An improved prosthetic leg device outfitted with a number of external sensors, representing foot touch and pressure and knee joint angle, transmit sensory signals back to the nervous system through a set of stimulation electrodes implanted into the tibial nerve.
The authors of a new paper published in Nature Medicine found that, for two patients with lower-limb amputations, the use of this prosthetic improved walking performance and boosted endurance in both a laboratory setting and a real-world environment. In addition, using the prosthetic reduced the patients’ phantom limb pain.
An international team of researchers led by ETH Zurich and Lausanne-based start-up company SensArs Neuroprosthetics has now developed an interface to connect a leg prosthesis with the residual nerves present in the user’s thigh, thus providing sensory feedback. In a study conducted in collaboration with the University of Belgrade, the scientists tested this neurofeedback system with two volunteers who have an above-knee leg amputation and use a leg prosthesis.
The solution benefited the amputees in a variety of ways, as reported in a paper titled, “Sensory feedback restoration in leg amputees improves walking speed, metabolic cost and phantom pain.”
“This proof-of-concept study shows how beneficial it is to the health of leg amputees to have a prosthesis that works with neural implants to restore sensory feedback,” said Stanisa Raspopovic, PhD, professor at the Institute of Robotics and Intelligent Systems at ETH Zurich.
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’s electronic knee joint.
For the three months that the experiment lasted, surgeons placed tiny electrodes in each volunteer’s thigh and connected them to the residual leg nerves. “The goal of the surgery was to introduce electrodes in the right places inside the nerve to allow the restoration of lifelike sensory feedback, and to allow the stability of the electrodes,” said Marko Bumbasirevic, MD, PhD, professor and orthopedic microsurgeon at the Clinical Centre of Serbia in Belgrade, who was the clinician responsible for the electrode implant.
The research team developed algorithms to translate the information from the tactile and motion sensors into impulses of current—the language of the nervous system—which were delivered to the residual nerve. The signals from the residual nerves are conveyed 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. Walking with neurofeedback was physically much less demanding, as shown by the significant reduction in the volunteers’ oxygen consumption while walking.
Ambulation with neurofeedback was also mentally less strenuous, as the researchers showed with brain activity measurements during the trials. The volunteers didn’t have to concentrate as hard on their gait, which meant that they were able to devote more of their attention to other tasks.
In one test, the volunteers had to walk over sand, and again the feedback enabled them to walk considerably faster. In surveys, the volunteers stated that the neurofeedback greatly increased their confidence in the prosthesis.
The interface with the nervous system can also be used to stimulate the nerves independently of the prosthesis. Before they started the trial, both volunteers complained of phantom limb pain. Over the course of a one-month therapy program with neurostimulation, the scientists managed to considerably reduce this pain in one of the volunteers; in the other the pain disappeared completely.
The scientists view these outcomes optimistically. However, they point 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. “At SensArs, we’re planning to develop a wireless neurostimulation device that can be fully implanted into the patient like a pace-maker, and that can be brought to the market,” said Francesco Petrini, PhD, CEO of SensArs.