proceeds on the same hardwired cables, essentially unchanged by the new equipment.

For example, the authors note, transferring the median nerve to a segment of pectoralis muscle provides a hand-close myoelectric signal; the patient thinks about closing her hand and the median-nerve-reinnervated section of the pectoralis muscle contracts. The myoelectric signal then provides control input for a motorized hand.

"By transferring multiple nerves, TMR myoelectric signals allow intuitive, simultaneous control of multiple joints in an advanced prosthesis," the authors write.

Similarly, the transfer of residual hand nerves in concert with sensors in a prosthetic hand could provide the patient with a sense of touch in the missing limb.

New Challenges

The patient described in the current report is actually the fifth patient to receive this procedure. The first, Jesse Sullivan, made headlines in 2006. Since then, the researchers have used the procedure in 3 other patients, with success in 2. The last case was unsuccessful due to nerve damage that was not apparent until the time of operation, Dr. Kuiken noted.

However, this patient, a 24-year-old woman with a left arm amputation at the humeral neck due to a motorcycle accident, "presented some new challenges," he said. First, in previous patients, all fat was removed from pectoral muscles so that the electrodes would have proximity to the reinnervated muscles. For a female patient, this would essentially mean a mastectomy. In this case, then, they developed a new approach using muscles above and to the side of the breast, he said.

In addition, they hoped to redirect nerves to create targeted sensory reinnervation, the sense of touch that had previously developed unexpectedly in their first patient.

In this patient, the ulnar, media, musculocutaneous, and distal radial nerves were transferred to separate segments of her pectoral and serratus muscles. In addition, 2 sensory nerves were cut and the distal ends anastomosed to the ulnar and median nerves. After she recovered, she was fitted with a new prosthesis using these additional TMR sites. The patient described the control as "intuitive," they note; "she thought about using her hand or elbow and the prosthesis responded appropriately."

She had "substantial improvements" in functional testing; for example, mean scores on the "blocks-and-box" test increased from about 4 with the conventional prosthesis to 15 with the new prosthesis. After reinnervation with both ulnar and medial nerves, the patient felt her hand was touched when this skin on her chest was touched, with near-normal thresholds in all sensory modalities, they noted.

"This patient and our other 3 patients represent early application of the targeted reinnervation technique," the authors conclude. "Whether the improved function is enough to keep these patients wearing their devices in years to come, or whether they adapt to their new control even better and show greater functional gains, remains to be seen. Long-term follow-up is also needed to see how our patients' transfer sensation evolves."

In the meantime, Dr. Kuiken says he'd like to see the technique be brought into wider use in other centers. "I think we've had successful enough results that we're ready to say this is no longer experimental, and we can start doing this just for the clinical benefit to people, without big grants and studying it from top to toe. We want it to be tried in other places by other people so that patients get the advantage of it and we learn more."

If it is successful, this could change the way amputation surgery is approached, leaving nerves a bit longer rather than cutting them as short as possible, for example. In addition, there's "a lot of exciting neuroscience" to go along with it in terms of brain plasticity in response to this procedure, he said.

Toward "Seamless Integration" of Prosthetic Limbs

A Commentary accompanying the paper is provided by Leigh R. Hochberg, MD, from the Department of Veterans Affairs Center for Restorative and Regenerative Medicine and Rehabilitation and Development Service, in Providence, Rhode Island, and Massachusetts General and Brigham and Women's Hospitals, Harvard Medical School, in Boston, and Dawn M. Taylor, PhD, from Case Western Reserve University and the Cleveland VA Medical Center, in Ohio.

For the young woman in this study, they write, "the new procedure enabled substantially better performance in functional movement tests than did her conventional myoelectric prosthesis. In the most important laboratory, the patient's home, she reported using her TMR prosthesis for many hours a day the ultimate compliment for a new technology and for its developers."

These initial reports of targeted reinnervation, they write, "are an important step forward in the seamless integration of replacement limbs into the body. Progress here should inspire a surge of research in high-density recording and stimulation interfaces, microelectricomechanical systems, microscale implanted wireless systems, optimum signal extraction and processing methods, osteointegration, and methods of reinforcing beneficial sensorimotor cortical representations while preventing aberrant pain signals.

"As these fields of biology and technology come together, disability because of amputation could someday be greatly reduced and perhaps eliminated." These technologies could also potentially benefit those with paralysis from injury or neurologic disease, they conclude