Intraoperative demonstration of selective stimulation of the common human femoral nerve with a FINE

Abstract

We have tested the hypothesis that the Flat Interface Nerve Electrode (FINE) can selectively stimulate each muscle innervated by the common femoral nerve of the human, near the inguinal ligament in a series of intraoperative trials. During routine vascular surgeries, an 8-contact FINE was placed around the common femoral nerve between the inguinal ligament and the first branching point. The efficacy of the FINE to selectively recruit muscles innervated by the femoral nerve was determined from electromyograms (EMGs) recorded in response to electrical stimulation. At least four of the six muscles innervated by the femoral nerve were selectively recruited in all subjects. Of these, at least one muscle was a hip flexor and two muscles 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 (FES). 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.

Fig. 1

Experimental setup for testing the FINE on the femoral nerve. A 4-channel stimulator is used to deliver the stimulus to one of the eight contacts in the FINE. The cable connecting the stimulator to the FINE must be switched between two cables to control the contacts either on the top or the bottom of the electrode, respectively. Differential EMG is collected from the six innervated muscles.

Fig. 2

A FINE similar to the one used in intraoperative experiments. Dimensions: 10 mm wide, 1.5 mm tall, 7 mm deep. Offset contacts maximized the spatial volume that was stimulated. Scale (right) in mm.

Fig. 3

The selectivity within each subject when costs were a function of all non-target muscles and did not exceed 10% for any non-target muscle (dark bars) and when cost were a function of all non-agonist muscles and did not exceed 10% for any non-agonist muscle (light bars).

Fig. 4

The maximum estimated knee extension moment increased as the acceptable estimated hip moment increased (left). No knee extension moment was estimated without hip flexion moment. The dashed line marks the threshold for the sit-to-stand transition. The maximum estimated hip flexion moment increased as the acceptable estimated knee moment increased (right). Hip flexion was estimated to occur without knee extension in three subjects. Hip flexion sufficient for gait was marked by the dashed line. Single contact stimulation.