Humans are able to naturally sense where their limbs are, how fast they’re moving and the torque, all without looking. Being able to sense where limbs are, known as proprioception, allows humans to control body movements precisely. However, researchers have been unable to recreate the sensation in people with artificial limbs, limiting their abilities to accurately control movements.
The MIT researches developed a paradigm known as the agonist-antagonist myoneural interface (AMI) that is a surgical approach to limb amputation that involves preserving dynamic muscle relationships in the amputated limb. The method was testing in extensive preclinical experimentation through MIT before it was implemented in a human patient.
In the study, information regarding joint position, speed and torque were relayed from a prosthetic limb into the nervous system.
“Our goal is to close the loop between the peripheral nervous system’s muscles and nerves, and the bionic appendage,” Hugh Herr, a senior author on the study, said in a press release.
To get the information to relay to the nervous system, the researchers used the same biological sensors that the body naturally creates in proprioceptive sensations. The AMI method uses two opposing muscle-tendons that are surgically connected in series. When one muscle contracts and shortens, the other stretches and vice versa.
The movement allows for natural biological sensors in the muscle-tendon to transmit electrical signals to the central nervous system. As a result, muscle speed, length and force information is interpreted by the brain as a natural joint proprioception, just like how muscle-tendon proprioception works naturally in human joints.
“Because the muscles have a natural nerve supply, when this agonist-antagonist muscle movement occurs information is sent through the nerve to the brain, enabling the person to feel those muscles moving, both their position, speed and load,” Herr said.
Agonist-antagonist myoneural interface are connected with electrodes that allow the researchers to detect electrical pulses from the muscle or allows them to apply electricity to the muscle to make it contract.
“When a person is thinking about moving their phantom ankle, the AMI that maps to that bionic ankle is moving back and forth, sending signals through the nerves to the brain, enabling the person with an amputation to actually feel their bionic ankle moving throughout the whole angular range,” Herr said.
However, the electrical language that happens during proprioception in the nerves is difficult to decode, according to the researchers.
“Using this approach, rather than needing to speak that electrical language ourselves, we use these biological sensors to speak the language for us,” Tyler Clites, the first author on the study, said. “These sensors translate mechanical stretch into electrical signals that can be interpreted by the brain as sensations of position, speed and force.”
The AMI was first surgically implemented in a human at Brigham and Women’s Faulkner Hospital in Boston. During the operation, two AMIs were constructed in the residual limb at the same time as primary below-knee amputation. One AMI controls the prosthetic ankle joint and the other controls the prosthetic subtalar joint.
“We knew that in order for us to validate the success of this new approach to amputation, we would need to couple the procedure with a novel prosthesis that could take advantage of the additional capabilities of this new type of residual limb,” Matthew Carty, one of the authors on the study and the surgeon who performed the surgery, said. “Collaboration was critical, as the design of the procedure informed the design of the robotic limb, and vice versa.”
MIT researchers also built an advanced prosthetic limb that was electrically linked to the patient’s peripheral nervous system using electrodes over AMI muscles after the amputation surgery. The researchers could ten compare the movement of the AMI patient with four other patients who had traditional below-knee amputations with the same prosthetic limb. They found that the AMI patient had stable control of the movement of the prosthetic device and could more more efficiently than the patients who had traditional below-knee amputation. AMI patients also have more natural, reflexive behaviors like extending the toes toward the next step when walking down stairs.
AMI patients also reported feeling that the bionic ankle and foot were part of their body whereas the traditional amputation patients felt disconnected from the prosthesis.
“This is pretty significant evidence that the brain and the spinal cord in this patient adopted the prosthetic leg as if it were their biological limb, enabling those biological pathways to become active once again,” Clites said. “We believe proprioception is fundamental to that adoption.”
Patients who undergo lower limb amputation surgery have difficulty gaining a sense of embodiment with their artificial limb.
“This is groundbreaking. The increased sense of embodiment by the amputee subject is a powerful result of having better control of and feedback from the bionic limb,” said Daniel Ferris, a professor of engineering innovation at the University of Florida, who was not part of the research. “I expect that we will see individuals with traumatic amputations start to seek out this type of surgery and interface for their prostheses – it could provide a much greater quality of life for amputees.”
The researchers have implemented the AMI technique in nine other below-knee amputees and plan to further develop the technique to accommodate above-knee, below-elbow and above-elbow amputations.
“Previously, humans have used technology in a tool-like fashion,” said Herr. “We are now starting to see anew era of human-device interaction, of full neurological embodiment, in which what we design becomes truly part of us, part of our identity.”
The research was published in the journal Science Translational Medicine.