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Alumni Magazine

New neural-controlled ankle prosthesis improves stability

Illustration of ankle prosthetic.

A new prosthetic ankle controlled by the brain is showing promising improvements in patients’ balance and stability, helping them to feel more control when doing activities like picking up an object from the ground or navigating tasks that might cause them to fall.

Surface electrodes placed over two residual calf muscles sense electrical signals from the brain telling the muscles to contract. The prosthetic ankle processes the electrical activity, and as the patient is telling their ankle to flex, the prosthesis makes the movement.

This type of neural control, called direct electromyographic control, allows the user to continuously control the limb, mimicking the way in which people without amputations move their lower limbs.

Aaron Fleming, who graduated last December with a Ph.D. from the UNC/NC State Joint Department of Biomedical Engineering (BME), was first author on a case study published last year. Along with Helen Huang, Jackson Family Distinguished Professor in BME, he has worked with five other participants.

Most research into robotic ankle prostheses has focused on autonomous control, which relies on algorithms to make movements while walking or standing.

“Early on, we didn’t know what was going to happen,” Huang said. “There wasn’t anything that we could build upon.”

Illustration of ankle prosthesis interacting with the human body and brain.

To use the device, participants worked with a physical therapist to train their residual muscles so that they’d be using them simultaneously with their non-amputated leg’s muscles. These exercises — such as squatting to pick up an object or standing with their eyes closed — tested their stability and posture control.

“Oftentimes, when an amputee reaches to pick something off of the ground, they actually will rotate, like lean their prosthesis forward. They’re on the toes of the prosthesis because they don’t have the range of motion in their daily device,” Fleming said.

But with the neural-controlled device, results showed that participants had a wider range of motion, more control and more power in their limb compared to when they were using their daily prothesis.

Huang and Fleming hope that the improved control people have when using this prosthesis will empower them to feel more comfortable and confident participating in their communities.

“When people are using neural control, there’s a real emotional impact,” Fleming said. “They say they feel like they have their foot back.”

While results have been promising, further evaluation is needed. Huang and Fleming are developing a proposal for a clinical trial. There is also ongoing work in the field to improve electrodes, including development of injectable ones the size of rice grains that can provide reliable signals for weeks or months at a time.