Friday, February 02, 2007

A Prosthetic Arm That Acts Like a Real One

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

By Emily Singer

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

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.

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.