Biomechanical response in the ankle to stimulation of lumbosacral nerve roots with spiral cuff multielectrode--preliminary study.

Biomechanical response in the ankle to tetanic stimulation of the lumbosacral root was investigated to assess the potential for lower limb functional neurostimulation. Myotomal response in the leg was measured as the three-dimensional isometric torque in the ankle after extradural tetanic stimulation of the L3-S1 roots exposed surgically for herniated disc removal in five patients. The cuff multielectrode was employed to investigate functional topography of the roots by monopolar, bipolar, and tripolar electrode configurations. Four response patterns in the direction of three-dimensional torque vectors were observed. The L-5 and S-1 roots had the same response pattern, but S-1 roots produced stronger torques. Dorsiflexion torque was not obtained by stimulation of L-5 roots despite coactivation of the tibial anterior and peroneal muscles. Dorsiflexion torques were produced only by stimulating the L-4 roots. More selective bipolar and tripolar stimulations recruited force at higher thresholds and less gain. Additionally, some muscles were not activated by tripolar stimulation of the same root. In one L-4 root, the torque at lower electrical threshold was replaced by inverse torque at higher threshold, providing indirect evidence that different muscles may have motoneuron populations that differ in diameter or location within the root. Although dorsiflexion and plantarflexion torques are functional per se, they are accompanied by foot inversion and leg rotation torques (as well as proximal muscle contractions). Further experimental investigations on direct extradural stimulation of lumbosacral roots, either single or in combination, are recommended to explore the potential of lumbosacral nerve root stimulation for restoration of leg function.