A new prosthesis powered by microscopic electrodes implanted in the thigh muscles of the wearer transmits natural sensations when the limb is touched, as well as with the movement of the limb. This allows the body to react as it usually does during walking to compensate for any gait irregularities even while the amputee is thinking of something quite different. Not having to think about controlling the prosthesis all the time makes all the difference for users with this new device, which was reported in the journal Science Translation Medicine, October 2, 2019.
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Amputee Djurica Resanovic testifies how she could feel when the leg was touched, anywhere along the limb, or the knee flexed. She says,
First time that I feel my leg, my foot. It was very interesting, like my own leg, after several years.”
This new type of prosthesis is called a neuroprosthesis. It is aimed not just at restoring mobility, but allowing leg amputees to feel how the limb is touching the ground or flexing at the knee joint with each step. No longer is intense concentration needed to take a step correctly. The person can just keep the eyes looking forward, rather than downward to position the next step of the prosthetic leg.
The concept behind the neuroprosthesis
The researchers first use a specially made insole under the foot of the prosthetic limb, which contains arrays of sensors. The knee of the artificial limb is also packed with sensors. These pick up the pressures at the insole and the knee, which are then converted into electrical impulses that travel to the remaining stump of the severed tibial nerve through microscopic electrodes inserted into the nerve tissue transversely. Thus the nerve is now in connection with the artificial limb, both sensing touch and any movement in the artificial foot or knee joint.
At this point, the rest of the intact nervous system takes over, detecting the events at the prosthesis at the level of the brain and adjusting the gait in response, just as with the natural limb. In effect, the sensor-electrode combination restores the circuit. This is of huge benefit to the amputee because being able to feel one’s artificial limb while walking allows automatic adjustment of gait to avoid falls.
Advantages of a neuroprosthesis
In general, even if a leg amputee is fitted with an artificial limb, the healthy leg is felt to be and treated as more reliable for mobility, leaving the patient with only one ‘real’ leg. The lack of sensation prevents the prosthesis from ever being a natural part of the phenomenon of walking. Instead it is a mobility aid. However, the integration of the natural nerve stump with the bionic sensors implanted in the prosthesis allows the brain to consider the limb as a part of the body. This is essential to help the users to have confidence in the prosthetic as well as natural limb. In turn, this will promote much wider acceptance and use of this pioneering technology.
Researcher Stanisa Raspopovic says,
We developed the first leg with feelings for highly disabled, above-knee amputees, which enabled them to surpass unexpected obstacles without falling, or to climb stairs much faster. These two tasks are extremely difficult, if even possible, for amputees wearing commercial prostheses.”
In other words, the neuroprosthesis is felt to be a real limb, a body extension, as proved by elaborate measurements of brain waves related to the use of the limb, and psychophysical tests.
Future research by the same team will focus on a longer follow-up period, using the technology in a larger number of participants who will also be assessed on their function in their home situations. This will tell researchers more about the way the bionic limb enhances the health and quality of life of its users, according to marketing expert Francesco Petrini who is in charge of selling these unique prostheses via the startup SensArs Neuroprosthetics.
Francesco Maria Petrini et al. Enhancing functional abilities and cognitive integration of the lower limb prosthesis. Science Translational Medicine. Vol. 11, Issue 512 https://stm.sciencemag.org/content/11/512/eaav8939