Implantable Stimulator Shows Promise for Chronic Knee Pain

NATIONAL HARBOR, Maryland — Stimulation of the infrapatellar branch of the saphenous nerve with an implantable electrical device is a potentially effective treatment for chronic, intractable knee pain.

In a small case series consisting of five patients with chronic knee pain, pain intensity scores on the visual analog scale (VAS) dropped from an average of 8 out of 10 before the implant to 1.4 out of 10 when measured 6 months afterward.

Pain relief was also long lasting, with an average score at 2 years still significantly reduced from baseline, at 3 out of 10 on the VAS.

"We have a lot of patients with chronic knee pain, and unfortunately, our hands are tied in terms of what we can do for them," lead author Kwo Wei David Ho, MD, PhD, Stanford University, California, told Medscape Medical News.

"They can use NSAIDs, physical therapy, some get steroid injections, or genicular nerve blocks, but they don't work that well. Some have knee replacement surgery, and can still have persistent knee pain after the operation, so here we are using an alternative therapy called peripheral nerve stimulation of the saphenous nerve. This provides a way to relieve pain without nerve destruction or motor dysfunction," Ho said.

The findings were presented here at the American Academy of Pain Medicine (AAPM) 2020 Annual Meeting.

Patient Controlled

For the study, the investigators surgically implanted five patients with intractable knee pain with the StimRouter™ (Bioness, Inc).

The device takes about 15 to 30 minutes to implant, much like a pacemaker, and reduces pain by delivering gentle electrical stimulation directly to a target peripheral nerve, in this case the saphenous nerve, to interrupt the pain signal, Ho said.

"A thin, threadlike lead, or noodle, is implanted below the skin next to the target peripheral nerve responsible for the pain signal under ultrasound guidance, and then a patch or external pulse transmitter (EPT) is worn on top of the skin. This sends electric stimulation through the skin to the lead," he explained.

The patient can then control the EPT and adjust stimulation with a wireless handheld programmer.

"Some patients turn it on at night for a couple of hours and then turn it off, some leave it on for the entire night, or the whole day if they prefer. What we've been noticing in our series is that after a while, patients are using less and less, and the pain gets better and better, and eventually they stop using it entirely because the pain completely resolves," Ho said.

Good candidates for this treatment are post-knee replacement patients with residual pain, he added.

Durable Effect

Of the five patients in the case series, four had previous knee arthroplasty.

To determine the chances of a good response to the implant, study participants underwent a diagnostic saphenous nerve block, with the rationale that if the block successfully reduced knee pain by 50% or more in the short term, patients would likely respond well to the implant.

Before the peripheral nerve stimulation implant, the average pain intensity was 7.8 out of 10 on the VAS. After stimulator implantation, the average pain intensity was 1.4 at 6 months (P = .019, in 5 patients). At 1 year, the average pain intensity score was virtually the same, at 1.5 on the VAS, (P = .0032, in 4 patients). At 2 years, the average pain intensity score was 2.75 (P = .12, in 2 patients).

"This study provides preliminary evidence that stimulation at the saphenous nerve may be effective for selected patients with chronic knee pain," Ho said. 

Commenting on the findings for Medscape Medical News, Patrick Tighe, MD, MS, University of Florida, Gainesville, said that chronic knee pain continues to present "numerous diagnostic and therapeutic challenges for many patients."

"It may be surprising, but there is still so much we don't know about the innervation of the knee, and we are still learning about different ways to alter the behavior of those nerves," said Tighe, who was not involved with the current study.

"This work points to some exciting opportunities to help patients suffering from chronic knee pain. We certainly need more research in this area to figure out the optimal approach to applying these findings more widely," he said.

Ho and Tighe have disclosed no relevant financial relationships.

American Academy of Pain Medicine (AAPM) 2020 Annual Meeting: Abstract 410. Presented February 29, 2020.

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Cite this: Implantable Stimulator Shows Promise for Chronic Knee Pain - Medscape - Mar 05, 2020.