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Prosthetics Re-Wired for Sense of Touch

Researchers in Chicago say they've helped amputees to feel sensation as if they still had their missing hands by surgically re-routing the nerves that transmit that feeling. As Stephanie Lecci reports, they say that renewed sense of touch could lead to better prosthetic limbs.

It had been 19 months since Claudia Mitchell lost her arm in a motorcycle accident. But one day in 2005, she suddenly was able to feel her missing hand, through the skin of her chest. "So I'm in the shower," she recalls, "normal temperature hot water. And it hit my chest and I could actually feel my hand, and I was like, 'Oh wow, it feels like something hot is on my hand.'"

That sensation returned months after surgeons re-routed nerves that once went to her arm into her chest, to help her move her specially-designed prosthetic arm more smoothly. After the operation, just thinking about a specific movement, such as bending her elbow, transmitted a nerve impulse to her chest muscle, which contracted, activating the appropriate motors to make her bionic arm bend.

A similar procedure on another patient had also brought back the feeling of the missing hand. Doctors hoped it would do the same for her. And it did; through the former hand nerves, now in her chest, Mitchell can feel heat and cold, soft and hard pressure, and even the sensation of her finger being bent back.

"It's not the exact same feeling of touch on a normal part of my body," she explains. "But I can identify there's this area on my chest, okay, that's my index finger, that's my pinky, that over there is my thumb." She compares the sensation of pressure to the tingly feeling you get when your arm or leg goes to sleep and then wakes up.

Mitchell has been working with sensory neurophysiologist Paul Marasco of the Rehabilitation Institute of Chicago. He's co-author of a recent study on the restored hand sensations that occur after the nerve transfer surgery, published in the Proceedings of the National Academy of Sciences.

He says he and his colleagues were surprised by the unexpected side effect of the surgery, which was meant to improve physical control of the bionic arm. "It appears that after being rewired the nerves that brought in the sensation, they actually grew through the muscle out into the skin and then re-established the sensation from the missing limb in the skin of the chest." Interestingly, he notes, the sensation in the chest isn't lost, so to the patient, touching the skin where the hand nerve has been relocated feels like a touch on both the chest and the hand.

Marasco says the rewiring surgery and the sensory feedback it restores could lead to better prosthetic limbs, by allowing improved connections between mechanical sensors on the prosthesis and an amputee's remaining nerves.

He says it's possible that one day, an artificial hand could give its owner something like the kind of feedback we get from a natural hand. "We have sensors in fingertips of the experimental arms that we've worked with. And when they grab onto an object or touch an object, it sends a signal to this little plunger on their chest that pushes into, you know, say, the thumb and the pointer finger, and gives them a sense that they're squeezing."

Claudia Mitchell calls that a revolution, adding that sensors that relay feelings of touch would give her and other amputees more control over their prosthetic limbs. "You're not going to squeeze that banana until it gushes before you can open it. You would be able to feel a glass sliding in your hand with the condensation rather than it completely falling and breaking before you even knew it was going to fall. Being able to know that something you picked up with your prosthesis that you can't feel is hot, so you shouldn't reach over and grab it with your other hand. All those types of things that we really just don't even think about, it'll be nice to be able to have that back."

Researchers are working on ways to achieve that level of sensitivity and control, and orthopedic surgeons, like Douglas Smith at Seattle's Harborview Medical Center, are watching developments carefully. Smith, who was not involved in the study, has performed seven nerve relocation surgeries. He says even if prostheses capable of such sophisticated movements are a while off, just being able to feel their missing hands is a true advance for amputees right now.

"The individuals that have developed this new sensation, it has been something very special to them," he says. "It doesn't bring the part back by any means, it doesn't begin to make up for the loss that they have, but it's been really exciting to see a small way to connect with the part that's gone."