Intraoperative evaluation of the spiral nerve cuff electrode on the femoral nerve trunk

Abstract

Evaluation of the Case Western Reserve University spiral nerve cuff electrode on the femoral nerve trunk was performed intraoperatively in four subjects undergoing femoral-popliteal bypass surgery. The threshold, nerve size and selective activation capabilities of the electrode were examined. The activation thresholds for the first muscle to be recruited were 6.3, 9, 10.6, and 37.4 nC with pulse amplitudes ranging from 0.3 to 1 mA. The femoral nerve was found to have an elliptical cross-section with a major axis average length of 9 mm (8-12 mm) and a minor axis length of 1.5 mm. In all four subjects selective activation of the sartorius was obtained. In two subjects, the rectus femoris could also be selectively activated and in one subject the vastus medialis was selectively activated. Each electrode had four independent contacts that were evaluated separately. Small air bubbles were formed in the space over some contacts, preventing stimulation. This occurred in one contact in each electrode, leaving three effective stimulation channels. This issue has been corrected for future studies.

Figure 1

Schematic of spiral cuff electrode. (A) Spiral electrode coiled, resulting in two full wraps. (B) Electrode unwrapped to show contact layout. The four independent contacts are located at 90° around the nerve. (C) Photo of the electrode. Notice tabs added to the leading edge of the cuff to facilitate implantation.

Figure 2

Schematic of the experimental setup. An EMG reference patch electrode was placed on the opposite hip.

Figure 3

Pulse width modulation recruitment with a pulse amplitude of 0.3 mA from subject 3. (A) Stimulation on channel 4. Sartorius was recruited to 65% activation before any other muscle reached threshold. (B) Stimulation on channel 3. Rectus femoris was recruited to 34% activation before any other muscle reached threshold but to 68% if 20% activation of other muscle was allowed. This was defined as preferential activation. Error bars show the standard deviation of each set of three twitches.

Figure 4

Selective activation from each subject (solid bars). A single muscle could be selectively activated to at least 50% in all subjects. The hatched bars represent preferential activation of muscles.

Figure 5

Example of functional selectivity: selective knee extension from activation of both the vastus lateralis and the vastus medialis in subject 3 using a pulse width modulation of 0.5 mA on channel 2. The vastus medialis reached 37% and the vastus lateralis reached 42% before either hip flexor reached threshold. Error bars show the standard deviation of each set of twitches.