The principle behind functional electrical stimulation (FES) is similar to the one that enabled Luigi Galvani, the 18th Century scientist now memorialized in the term “galvanic response,” to make disembodied frog legs “kick” on his laboratory table. Galvani found that he could make frog leg muscles contract simply by sending a current through them. In life, the frog’s (or any organism’s) muscles contract when the brain sends neurologic impulses to them by way of the spine.
Spinal cord impairment (SCI), whether it’s the result of injury or disease, interrupts the flow of nerve impulses from the brain to the muscles. But that isn’t the end of the story as far as muscle movement goes for some people with SCI, thanks to the neuroprosthetic technology of FES.
“FES is the result of more than 25 years of activity that largely has taken place in Cleveland at our center,” explains Dr. P. Hunter Peckham, director of the Functional Electrical Stimulation Center at the Cleveland Veterans Administration and professor of Biomedical Engineering at Case Western Reserve University in Cleveland, Ohio. Dr. Peckham, along with Kevin Kilgore, PhD, and hand surgeon Dr. Michael Keith Keats, leads the Cleveland Center’s Quadriplegic Hand Research Team, which is working on a system that will enable certain quadriplegics to grasp items at will in their own hands.
Peckham cites several important conditions that drive the design of every FES device. “The motion must be functional and fluid, and it must be able to be controlled by the user.” And ideally, the technology must be durable enough to be used all day every day for as long as possible.
To stimulate the motion in the hands, Peckam explains, “electrodes are placed near the nerves that go to the muscles that provide hand-grasp and elbow extension. There are, fundamentally, three different methods that you can use to stimulate muscles. One way is that you can put the electrodes on the skin’s surface, over the muscles. Another way is, you can have the electrodes penetrating the skin, in a method called percutaneous implantation. This method delivers the stimulus more closely to the nerve. The third way is for the device, including the generator of the stimulation, to be implanted inside the body, and then all of the leads can be subcutaneous, or just under the skin’s surface.
“We’ve tested all three methods of delivery,” Peckham continues. “Our experience has been that the surface stimulation systems are not reliable for day to day use, because there are too many vagaries with the placement of the electrodes, and they’re hard to put on. For a device that’s intended for function, where the user independently has to make it work from day to day, those surface devices have not been sufficient, so we don’t use them. Percutaneous systems work quite well for testing purposes, but not for the long-term, because you have to maintain the interface between the device and the skin. With an implanted device, a lot of the paraphernalia is safely placed underneath the skin.”
These implantations require a surgical procedure. “The upside to this is that it is highly reliable for daily use,” Peckham says.
The surgery is not for everybody; it is intended for people with injuries in between C-5 and C-7 (different FES devices have different requirements, depending on which muscles in the body they’re designed to stimulate). “They can be complete or incomplete. Often, physicians don’t like to perform the procedure until we know a person’s injury is stable. We prefer to do it as soon as we know what level of function they’ll end up with. Patients should have a goal-directed desire for how they want to use their hand. Muscles have to have an intact nerve supply to the spinal cord. With SCI, these can be damaged. A person must also be able to achieve full range of movement. Sometimes, people have contractures in their joints, so even if the muscle is activated, the hand can’t open and close fully. We have to overcome those with therapy.”
The stimulator—the device that sends the electric signal down the wires—is placed in the upper chest, so users can feel if something goes wrong, such as a broken electrode. A small incision is made, and electrodes are put directly on the muscles under the skin. Next, the incision for the stimulator is made, and the wires leading from it are connected with the electrodes. The procedure takes four to five hours.
Patients are in soft casts for two to three weeks following the procedure, and then they undergo outpatient sessions to program the device for strengthening implanted muscles. Sessions to learn how to make the implant functional follow.
The stimulator is operated by an external controller, placed on the skin above the stimulator, that sends a radio signal on different channels, depending on which electrode (or muscle) the user wishes to stimulate.
FES is entirely controlled by the user. The control allows users to choose between palmar (the type of grasp used to pick up a cup) or lateral (the type used to pick up a pen) grasp. “In the external control unit, we have the ability to tailor how the device is programmed, in order to be responsive to how the person wants his or her hand to be used,” Peckham says. “Once it’s programmed, the user has total control of the device.”
The very same principles and devices used to make a quadriplegic’s hand able to grasp can be applied to help those with SCI from levels C-6-T-12 stand. “We position the electrodes in the muscles of the lower extremities. This is mainly intended for standing and transfers. The electrodes go to the muscles that extend the knee and hip, and those that cause the back to straighten. In this case, the control works a little differently. The person hits a switch, and it turns all of those muscles on gradually, so that they’ll extend. A person can do crude walking, called a swing-through gait, where the body is stiff. You move a walker forward, and then you extend the elbows to lift the legs and swing the body forward.”
Other FES applications developed at Cleveland and elsewhere have been designed to assist with breathing, bowel and bladder regimens, cough control, spasticity, sexual function, and even prevention and treatment of pressure ulcers.
For his work with FES, Peckham was named 1999’s Engineer of the Year by Design News magazine. “It’s a nice honor,” he says. “They identified this technology as being innovative in design. I think of myself as part of a team.” The center is primarily funded by grants from the National Institute of Health and the VA’s Rehabilitation Research and Development program’s Merit Review Awards. “That’s what allows us to do these things.” The center also does research regarding, for example, the implementation of percutaneous systems to test muscle function for higher SCI levels.
Thanks to FES, patients with SCI can grab opportunities in life that they’ve long been denied.
Lori A. Wood is a frequent contributor to Orbit.
