New spinal implant gets paralyzed people up and walking

Spinal cord injuries are life-altering, as they prevent the transmission of nerve impulses past the point of injury. That means no sensory inputs make it to the brain, and no signals from the brain make it to the muscles normally controlled by the brain. But improvements in our understanding of neurobiology have raised the hope that we can eventually restore some control over paralyzed limbs.

Some of these efforts focus purely on nerve cells, attempting to get them to grow through the damage at the site of injury and restore a functional spinal cord. Others attempt to use electronics to bypass the injury entirely. Today, there was very good news for the electronics-focused effort: researchers have designed a spinal implant that can control the leg muscles of paralyzed individuals, allowing them to walk with assistance within hours of the implant being activated.

Skipping the brain

Much of the spinal cord is composed of long extensions made by nerve cells, termed axons. These axons allow nerve impulses to travel long distances, which is necessary for information to travel back and forth to the brain. Sensory inputs, like pain in your elbow or tickling of your feet, ride axons up the spinal cord into the brain. The brain in turn sends signals back down the spinal cord, controlling your breathing or moving your arms.

Injuries to the spinal cord can physically sever these axons, disrupting this communication. Cells have the ability to regrow axons in many cases, but in the spine, heavy scar tissue develops that blocks this process, meaning lost communication may never be restored. In some cases, the injury is partial, and the remaining function can be re-trained to work with the function that's left. But in other cases, the damage is severe enough that very little nerve function is retained, and paralysis is permanent.

Or at least permanent in the absence of medical intervention. Many researchers are working to find ways to limit or eliminate the scar tissue and induce regrowth of the severed axons. But this new research doesn't involve any of this.

An alternative to this type of biological repair is what you might consider an electronic bypass. In its most sophisticated form, this would involve an implant that registers neural activity, located either in the brain or in the spinal cord closer to the brain than the injury. This is then paired with some sort of hardware—potentially another implant—on the far side of the injury that stimulates the nerves based on the information read by the other implant.

A less sophisticated version of this is to simply have pre-programmed behaviors you want to control, such as the leg movements involved in walking. That is the approach used here. But, as will become very clear, "less sophisticated" leaves a whole lot of space for some very sophisticated work.

A model spine

To control walking, the axons that bring nerve signals to the leg muscles have to exit the spine; decades of research have identified the specific bundles of nerve fibers in the lower spine where the axons make their exit. Different bundles ennervate different muscle groups, allowing the potential for fairly precise control. But so far, that potential has been unrealized.

The team started by looking in detail at these bundles in 27 different individuals (some of them cadavers, the rest CT or MRI scans) and found that there's a fair amount of variability in the details of the spine's structure, although the size of the nerve bundles doesn't change much. But the researchers were able to use this data to build a model of the spine and virtually experiment with electrode location and size in order to see which nerve bundles they would stimulate. This eventually led to the design of an implant with 16 individual electrodes that should allow control over which nerve bundles were activated.

At this point, the three volunteers for this trial, each of whom had lost use of their legs, got involved. They were placed in an MRI tube to monitor neural activity while their legs were moved. The movement caused their leg muscles to send signals back to the spine regarding their altered tension, setting off activity in the nerve bundle that enervated the muscle. Reading this activity using the MRI allowed the researchers to figure out which nerve bundles were associated with specific muscles.

Using the mix of anatomical and activity data that resulted, the researchers built a computer model for each of the three individuals. They then used this model to control the stimulation of the leg muscles, testing out different potential motions while the subjects were lying down and then fine-tuning the model based on any unwanted movements that resulted. Overall, this process took about an hour.

The results were pretty astonishing. Prior to activating the implant, none of the three participants could initiate any sort of muscle activity when attempting to take a step. The same day that the model was trained, all of them could take steps on a treadmill if they were supported. The model was able to generate the right series of currents to stimulate the leg muscles appropriately.

Out for a walk and more

With three days of fine-tuning, the participants were able to walk around a room if given sufficient support. Eventually, they were able to stand unaided and walk supported only by a walker—their legs were controlled via an implant in their abdomen, which responded to triggers on the handles of the walker. One was even able to go up stairs.

Separate programs were also developed that allowed them to ride recumbent bicycles or to paddle a kayak.

One striking thing is that two of them actually regained the ability to exert a bit of voluntary muscle control in their formerly paralyzed limbs. Apparently, a bit of weak connectivity was still present but unable to provide a signal strong enough to trigger muscle activity. With extended activity, those weak connections were gradually strengthened, providing a complete pathway from brain to muscle.

Obviously, this is a pretty intensive medical intervention. But most people with spinal damage face years of intensive therapy in any case—and it often makes a minimal difference. In this case, the implant offered same-day progress and an end point of independent mobility. While the implant didn't restore a normal stride, it was flexible enough to handle multiple motions, allowing the participants to engage in a number of activities that require coordinated muscle action in the lower body.

The research team is clearly thinking that this is a potentially general solution, since they discuss that they'll likely need additional electrode arrangements to serve the full diversity of human anatomy. They also mention the possibility of personalizing the electrode arrangement based on imaging, which they suspect would allow better control over muscle activity. And some potential improvements could be gained from updating the model in response to performance.

So, this is definitely a technology that's going to be worth watching over the coming years. The work was part of a clinical trial, so we'll likely see more news soon.

Nature Medicine, 2022. DOI: 10.1038/s41591-021-01663-5  (About DOIs).