In practice, the act of walking doesn’t involve much more than repeatedly putting one foot in front of the other. However, a lot happens behind the scenes both at a conscious and subconscious level. Once one foot is off the ground, sensory feedback from the soles of both feet travels to the spinal cord, which processes this information and coordinates the necessary muscle contractions to realign the body’s weight over the feet, maintaining balance as body weight shifts forward. People who have undergone a lower limb amputation lose this sensory feedback and are at an increased risk of falls. As a result of missing sensory feedback, some people experience phantom limb pain, or pain coming from their missing limb.
In a recent study published in Nature Biomedical Engineering, researchers at the University of Pittsburgh stimulated the spinal cords of three people with below-the-knee amputations to generate sensations that felt like they originated from the missing foot (1). This restored sensory feedback, improved balance, and reduced phantom limb pain in all three participants. The findings could lead to a better quality of life for the 150,000 people who undergo amputation of a lower limb in the United States every year (2).
“What really excites me is the fact that they're [using a spinal cord stimulator] that is already available on the market, and they're using it in a way that reduces pain,” said Jacob George, a biomedical engineer at the University of Utah who was not involved in the study.
George explained that the improvements in pain might make insurance companies willing to cover a patient’s treatment costs since reducing pain would save insurance companies money while providing an obvious patient benefit. “There's a pathway to potentially get this technology to patients not only because the hardware exists, but also because the reimbursement could exist,” he said.
For their experiments, the researchers used spinal cord stimulation electrodes that are commonly used for treating low back and proximal limb pain in around 50,000 people every year (3). However, adapting the electrodes for treating amputation-related pain required some modifications and extensive reengineering.
For one, traditional spinal cord stimulation targets the peripheral nerves, such as those found in the arms or legs. “We put [the electrodes] in a slightly different spot near the spinal cord than they would traditionally be used so we could get sensations in the feet,” said biomedical engineer and study author Lee Fisher. This generated a feeling comparable to pressure in the missing foot upon spinal cord stimulation.
People want solutions that can [address their] phantom limb pain. The reaction that we saw from people when we turned the stimulator on was the most rewarding part of what we've done.
– Lee Fisher, University of Pittsburg
Unlike standard stimulators that deliver constant stimulation, this study's protocol required dynamic adjustments to the stimulation parameters based on real-time pressure changes. “Every 20 to 50 milliseconds we needed to change what the stimulator was doing based on the signals we were measuring from the insole,” Fisher said. Participants wore a prosthetic limb fitted with a shoe insole that could detect pressure changes, ensuring that stimulation mimicked real foot pressure sensations whenever the prosthetic made ground contact. This immediate feedback was crucial because it prevented the distracting lag that would otherwise occur, similar to what would happen when watching a movie with a sound delay.
The study subjects only received spinal cord stimulation while they were in the lab as the device is not ready for at home use. Before they can make the device available to patients, Fisher’s research team is working on finetuning it. They are currently building a next generation spinal cord stimulation electrode that generates more targeted sensations in the toes and heels for greater improvements in balance and phantom limb pain. They are also working on a spinal cord stimulator system that patients can use at home so that they can conduct a longer study to examine the long-term effects of spinal cord stimulation. “People want solutions that can [address their] phantom limb pain,” said Fisher. “The reaction that we saw from people when we turned the stimulator on was the most rewarding part of what we've done.”
References
- Nanivadekar, A. C. et al. Restoration of sensory feedback from the foot and reduction of phantom limb pain via closed-loop spinal cord stimulation. Nat Biomed Eng 1–12 (2023). doi:10.1038/s41551-023-01153-8
- Sexton, A. T. & Fleming, L. L. in Med. Manag. Surg. Patient Textb. Perioper. Med. (eds. Walker, H. K., Lubin, M. F., Spell, N. O., Smith, R. B. & Dodson, T. F.) 741–743 (Cambridge University Press, 2006). doi:10.1017/CBO9780511544590.119
- Kumar, K. & Rizvi, S. Historical and present state of neuromodulation in chronic pain. Curr Pain Headache Rep 18, 387 (2014).
