A pioneering surgical procedure is providing 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, published in the journal *Nature Medicine*. Including these seven, approximately 60 individuals worldwide have undergone this procedure, which can be used to install either bionic legs or arms.
This groundbreaking procedure, known as Targeted Muscle Reinnervation (TMR), represents a significant advancement in prosthetics. “This is the first prosthetic study in history that shows a leg prosthesis under full neural modulation, where a biomimetic gait emerges,” said Hugh Herr, co-senior study author and a professor of media arts and sciences at MIT. In essence, the synthetic prosthesis is able to fill in for the lost function of the missing limb, resulting in 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 added.
The surgery itself involves reconnecting muscles in a patient’s residual limb after a below-the-knee amputation (for bionic legs). Electrical signals from the nervous system, which carry instructions for movement, can then pass between these muscles and be detected by electrodes in the newly installed prosthetic limb. These signals are picked up by a robotic controller in the prosthesis, enabling it to control a patient’s gait, or way of walking. Importantly, signals about the position and movement of the prosthesis are then fed back to the nervous system, providing the user with sensory feedback.
In a series of experiments, the seven patients who received TMR surgery were able to walk faster than those who received the same type of prosthetic limb but had traditional amputations. Some patients even walked at the same rate as people without amputations. They could also avoid obstacles and climb stairs more naturally than patients who underwent traditional amputations.
While current prosthetic limb technology enables amputees to achieve a natural walking gait, these limbs rely on robotic sensors and controllers to move in a predefined, algorithmic pattern. TMR, on the other hand, 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 explained.
The patients who underwent TMR also experienced less pain and muscle atrophy. The procedure can also be used for people with arm amputations, and it can be performed either during the initial 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,” said Dr. Levi T. Hargrove, co-senior study author and an associate professor of surgery at Harvard Medical School.
