Artificial legs you can feel with

25 September 2019

WALKING

Walking on a trail

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Being able to get some sensations back from the missing leg when using a prosthesis seems to increase confidence when walking and to reduce tiredness and phantom limb pain.

In a paper recently published in Nature Medicine, an international team of researchers report on a way of connecting physical sensors from a leg prosthesis to the human nervous system and how restoring some feeling from the leg would affect the health of the users.

The researchers previously worked on creating hand prostheses that can provide some feedback for touching and grasping objects, but in this new project, they focused on improving prosthetics for upper knee amputations - where the amputation is at thigh level.

“Even the simple actions that we are able to perform with our healthy legs are very tiring for lower leg amputees. So over the long term, many of them are having 120% higher probability to get a heart attack because they are suffering while wearing the prosthesis,” says Stanisa Raspopovic from ETH Zurich, one of the leading researchers on the project.

This is because when using typical prosthesis, leg amputees are not receiving any feedback about, for example, the type of ground that they're walking on, or the position of the sole of the artificial leg on the ground, so they have to concentrate much harder. Moreover, the higher the amputation, the harder walking becomes.

The participants in this study were two above-knee amputees and they were observed over a period of 3 months as they alternated between using a normal prosthesis and the prosthesis that is able to provide sensory feedback for the leg.

By measuring the participants’ breathing and brain activity, Raspopovic and his colleagues found that with the enhanced prosthesis the two amputees “could walk at a higher pace, were feeling less exhausted, had less fatigue and less mental effort, and had diminished phantom limb pain.”

Leg amputees often suffer from phantom limb pain, where the parts of the leg that they no longer have still continue to hurt. In the first month of the study, the remaining leg nerves of the participants were stimulated while they were resting, and that seemed to have helped alleviate phantom limb pain

“In our two volunteers we have observed that, for one of them, the phantom limb pain was completely diminished after approximately three weeks, while in the other it was reduced by 80%,” explains Raspopovic. It seems like it could become a potentially effective form of therapy.

In order to create the link between the prosthesis and the leg nerves, the team surgically attached electrodes to the remaining nerve inside the thigh. These can then be used to stimulate the nerves through small cables coming out of the backs of their thighs, through the skin, to connect to a small device on the prosthetic. This device translates signals from sensors on the prosthetic’s foot and knee into nerve signals.

Next they had to map the signals, so that - for example - the sensor under the prosthetic heel being pressed down corresponds to the feeling of the heel touching the ground. The implanted electrodes would vary in intensity, duration and frequency of their electric pulses, and each time the participants described what they felt.

To complete the feedback pathway from the sensors to the nerve, the researchers mapped the numbers given by the sensor readouts, such as the pressure of the sole and the angle of the knee, to the appropriate electrode stimulation as they had established it previously. At the end of this process, the users of the prosthesis could feel sensations in areas of the leg that have been amputated, but were now corresponding to the position of the sensors.

Raspopovic and his team are very encouraged by the health benefits of restoring some sensation in a lost leg. Their end goal is to develop a fully implantable version, a bit like a ‘neuro-pacemaker’ that connects to our nerves, and make the technology available for all of those who need artificial limbs.

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