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Tiny Implants Combat Chronic Pain

Spinal pacemakers mask discomfort from nerve damage

3 min read

Back in 2006, Adam Hammond, a U.S. Army skydiver, experienced every jumper’s worst ­nightmare when his parachute failed to deploy. ”I was basically in free fall,” Hammond says of his ­accident. ”When I hit the ground, my ­helmet shattered and my shoes flew off my feet.” Hammond was completely ­immobilized by chronic pain for two years. Earlier this year he received an implanted device that electrically ­stimulates his ­spinal cord. Now, instead of ­feeling a ­stabbing pain in his tailbone, he ­experiences just a tingling sensation. ”My life has done a complete 180,” he says.

Smaller, longer-lasting implants are broadening the appeal of pain ­management devices for patients who have not been well served by ­conventional medications. With smaller sizes, surgeons have more flexibility with where to place the implant. And with better techniques for transferring and storing energy, the implants can last ­longer and be placed deeper in the body, which increases their cosmetic appeal.

The device that Hammond received, Advanced Neuromodulation Systems’ Eon Mini, is the smallest neuro­stimulator on the market. According to ANS, of Plano, Texas, depending on the power output used to block pain, patients can go between one week and a few months before the implant needs to be recharged—wirelessly—and the ­battery is expected to last 10 years. The device delivers the stimulation through 16 ­electrodes, which a physician can adjust individually to produce pulses of different intensities and frequencies. A patient can control the stimuli with an inductively coupled programming wand.

”What’s unique about pain is that it’s entirely subjective,” says Tom Hickman, the vice president of ­product management at ANS, a division of St. Jude Medical. ”What feels good for one patient won’t feel good for another. But the outcomes, the return of ­activity and quality of life, are objective.”

Physicians estimate that each year as many as 40 000 people ­worldwide receive spinal-cord-stimulation implants. That’s a small fraction of the roughly 5 million patients who might ­benefit from the treatment, say ­analysts at Medtech Insight, a market research firm in Irvine, Calif. But the ­mechanisms of why the therapy works are not well understood, and only about half of the implant recipients experience some relief from pain, says Linda Porter, a ­program director at the National Institute of Neurological Disorders and Stroke, in Bethesda, Md. The ­uncertainty, coupled with the ­approximately US $20 000 price tag, have made many physicians and hospitals wary.

Though surgeons have been using these stimulators for pain for about 30 years, technical innovation is ­fueling new research, and stimulators are currently the focus of a half dozen clinical trials in the United States.

Two factors have limited the ­efficacy of the device: the lifetime of the ­battery and the implant’s ­overall size. Occasionally, patients report that the pulse generator can shift and erode through the skin. The ­electrodes, too, can move, and their leads can break under stress from the frequent ­movements of the back. Few ­medical ­centers are prepared to ­handle ­migrating neuro­stimulators, which means patients might need to travel long distances for ­surgery to adjust the implant.

”The size of the battery is ­getting smaller, and with that, the less painfully [the device] nicks away under the skin,” says Krishna Kumar, a neurosurgeon at Regina General Hospital, in Canada. Improvements in the devices power supplies are helping, too. ”Three years ago, you had to throw the battery away every two years or so,” Kumar says.

Before a recent redesign, ANS’s implant consisted of a titanium can encasing the electronics, with a charging coil attached to the outside of the can. Like most neurostimulators, it was bulky.

The key to shrinking the device was to use both a smaller battery and a smaller charging coil, which was tucked inside the casing. Then the new coil was tuned so that it could receive energy efficiently through the ­titanium-alloy can without slowing down the battery’s recharging rate or sacrificing the depth at which the implant could be embedded. The new device is about half the thickness of its predecessor.

Though spinal cord stimulation has been approved in the United States to treat people with chronic limb pain and those who have had unsuccessful back surgery, the therapy is also used in Europe for patients with vascular disease and angina and is now being explored for other conditions, including migraines and obesity.

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