Surgical technique improves sensation, control of prosthetic legs

A new surgical technique could help people with artificial legs accurately sense and control their movements.

Two agonist-antagonist myoneural interface devices were surgically inserted in a patient’s residual limb -- to the robotic ankle joint and robotic subtalar joint. Image by Stephanie Ku/MIT Media Lab/Biomechatronics group
Two agonist-antagonist myoneural interface devices were surgically inserted in a patient’s residual limb — to the robotic ankle joint and robotic subtalar joint. Image by Stephanie Ku/MIT Media Lab/Biomechatronics group

Researchers at MIT Media Lab have invented the neural interface and communication system that sends movement commands from the central nervous system to a robotic prosthesis. In turn, the limb relays proprioceptive feedback describing movement of the joint back to the central nervous system.

The researchers findings were published this week in Science Translational Medicine.

“This is groundbreaking,” Daniel Ferris, a professor of Engineering Innovation at the University of Florida, who was not involved in the research, said in a press release. “The increased sense of embodiment by the amputee subject is a powerful result of having better control of and feedback from the bionic limb.”

Ferris said he expects amputees to seek out this new method.

“It could provide a much greater quality of life for amputees,” Ferris said.

Surgeons inserted two agonist-antagonist myoneural interface devices in the patient’s residual limb, linking one to the robotic ankle joint and the other robotic subtalar joint.

“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,” said senior author and project director Hugh Herr, a professor of media arts and sciences at the MIT Media Lab.

Herr said it is the first time information on joint position, speed and torque has been fed from a prosthetic limb into the nervous system to a human patient at Brigham and Women’s Faulkner Hospital.

“Our goal is to close the loop between the peripheral nervous system’s muscles and nerves, and the bionic appendage,” Herr said.

The AMI systems consist of two opposing muscle-tendons, an agonist and an antagonist — when one muscle contracts and shortens, the other stretches.

The coupled movement transmits the muscle length, speed and force information as natural joint sensation — just how it works naturally in human joints, Herr said. Connecting the AMI with electrodes, the researchers can detect electrical pulses from the muscle or apply electricity to the muscle for it to contract.

“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,” he said.

Paper author Matthew Carty, a surgeon in the Division of Plastic and Reconstructive Surgery, inserted the AMI into a patient’s residual limb below knee amputation.

“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,” Carty said. “Collaboration was critical, as the design of the procedure informed the design of the robotic limb, and vice versa.”

The researchers compared the movement of the AMI patient with four people who had undergone a traditional below-knee amputation and received the same prosthetic limb.

They found the AMI patient’s control over movement of the prosthetic device was more stable and efficient than those with the conventional amputation and the patient quickly displayed natural, reflexive behaviors.

With more natural behavior, patient said it felt as though the bionic ankle and foot had become a part of the body.

“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.”

The researchers later carried out the AMI procedure on nine other below-knee amputees, and are planning to adapt the technique for those needing above-knee, below-elbow, and above-elbow amputations.

“Previously, humans have used technology in a tool-like fashion,” Herr says. “We are now starting to see a new era of human-device interaction, of full neurological embodiment, in which what we design becomes truly part of us, part of our identity.”

By Allen Cone