Up to half of all people with an upper limb amputation decide not to wear a prosthetic device, mainly because prosthetics are often uncomfortable and don’t bring back the lost sensation that patients desire. But just because someone has lost a limb doesn’t mean they’ve permanently lost sensation. The feeling of water running over their hand, the sensation of tapping, the brush of sandpaper–this can all be brought back, according to research from Case Western Reserve University and the Cleveland VA.
In a paper published in Science Translational Medicine, researchers describe a prosthetic hand system that has allowed two amputees to feel normal hand sensations that are so sensitive they can pick up delicate objects like cherries and grapes. And, there’s a big bonus: the phantom limb pain so many amputees feel essentially disappeared for the patients during testing. The patients tested the system, which can only be used in the lab for now, for about two years each.
The idea of restoring sensation to people with limb loss has been around since the 1970s, when researchers first proved that stimulating nerves could produce a semblance of sensation in a missing limb. But they couldn’t control or localize the sensations that patients experienced, and the research went dormant for decades. Now it’s back.
To bring back sensation in the latest study, the researchers placed electrodes around the outside of the nerves in each patient’s upper arm. These nerves used to carry signals to the hand. When the limb disappears, the signals go along with them–but the wires that carried those signals are still there.
When they connected the electrodes to an electrical current-generating machine, patients felt like the sensations were coming from their prosthetic hands. “We’ve learned the language the brain has to have. When you excite the wires, in the past, people got a feeling of tingling pins and needles. By doing patterned stimulation intensity, slightly altering the pattern of the current in specific ways, we can change that to normal and natural pressure sensations,” says Dustin Tyler, director of the research and associate professor of biomedical engineering at Case Western Reserve. Depending on the pattern, researchers could create sensations of different textures, like sandpaper and smooth surfaces.
In addition to bringing back sensation, the prosthetic hand system also eliminated the episodic, uncontrolled pain that both study participants previously experienced. The researchers aren’t quite sure why this happened, but they plan on investigating further in a future study. “The pain is usually representative of the way they lost the limb in the first place. We think it’s psychological, a lack of input,” says Tyler.
There have been other prosthetic limb systems that allow patients to feel sensation in the past, but this one may be the most advanced.
The prosthetic hand still has a ways to go before it can be brought out of the lab and into people’s lives. The current system requires electrodes to penetrate the skin–an easy way to get an infection–but Tyler hopes to develop a fully implanted system, where users can put on their prosthetic arm and connect via Bluetooth (or something else) to the internal system. That system could be available within a decade.