A Lifelike Prosthetic Arm
People who have lost an arm have not traditionally had much hope of ever regaining meaningful function. Prosthetic arms have been controlled in a rudimentary way, by transforming residual shoulder movements or muscle signals into the simplest movement commands. These artificial arms cannot do two things at once, much less three or four. Amputees often toss them in the closet out of sheer frustration, somewhat stung by the fact that leg amputees have far better products available to them.
But the situation is starting to change, thanks to a team led by Todd Kuiken, director of the Rehabilitation Institute of Chicago’s Center for Bionic Medicine. Kuiken has developed a novel surgical technique that, when paired with both motorized prosthetic arms already on the market and experimental bionic arms developed through a Defense Advanced Research Projects Agency (DARPA) program, affords amputees a remarkable degree of dexterity. Claudia Mitchell, who lost her arm in a motorcycle wreck in 2004, remembers putting on a prosthesis after undergoing Kuiken’s procedure and seeing it work for the first time: “You could not wipe that grin off of my face. I can now iron a shirt again like nobody’s business.” Mitchell has become a hit at parties. “People can’t believe how this thing works,” she says. “They want to see me do things with it.”
The device is activated by commands from surviving arm nerves that have been transplanted and rewired to muscles elsewhere–typically, as in Mitchell’s case, in the chest. The nerves send electrical signals to control the prosthetic arm, with results so natural that observers often don’t realize the arm is bionic until they listen closely for the sound of whirring motors. Called targeted muscle reinnervation, the procedure is unique because it permits intuitive control over the robotic limb. After about six months of healing, patients can move the arm merely by thinking about what they want it to do, just as they once did with their real arms. Tell Mitchell “Bend your arm,” and the muscles in her chest flinch instantaneously–a most peculiar sight. But she is not thinking about moving her chest muscles. Rather, she is thinking about bending her arm, and that thought moves the chest muscles to make the robotic arm do her bidding.
Kuiken recently published promising test results in the Journal of the American Medical Association, showing that five patients told to perform 10 different arm movements with a virtual prosthesis could do so almost as readily as non-amputees in a control group: their response time was less than a quarter of a second longer. (The virtual prosthesis allows scientists to more easily figure out the speed and level of control that can be gleaned from muscle signals. Researchers performed similar experiments with mechanical arms.) In an accompanying editorial, the pioneering biomedical engineer Gerald Loeb wrote, “The speed as well as the accuracy of the movements represent substantial improvements over previous systems. Even more important, however, is the ease with which patients learned to perform tasks requiring coordinated motion in more than one joint.” He concluded, “With increasing functional capabilities, patients with upper-extremity amputations may derive exceptional benefit from prosthetic arms, just as legions of patients with lower-extremity amputations now lead remarkably normal and even athletic lives.” (Leg prostheses have been further along in development because there is a bigger market for them: 90 percent of amputees have lost lower limbs. Also, legs don’t require as much dexterity as arms.)
The journey from initial consultation in Chicago to full functionality–say, the ability to slice a lemon with the prosthetic hand while holding it with a natural hand–often takes a year or longer. Patients first undergo a two-hour surgery performed by Greg Dumanian, a Chicago plastic and hand surgeon who has worked closely with Kuiken in developing the procedure. Dumanian identifies the surviving portion of the nerves that previously conducted electrical signals from the spinal cord to the lost limb; then he transfers them to muscles in the chest or upper arm. The nerve that would normally trigger the hand to close might be transferred to part of the chest muscle, for example. (The exact procedure varies according to the patient’s injuries.) When the robotic arm is in place, an electrode on the chest detects contractions in this muscle and sends the signal to the prosthesis. The prosthesis is programmed to interpret that signal as a command to close the hand, and the action typically takes place less than half a second after the chest muscle moves.
The experimental bionic arms are also programmed with pattern recognition algorithms to decipher the rapid series of nerve signals that govern hand and wrist motions. The more than 30 patients who have had the procedure report that they are easily able to slice hot peppers, open a bag of flour, put on a belt, operate a tape measure, or remove a new tennis ball from a container.
Among several experimental approaches to improving prosthetic arms, including transferring nerves directly to a prosthesis and decoding movement signals directly from the brain, Kuiken’s technique is the one that has made the most progress. The former has yet to be tested in humans, and the latter is currently considered too dangerous for most patients, since it requires brain surgery. Kuiken says he views targeted reinnervation as a quicker, more practical way of restoring crucial functions. (His approach won’t help quadriplegics, however, because the nerves need to be intact for the procedure to work.) So far, the procedure is performed only at Kuiken’s rehab center; in ongoing studies, the center is offering it to any patient for whom it’s deemed medically appropriate.
Advanced though it is, the experimental prosthetic is still missing a major function: sensation. If Mitchell were to place her bionic hand on a hot pan, she would have no way of knowing its temperature. Giving the prosthetic sensory capabilities similar to those of a real limb is more complicated than restoring movement. But it’s not impossible. While Kuiken’s procedure focuses on moving motor nerves, which conduct nerve signals from the brain to the muscles, it appears that sensory nerves, which carry signals from the skin to the brain, are affected as well. Patients, including Mitchell, have reported that when certain areas of their rewired chest muscles are touched, they feel as if their missing hand is being touched. Place an ice cube on the chest, and a phantom hand gets cold.
Kuiken, Loeb, and others are studying ways for the bionic arm to make use of this sensory information. For starters, they’ll need sensors that can stand up well to moisture, heat, and the other physical eventualities of daily living. They’ll then need to deliver that sensory information to the wearer.
But what’s clear now is that for the first time, a useful prosthetic arm is in sight. “We’re not trying to make a bionic person who can leap tall buildings and pick up cars,” Kuiken says. “We’re trying to make something that restores a fraction of the incredible function and power and efficiency of a human limb.” For arm amputees like Claudia Mitchell, that means getting a chance that leg amputees have had for years.
Michael Rosenwald is a staff writer at the Washington Post.
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