Selective stimulation of the human femoral nerve with a flat interface nerve electrode

Abstract

In humans, we tested the hypothesis that a flat interface nerve electrode (FINE) placed around the femoral nerve trunk can selectively stimulate each muscle the nerve innervates. In a series of intraoperative trials during routine vascular surgeries, an eight-contact FINE was placed around the femoral nerve between the inguinal ligament and the first nerve branching point. The capability of the FINE to selectively recruit muscles innervated by the femoral nerve was assessed with electromyograms (EMGs) of the twitch responses to electrical stimulation. At least four of the six muscles innervated by the femoral nerve were independently and selectively recruited in all subjects. Of these, at least one muscle was a hip flexor and at least two were knee extensors. Results from the intraoperative experiments were used to estimate the potential for the electrode to restore knee extension and hip flexion through functional electrical stimulation. Normalized EMGs and biomechanical simulations were used to estimate joint moments and functional efficacy. Estimated knee extension moments exceed the threshold required for the sit-to-stand transition.

Figure 1

A FINE similar to the one pictured here was used in intraoperative experiments. Contacts were offset to maximize the spatial volume that was stimulated. Top view (top) shows the offset contacts. Side view (middle) shows the lumen through which the femoral nerve passed. Side view (bottom) shows the open FINE and the button designed to keep the FINE closed. Scale on right is in mm.

Figure 2

The target location for FINE placement is along the femoral nerve distal to the inguinal ligament and proximal to nerve branching (left) (Gray 1918). The exposed femoral nerve prior to implanting the FINE remained oblong in cross section (upper middle). The FINE placed around the exposed femoral nerve (upper right). Histological examination of a cadaver’s femoral nerve shows that its width is greater than its height (lower right) (Schiefer et al 2008).

Figure 3

Experimental setup for testing the FINE on the femoral nerve. A custom current-controlled stimulator delivered stimulus pulses to the FINE. Differential EMG was collected from each of the six muscles innervated by the femoral nerve. EMG was referenced to a ground patch placed on the contralateral leg, amplified, filtered and collected.

Figure 4

Protocol used to quantify the EMG response to nerve stimulation. The twitch response of the six muscles innervated by the femoral nerve (left). The twitch response of the sartorius (upper right). The EMG was rectified and integrated from 3 to 40 ms (dashed vertical lines) to quantify the twitch (lower right).

Figure 5

Recruitment curves generated for subject 4 at each contact. Each curve was obtained independently with only a single contact active. Contacts 1–3 and 6–8 were fixed at 0.3 mA. Contacts 4 and 5 were fixed at 10 µs. The dashed line at 10% represents threshold.

Figure 6

The maximized selectivity (lower of stacked bars) and corresponding recruitment (higher of stacked bars) for each muscle within each subject is shown under two regimes: (1) costs were a function of all non-target muscles and did not exceed 10% for any non-target muscle (left of grouped bars) and (2) costs were a function of all non-agonist muscles and did not exceed 10% for any non-agonist muscle (right of grouped bars). Subject 7 did not exhibit pectineus twitches.

Figure 7

Same as figure 6, except that costs were limited to 20% instead of 10%, illustrating an increase in recruitment for many muscles. The largest increase occurred for vastus lateralis recruitment in subject 6 when costs were a function of all non-target muscles.

Figure 8

The maximum estimated knee extension moment increased as the acceptable estimated hip moment increased (left). The dashed line marks the sit-to-stand transition threshold, which all subjects exceeded with minimal hip flexion moment. The maximum estimated hip flexion moment increased as the acceptable estimated knee moment increased (right). Hip flexion was estimated to occur with a negligible knee extension in three subjects. Hip flexion sufficient for gait was marked by the dashed line. Curves were constructed with single contact stimulation.