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New procedure lets prosthetic arms listen to the brain

Tests show connecting residual nerves from an amputated arm to the chest wall enables the prosthetic to perform more movements and react almost as quickly as the real thing.

February 14, 2009|Thomas H. Maugh II

By rerouting truncated nerves from an amputated arm to chest muscles where nerve impulses can be detected with sensors, Chicago researchers have made it possible to control prosthetic arms using only the patient's thoughts.

Tests on five patients show that the arms move almost as quickly and accurately as healthy arms, Dr. Todd A. Kuiken of the Rehabilitation Institute of Chicago reported Wednesday in the Journal of the American Medical Assn.

"I was amazed at the level of hand function and how fast I was able to control the arm and hand," said Amanda Kitts, who was fitted with the device after losing an arm in a car accident. "I was able to pick up a penny off the table and could catch an object in motion, like a checker that was rolling across the table," she said in a statement.

Existing prosthetic arms are biomechanical and so clunky that many amputees do not even bother with them. By flexing a back muscle, for example, the patient activates cables and pulleys to bend the arm. Flexing the bicep can close the hand. Motions are slow and in most cases only three motions are possible.

In contrast, the new prosthetic arm, developed by Johns Hopkins University for the Defense Advanced Research Projects Agency, is electrically operated and has as many as 10 motions. The problem has been controlling the motions.

In 2007, Kuiken announced a new way to do so. Nerves from the residual arm are attached to chest muscles, whose movements amplify the nerve signal. Sensors placed over the new locations on the chest pick up impulses from the brain and transmit them to the prosthetic arms, a process called targeted muscle reinnervation.

In the new study, Kuiken and his colleagues studied five amputees who had had the procedure and compared them with five non-amputees, measuring the time it took to initiate and complete arm motions.

They found that it took the amputees only slightly longer: Initiating a motion, such as flexing or rotating the wrist, took about 0.22 second for the amputees, compared with 0.16 second for the controls. Completing a task took 1.29 seconds for amputees and 1.08 seconds for controls. Conventional prosthetic arms require much longer times.

The primary drawbacks to the new technique are that it will be expensive -- although the cost is not yet known -- and requires extensive surgery and recuperation. And, Kuiken said, it will be years before it is available for widespread use.

Nonetheless, Dr. Gerald E. Loeb of USC wrote in an editorial accompanying the study that the work is "exciting and promising." It's still a long way to the fully restored hand of Luke Skywalker in the "Star Wars" films, he added, but it's a step in the right direction.


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