Chronic stability and selectivity of four-contact spiral nerve-cuff electrodes in stimulating the human femoral nerve

Abstract

This study describes the stability and selectivity of four-contact spiral nerve-cuff electrodes implanted bilaterally on distal branches of the femoral nerves of a human volunteer with spinal cord injury as part of a neuroprosthesis for standing and transfers. Stimulation charge threshold, the minimum charge required to elicit a visible muscle contraction, was consistent and low (mean threshold charge at 63 weeks post-implantation: 23.3 +/- 8.5 nC) for all nerve-cuff electrode contacts over 63 weeks after implantation, indicating a stable interface with the peripheral nervous system. The ability of individual nerve-cuff electrode contacts to selectively stimulate separate components of the femoral nerve to activate individual heads of the quadriceps was assessed with fine-wire intramuscular electromyography while measuring isometric twitch knee extension moment. Six of eight electrode contacts could selectively activate one head of the quadriceps while selectively excluding others to produce maximum twitch responses of between 3.8 and 8.1 N m. The relationship between isometric twitch and tetanic knee extension moment was quantified, and selective twitch muscle responses scaled to between 15 and 35 N m in tetanic response to pulse trains with similar stimulation parameters. These results suggest that this nerve-cuff electrode can be an effective and chronically stable tool for selectively stimulating distal nerve branches in the lower extremities for neuroprosthetic applications.

Figures

Figure 1

(a.) The CWRU spiral nerve-cuff electrode. The electrode is designed to wrap twice around the nerve so that four contacts are equally spaced around the circumference. (b.) The unspiraled cuff with labels for contacts 1, 2, 3, and 4. (c.) A spiral nerve-cuff around the femoral nerve. The cuff is placed distal to nerve branches that innervate rectus femoris and sartorius, but proximal to branches that innervate the vasti.

Figure 2

(a.) An example of a typical normalized fine-wire intramuscular EMG response to a single stimulus pulse delivered via a nerve-cuff electrode contact. The triphasic M-wave response to stimulation occurs within the first 40 ms after stimulation (which occurs at t=0 s). (b.) A typical twitch knee extension moment response to a single stimulus pulse applied at t=0 s. The maximum of the response is calculated to create twitch recruitment curves. (c.) A typical tetanic knee extension moment response to a 3 second, 16 Hz train of stimuli (black bar). The middle one second of the three second response (gray box) is averaged to create tetanic recruitment curves.

Figure 3

Average stimulation charge threshold for bilateral nerve-cuff electrodes. Thresholds were measured three times, averages and standard deviations were calculated, and a one-way ANOVA was performed. Brackets and asterisks (*) denote statistically significant differences (p

Figure 4

Mean twitch EMG and isometric knee extension moment recordings when single stimulus pulses were applied three times via left nerve-cuff contacts 2 and 4 and right nerve-cuff contacts 2 and 4. Error bars are standard deviation. The horizontal black line on each plot marks 10% of normalized EMG, which is considered threshold for activation. All stimulus pulses were applied at a current amplitude of 1.4 mA.

Figure 5

Isometric recruitment curves in response to single stimulus pulses (right axes) and trains of stimuli (left axes) from left and right nerve-cuff electrodes. All stimulus pulses were applied at a current amplitude of 1.4 mA.