Biomedicine

A Prosthetic Arm That Acts Like a Real One

Patients use nerves left intact after amputations to control prosthetic limbs.

A new technique that capitalizes on remaining nerves allows amputees to intuitively control their prosthetic limb, providing them with a much better level of control than traditional prosthetics.

A new type of robotic arm responds to the same neural signals that a human arm would, giving its wearer better dexterity.

In a paper published today in The Lancet, scientists at the Rehabilitation Institute of Chicago described a procedure to surgically transplant nerves from the shoulder to the upper-chest muscle of a woman who had lost her arm in a motorcycle accident. The rerouted nerves then grew into the muscle, which amplified the messages once sent to muscles in the arm and hand; those signals are read by sensors on the prosthetic limb and translated into movement. The patient also developed a surprising degree of sensory perception in the upper chest, which scientists say will be key in the next generation of prosthetics.

“It’s encouraging to see that even after an amputation, the same intention to move the limb can be harnessed to control a prosthetic limb in much the same way that the limb was previously controlled,” says Leigh Hochberg, a neurologist at Massachusetts General Hospital, in Boston, who wrote a commentary accompanying the paper.

Most artificial arms are controlled by remaining muscles near the amputated limb. But the devices can be frustrating and slow: the user must consciously contract those muscles to trigger a movement, and only one movement can be performed at a time. Todd Kuiken and colleagues at the Rehabilitation Institute of Chicago developed a new, more intuitive method for controlling prosthetics that capitalizes on remaining nerves, which still carry neural signals meant for the lost limb.

The scientists transplanted to the upper chest both motor and sensory nerves that, prior to the amputation, would have traveled from the shoulder to muscles in the arm and hand. In the months after the surgery, the transplanted nerves grew into the chest muscle, eventually triggering twitches in the shoulder muscle when the patient thought about moving her hand or elbow. Scientists then mapped the precise pattern of muscle activity that occurred when the patient mentally executed specific movements, such as grasping or moving the elbow. Liberating Technologies, a prosthetic-device company, then made a specialized prosthetic limb, which was programmed to sense muscle activity generated by the transplanted nerves and use it to control movement of a motorized elbow, wrist, and hand.

The patient was able to use her new arm within a few days, becoming four times as fast on movement tests as she was with her traditional prosthetic. She reported that the new device was much easier and more natural to use, and she could move the hand, wrist, and elbow simultaneously. “This is a really innovative approach and has the potential to improve the control that people using these myoelectric prostheses have,” says Robert Kirsch , associate director of the Functional Electrical Stimulation Center at Louis Stokes Veterans Affairs Medical Center, in Cleveland.

Perhaps one of the most exciting findings was the surprisingly refined sensory ability the patient developed in her chest. (The patient described in the paper was the third to undergo the nerve-transplantation procedure, but she was the first to have sensory nerves transplanted in addition to motor nerves.) When the area was touched, she felt as if her missing hand had been touched, and she eventually developed a faint sensation of her middle finger when touched on a particular part of her chest.

Scientists say this sensory ability is an important step for the next generation of prosthetic limbs. Sensors or haptics technology could be placed in the fingers of a robotic arm and transmit signals to the chest, allowing the patient to feel the sensation encountered by the prosthetic limb. This would provide the sensory feedback–not present in standard prosthetics–that allows us to grip a Styrofoam coffee cup without crushing it or put down a cup of soup if it’s too hot. “Instead of doing commands like a robot, it might actually feel like a part of the body,” says Kirsch.

Other scientists are now developing similar implantable devices, potentially allowing a finer level of control. Kirsch, for example, is developing a device that would be implanted onto the muscle to directly detect muscle activity and then wirelessly transmit the activity signals to a prosthetic, an approach that he says will provide more-stable input to the robotic limb.

Richard Normann, a neuroscientist at the University of Utah who has pioneered the development of small electrode arrays that can record sophisticated neural signals, is working on a device that, when implanted onto the nerve, could record signals from individual axons within the nerve fiber, thereby providing a more nuanced set of control signals. He hopes to have a working version to test in some of Kuiken’s patients in about two years. “It is not unreasonable to believe an amputee could have an arm that he will come to believe and use just like an existing arm,” says Normann. “It’s not the reality today, of course, but it’s not a fantasy anymore.”

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