MIT researchers describe the first human implementation of a novel approach to limb amputation that helps improve sensation and control of a prosthetic limb in a new study published in Science Translational Medicine.
The researchers, from the Center for Extreme Bionics at the MIT Media Lab, have invented a new neural interface and communication paradigm that is able to send movement commands from the central nervous system to a robotic prosthesis, and relay proprioceptive feedback describing movement of the joint back to the central nervous system in return.
This new paradigm, known as the agonist-antagonist myoneural interface (AMI), involves a novel surgical approach to limb amputation in which dynamic muscle relationships are preserved within the amputated limb.
The AMI consists of two opposing muscle-tendons, known as an agonist and an antagonist, which are surgically connected in series so that when one muscle contracts and shortens—upon either volitional or electrical activation—the other stretches, and vice versa.
This coupled movement enables natural biological sensors within the muscle-tendon to transmit electrical signals to the central nervous system, communicating muscle length, speed, and force information, which is interpreted by the brain as natural joint proprioception.
This is how muscle-tendon proprioception works naturally in human joints, according to senior author and project director Hugh Herr, a professor of media arts and sciences at the MIT Media Lab, in a media release from MIT.
“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 says.
By connecting the AMI with electrodes, the researchers can detect electrical pulses from the muscle, or apply electricity to the muscle to cause it to contract.
In the study, the researchers describe the first human implementation of the AMI in a person with below-knee amputation.
In this operation, two AMIs were constructed in the residual limb at the time of primary below-knee amputation, with one AMI to control the prosthetic ankle joint, and the other to control 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,” says study author Matthew Carty, who performed the surgery. “Collaboration was critical, as the design of the procedure informed the design of the robotic limb, and vice versa.”
Toward this end, an advanced prosthetic limb was built at MIT and electrically linked to the patient’s peripheral nervous system using electrodes placed over each AMI muscle following the amputation surgery.
The researchers then compared the movement of the AMI patient with that of four people who had undergone a traditional below-knee amputation procedure, using the same advanced prosthetic limb.
They found that the AMI patient had more stable control over movement of the prosthetic device and was able to move more efficiently than those with the conventional amputation. They also found that the AMI patient quickly displayed natural, reflexive behaviors such as extending the toes toward the next step when walking down a set of stairs, the release continues.
These behaviors are essential to natural human movement and were absent in all of the people who had undergone a traditional amputation.
What’s more, while the patients with conventional amputation reported feeling disconnected to the prosthesis, the AMI patient quickly described feeling that the bionic ankle and foot had become a part of their own body.
The researchers have since 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, the release concludes.
[Source(s): Massachusetts Institute of Technology, EurekAlert]