Functional Electrical Stimulation (FES) can restore limb movements through controlled and coordinated activation of paralyzed muscles. The peripheral nerve is an attractive site for stimulation using cuff electrodes. To restore coordinated movements, many applications will require the electrode to selectively activate subpopulations of axons within a common nerve trunk. The chance of selective activation can be increased by using an electrode with multiple independent channels. However, currently implantable stimulators have a limited number of channels that can be devoted to an electrode. Therefore, it is desirable to design an optimal electrode that maximizes the likelihood of selectivity while minimizing the number of channels.
We hypothesized that through computer simulations we could find an optimal electrode design that would selectively activate the femoral nerve with a minimal number of contacts. Bio-inspired Finite Element Method (FEM) models were developed from cadaver femoral nerve specimens. Simulations investigated multiple Flat Interface Nerve Electrode (FINE) configurations to determine the optimal number and locations of contacts for the maximum muscular selectivity in the presence of naturally occurring anatomical variations. The performance of the FINE on the proximal femoral nerve for standing and stepping applications was estimated. It was concluded that eight channels
are sufficient for standing and stepping with an FES system using a FINE on the common femoral nerve. FINEs were manufactured for intraoperative testing based on the modeling results.
Four intraoperative studies – the first human trials using a FINE – were conducted to evaluate the efficacy of an 8-contact FINE on the femoral nerve. Data indicated that an 8-contact FINE can selectively activate most muscles innervated by the common femoral nerve. The selectivity values exceeded those previously obtained with a multicontact spiral nerve cuff electrode applied to the same location on the nerve. Estimated knee extension moment was sufficient for the sit-to-stand transition.
The results obtained during the intraoperative study correlated with those predicted by the computer modeling study. According to the modeling and intraoperative studies, a single 8-contact FINE with four contacts located above the nerve and four contacts located below the nerve and offset from those that are above can selectively stimulate most muscles innervate by the femoral nerve without a priori information about its subject-specific fascicular structure. These data support the use of the FINE in chronic trials of lower extremity neuroprostheses.