Spinal stimulators could give sensory feedback to prosthetic arms

For their study, published in eLife, four amputees received spinal stimulators which create the illusion of sensations in the missing arm.

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"What's unique about this work is that we're using devices that are already implanted in 50,000 people a year for pain - physicians in every major medical centre across the country know how to do these surgical procedures - and we get similar results to highly specialised devices and procedures," said study senior author Lee Fisher, Ph.D., assistant professor of physical medicine and rehabilitation, University of Pittsburgh School of Medicine (UPMC).

The strings of implanted spinal electrodes run along the spinal cord, where they sit slightly to one side, upon the same nerve roots that would normally transmit sensations from the arm.

Fisher's team sent electrical pulses through different spots in the implanted electrodes while participants used a tablet to report what they were feeling and where.

All the participants experienced sensations somewhere on their missing arm or hand, and they indicated the extent of the area affected by drawing on a blank human form. Three participants reported feelings localised to a single finger or part of the palm.

"I was pretty surprised at how small the area of these sensations were that people were reporting," Fisher said in a statement. "That's important because we want to generate sensations only where the prosthetic limb is making contact with objects."

When asked to describe not just where but how the stimulation felt, all four participants reported feeling natural sensations, such as touch and pressure, though these feelings often were mixed with artificial sensations, such as tingling, buzzing or prickling.

Although some degree of electrode migration is inevitable in the first few days after the leads are implanted, Fisher's team found that the electrodes, and the sensations they generated, mostly remained in place during the month-long experiment.

"Stability of these devices is really critical," Fisher said. "If the electrodes are moving around, that's going to change what a person feels when we stimulate."

The team’s next challenges are to design spinal stimulators that can be fully implanted rather than connecting to a stimulator outside the body, and to demonstrate that the sensory feedback can help to improve the control of a prosthetic hand during functional tasks. Shrinking the size of the contacts is another priority as it might allow users to experience even more localised sensations.

"Our goal here wasn't to develop the final device that someone would use permanently," Fisher said. "Mostly we wanted to demonstrate the possibility that something like this could work."