September 2019

Proof-of-Concept Prosthetic Leg Has Proprioception, Reduces Phantom Limb Pain

From left, researcher Giacomo Valle, co-author of the paper and a PhD student at Scuola Superiore Sant’Anna, Pisa, Italy, prepares volunteer Savo Panic for metabolic testing. Photograph by Federica Barberi, courtesy of ETF Zurich.

Two people with transfemoral amputations have been able to feel their prosthetic foot and knee in real time via 4 intraneural sensors that connect to residual nerves in the thigh. The resulting neurofeedback reduces the physical and mental strain for prosthetic users and has been shown to reduce phantom limb pain. An international team of researchers led by ETH Zurich (German: Eidgenössische Technische Hochschule Zürich), Switzerland; and Lausanne, Switzerland–based start-up company SensArs developed the interface used to connect the prosthetic leg with the residual nerves.

In a proof-of-concept study conducted in collaboration with the University of Belgrade, Serbia, the scientists tested this neurofeedback system with 2 volunteers over 3 months. To provide the nervous system with sensory information, the scientists began with a commercially available high-tech prosthesis. They attached tactile sensors to the sole of the prosthetic foot, and collected the data on knee movement provided by the prosthesis’ electronic knee joint. Surgeons placed tiny electrodes in each volunteer’s thigh and connected them to the residual leg nerves. The electrodes were developed by scientists from the University of Freiburg, Breisgau, Germany, and the prosthesis came from Össur, Reykjavik, Iceland; both were actively involved in the project.

The research team developed algorithms to translate the information from the tactile and motion sensors into impulses of current, which were delivered to the residual nerves. The signals from the residual nerves are conveyed via neurons to the person’s brain, which is thus able to sense the prosthesis and helps the user to adjust their gait accordingly.

As part of the study, the volunteers underwent a series of tests—alternating trials with and without neurofeedback. According to the results, walking with neurofeedback was physically less demanding, as measured by a decrease in oxygen consumption, and brain activity measurements indicated that ambulation with neurofeedback was less mentally strenuous. Results also showed that walking speed and self-reported confidence in the prosthesis increased. The study results were reported in a recent issue of Nature Medicine.

The interface with the nervous system can also be used to stimulate the nerves independently of the prosthesis, which has implications for the reduction or elimination of phantom limb pain. “Through sessions of purposely designed patterns of nerve stimulation we were able to diminish the phantom limb pain of the subjects until full suppression, after just 1 month of tests,” said Stanisa Raspopovic, PhD, professor, Institute of Robotics and Intelligent Systems at ETH Zurich, and a co-author of the paper.

The scientists said they view these outcomes optimistically. However, they pointed out the need for a longer investigation with in-home assessments and a greater number of volunteers, in order to provide more robust data that they can use to draw more significant conclusions. For the time-limited clinical study, the signals from the prosthesis were sent along cables through the skin to the electrodes in the thigh. This meant that the volunteers had to undergo regular medical examinations. To eliminate this need, the scientists intend to develop a fully implantable system.

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