Functional and biological test of a 20 channel implantable stimulator in sheep in view of functional electrical stimulation walking for spinal cord injured persons

A newly developed implantable stimulator with 20 output channels, mainly in tended for the stimulation of lower extremities in paraplegics, was implant ed in 6 sheep over a time period of 26 weeks. Five epineural electrodes eac h were used to contact various nerves at different locations to elicit hip and knee extension and flexion and to make carrousel and selective stimulat ion possible. Different electrode application strategies in view of paraple gic standing and walking were investigated. Additional implanted electrodes allowed M-wave monitoring for selectivity investigations in 3 sheep. Stimu lator, electrode leads, and electrodes proved to be reliable. Selective sti mulation with electrodes placed on the trunk of the sciatic nerve could be demonstrated but with bad reproducibility. Histological investigation of th e tissues surrounding electrodes and leads showed the expected stable forei gn body response. Strong hip and knee extension could be gained in all case s while only weak flexion forces could be elicited in most cases. Muscle bi opsies showed that daily stimulation for 8 h at threshold level caused an i ncrease in muscle Type I fibers and a decrease in Type IIc fibers. Implants and electrodes fulfill the most important functional and biological criter ia for their clinical application for paraplegic walking. The intention to provide selective flexion functions via epineural stimulation could not be demonstrated sufficiently in this animal model.