Spinal Implants Restore Both Movement and Sensation After Complete Paralysis

Published in Nature Biomedical Engineering. Corresponding author David Borton, Brown University and VA Center for Neurorestoration and Neurotechnology.

Spinal cord injuries sever more than the ability to move. They cut off the sensory feedback that tells the brain where limbs are in space -- the proprioceptive awareness that makes coordinated movement possible. Restoring motion without restoring sensation is like trying to walk with your eyes closed and your feet numb. It is technically possible but dangerously imprecise.

A team from Brown University, Rhode Island Hospital, and VA Providence Healthcare has now demonstrated, for the first time, simultaneous restoration of both motor control and sensory feedback across a complete spinal cord injury. In a clinical trial published in Nature Biomedical Engineering, three participants who were paralyzed from the waist down used implanted electrode arrays to activate leg muscles and receive sensory information about their limb positions -- at the same time.

Electrodes above and below the break

The approach is conceptually straightforward but technically demanding. Surgeons placed small electrode arrays at two locations along each participant's spinal cord: one below the injury site and one above it. The below-injury array delivered patterned electrical stimulation to spinal nerves that control leg muscles. The above-injury array stimulated sensory pathways that still connect to the brain.

Previous research had shown that patterned spinal stimulation below an injury can drive muscle control in non-human primates. Other work had explored sensory stimulation separately. But no one had combined motor stimulation below the injury with sensory stimulation above it in the same person at the same time.

David Borton, an associate professor of engineering at Brown and a biomedical engineer at the VA Center for Neurorestoration and Neurotechnology, described this as the first time simultaneous motor stimulation and sensory feedback have been demonstrated in people with complete spinal cord injuries. He called it an important step toward fully bridging the gap created by a spinal lesion.

The DJ board

Fine-tuning the stimulation required participant involvement. The research team built a control device -- which they nicknamed the DJ board -- with an array of knobs and sliders that allowed participants to personally adjust which parts of the spinal cord received stimulation, along with the speed and intensity of the signals.

Participants used the DJ board to find stimulation patterns that produced specific leg movements. Jonathan Calvert, the study's lead author and now an assistant professor at UC Davis, reported that participants found using the board enjoyable. They were given target leg positions and navigated the controls until they achieved the correct stimulation patterns. Being able to see their legs move again and having direct control through the interface was significant for them.

Data from the DJ board sessions then trained a machine learning algorithm developed by Thomas Serre, a professor of cognitive and linguistic sciences at Brown. The algorithm optimized stimulation patterns by efficiently searching through the enormous space of possible parameters -- far too large for trial-and-error approaches -- to find precise matches between desired muscle activity and stimulation settings.

Sensory replacement, not sensory restoration

The sensory component required a creative workaround. Because the neural wiring connecting leg sensation to the brain has been severed, the researchers could not directly stimulate the nerves normally associated with feeling in the legs or feet. Instead, they used a sensory replacement approach.

Stimulation above the injury generated sensations in parts of the body that still have intact connections to the brain -- the chest, arms, or back. Participants then learned to associate these substitute sensations with specific positions of their legs. A particular tingling in the chest, for example, could be mapped to a specific knee angle.

To test whether this feedback was useful, physical therapists positioned participants' legs at varying degrees of knee bend while the participants were blindfolded. Using only the sensory feedback from the spinal stimulation, participants accurately reported their knee angles. The sensations were not natural -- participants did not feel their feet touching the ground -- but the information was reliable enough to be functionally useful.

One participant described the experience: they could tell when their foot hit the treadmill based on feedback up to the chest level. It was not like feeling the foot hit directly, but it was close enough to be useful.

Walking on a treadmill

The culminating experiment combined both systems. Supported by ceiling-mounted harnesses and aided by physical therapists, participants performed walking movements on a treadmill while simultaneously receiving motor stimulation to engage leg muscles and sensory stimulation to provide positional feedback. The participants could walk while accurately reporting when their feet struck the ground.

Participants described the sensory feedback as genuinely helpful in coordinating their movements, and indicated that this type of feedback could be useful in daily life activities like transferring in and out of a wheelchair.

A two-week study with long-term implications

The trial was short -- a two-week, in-hospital study with only three participants. That is too small and too brief to establish clinical utility or long-term safety, though no device-related adverse effects were reported. The results demonstrate feasibility, not efficacy.

The sensory replacement approach, while functional, is fundamentally different from natural sensation. Whether participants can maintain learned associations between substitute sensations and limb positions over months or years, and whether those associations remain useful outside a controlled laboratory setting, is unknown.

The machine learning optimization, while promising, was performed within the constrained two-week window. Longer-term studies will be needed to determine whether the algorithms can adapt to changes in the spinal cord's response to stimulation over time, which is expected to shift as the body adjusts to the implants.

Rehabilitation effects -- whether coordinated stimulation across an injury site could produce lasting improvements in function even without stimulation -- were not explored in this study. Borton noted this as a direction for future work.

The team plans to recruit new participants for a longer-term study testing spinal stimulation outside the hospital setting. If the approach can be made portable and reliable enough for daily use, it could offer a meaningful complement to existing rehabilitation approaches for the estimated 300,000 Americans living with spinal cord injuries.