A patient tests a prototype prosthetic limb being developed by DARPA. Credit: DEKA Research and Development, and The Rehabilitation Institute of Chicago

By surgically rearranging the nerves that normally connect to the lost limb, physicians have developed an intuitive way for amputee patients to control a robotic arm.

Todd Kuiken and colleagues at the Rehabilitation Institute of Chicago first reported the technique in a single patient in 2007, and now they have tested it in several other patients. The patients could all successfully control the advanced prosthetic, which features motorized shoulders, elbows, wrists and hands. They could move the arm in space, mimic hand motions, and pick up a varietyt of objects, including a water glass, a delicate cracker, and a checker rolling across a table. (Three patients are shown using the arm in the video below.) The findings are reported today in Journal of the American Medical Association.

The motorized arm prostheses most commonly used today co-opt existing shoulder movements to control the hand, elbow or wrist on the limb. These 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.

Kuiken has developed an entirely new kind of interface. Using a surgical procedure called targeted muscle reinnervation, surgeons transfer nerves that previously carried signals to the amputated limb to muscles in the chest and upper arm. The rerouted nerves then grow into the muscles, which contract when the patient thinks about moving the lost limb. Those signals are read by sensors on the prosthetic limb and translated into movement.

Three patients who have undergone the surgery tested a prototype arm under development by the Defense Advanced Research Project Agency's (DARPA) Revolutionizing Prosthetics Program (see video below). They could reliably control the device within just two weeks.

"The speed as well as accuracy of the movements represent substantial improvements over previous myoelectric systems," writes Gerald Loeb, a physician and scientist in the department of biomedical engineering and neurology at the University of Southern California, in an accompanying editorial. "Even more important, however, is the ease with which patients learned to perform tasks requiring coordinated motion in more than one joint."

Kuiken and colleagues are working on adding sensory feedback to the system by transplanting nerves that once carried sensory signals from the amputated arm to the brain. This kind of feedback is especially important in determining, for example, how much force to use to grab a glass without breaking it. The researchers have already shown that patients who have had this nerve transplanted can feel sensations in the chest from the lost hand. They eventually aim to add sensors to the prosthetic fingers, which could translate tactile information to transplanted nerves, making the patient feel as if they had a real hand.