An amputation takes away bone and muscle, but the connecting nerves that once controlled the arm are still there, and they're capable of sending commands to the muscles and receiving sensation, even from a missing nerve. They are, in a sense, like data cables floating in space.
This is the basis for technology developed by Dr. Todd Kuiken, director of the Center for Bionic Medicine and director of Amputee Services at The Rehabilitation Institute of Chicago.
His prosthetics make use of limb-controlling nerves that remain in amputees to allow them to better control prosthetics just by thinking. He presented his latest advancements Thursday at the American Association for the Advancement of Science annual meeting in Washington, D.C. Glen Lehman, a retired sergeant first class who suffered a battlefield amputation in Iraq, showcased Kuiken's prototype.
“The arm is pretty much in tune with my thoughts,” Lehman said at the conference.
Here's how it works: Kuiken's group put the nerves once used in motor control of the missing limb on to spare muscles, usually, the pectoralis muscles of the chest. In a technique called “targeted muscle reinnervation,” the scientists cut out the nerves in the chest muscle and implanted in their place the main nerves that had controlled the now-missing arm. The arms nerves then began to grow within chest muscle.
“If you think 'hand closing,' it goes to your brain, down your spinal cord, through the hand-closing nerve, and that makes a little piece of your chest muscle contract,” he told CNN before the presentation.
The small contraction of the chest muscle sends an electrical signal picked up by tiny antennae implanted on the chest muscle, which in turn broadcast the message to a receiver in the prosthetic, telling the hand to close. It works the same with hand opening and moving the elbow up and down, with other nerves.
The surgery itself takes only a couple of hours, and can be an outpatient procedure. But it can take three months before the patient experiences the first twitches, and six months before the nerves reinnervate the muscle well enough to get fitted with a special arm.
There are about 50 patients around the world who are using this technology with conventional prostheses – in other words, more rudimentary devices currently available for use outside of the laboratory, Kuiken said. They are more rudimentary than the bionic arm that Lehman demonstrated. Conventional prostheses have hands that only open and close, elbows and wrist rotators; they don't have the ability to use the elbow and the hand at the same time, and can be confusing to use, Kuiken said. Kuiken's technology allows the patient to use the elbow and the hand simultaneously, in a more intuitive way.
The research also points to a potential mechanism for sensation feedback. For instance, if there were sensors on the prosthetic hand to detect how hard the patient is squeezing the hand, that sensory information can be fed back up through the arm and press on the corresponding skin (which is now on the chest) with a proportional amount of force.
The degree of sensation isn't as good as the human finger, but about as good as the chest skin not being used for this system. Patients can feel normal hot and cold, although it feels like it's in the missing hand, as well as pressure and vibration. Sometimes, though, instead of pressure there's a tingling sensation. About six patients have had this sensory part of the surgery; it's only been done in the laboratory and there's currently no way to clinically implement it, but that is an eventual goal.
In recent years, Kuiken and colleagues have been working on more nuanced hand movements, and they have been collaborating with the U.S. Army in helping patients. The scientists are comfortable trying their technique on patients under 50 who had upper-limb amputations as much as five or 10 years ago, Kuiken said.
They believe that this could also be a treatment for painful nerves, which don't have anywhere to go and form a tangled ball called a neuroma. This problem could be solved by cutting a motor nerve and letting it grow into a muscle, spreading out the nerve problems, this could solve the problem.
Theoretically, this could also be done with legs, and Kuiken has just started some research in this area, but the idea of motorized legs is much newer than motorized arms, which have been around for about 50 years. Also, more is at stake: “If you fall down because you thought wrong, that's a problem,” he said.