Bionic legs plugged directly into nervous system enable unprecedented 'level of brain control'

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A pioneering surgical procedure provides amputees with bionic limbs that are directly controlled by the nervous system, enabling patients to sense the limb's position in space. 

Scientists demonstrated the success of this technique in a new study of seven people who received bionic legs, which was published Monday (July 1) in the journal Nature Medicine. Including these seven, about 60 people worldwide have undergone this type of procedure, which can be used to install either bionic legs or arms. 

"This is the first prosthetic study in history that shows a leg prosthesis under full neural modulation, where a biomimetic gait emerges," Hugh Herr, co-senior study author and a professor of media arts and sciences at MIT, said in a statement. In other words, the synthetic prosthesis is able to fill in for the lost function of the missing limb and thus produce a natural gait.

"No one has been able to show this level of brain control that produces a natural gait, where the human's nervous system is controlling the movement, not a robotic control algorithm," Herr said.

Related: 'You can get the feeling that you are touching another human': New prosthetic device detects temperature

The surgery itself, known as agonist-antagonist myoneural interface (AMI), involves reconnecting muscles in a patient's residual limb after a below-the-knee amputation, in the case that the patient is getting a bionic leg. 

Electrical signals from the central nervous system, which relay instructions for movement, can then pass between these muscles, and be detected by electrodes in a newly installed prosthetic limb. The signals are picked up by a robotic controller in the prosthesis that enables it to control a patient's gait, or way of walking. Signals about the position and movement of a patient's prosthesis are then fed back to the nervous system. 

In a series of experiments described in the new paper, the seven patients who received AMI surgery were able to walk faster than people who received the same type of prosthetic limb, but who had traditional amputations. Some of the patients could even walk at the same rate as people without amputations. They could also avoid obstacles and climb stairs more naturally than patients who underwent traditional amputations. 

Current technology for prosthetic limbs already enables amputees to achieve a natural walking gait, according to the team who conducted the surgery. However, these prosthetic limbs rely on robotic sensors and controllers to actually move in a predefined, algorithmic pattern, the team said. AMI, in contrast, enables the limb to dynamically respond to signals from the body.

"The approach we're taking is trying to comprehensively connect the brain of the human to the electromechanics," Herr said.

The patients who underwent AMI also experienced less pain and muscle atrophy, the scientists reported. 

AMI can also be used for people who have arm amputations, the team said, and the surgery can be done either during a patient's original amputation or at a later date. 

"This work represents yet another step in us demonstrating what is possible in terms of restoring function in patients who suffer from severe limb injury," Dr. Matthew Carty, co-senior study author and an associate professor of surgery at Harvard Medical School, said in the statement.

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