Direct Motor Point FES demonstrates improved Range of Motion and Hand Dexterity in NHP Model

Introduction: Spinal cord injury (SCI) is a devastating chronic injury. One-third of SCI patients are tetraplegic due to cervical injuries, impairing daily activities. Brain-machine interfaces (BMIs) promise precise control for assistive devices, however they face challenges with functional restoration of fine motor skills and severe fatiguability. Our large animal model construct, direct motor point FES, targets peripheral nerve branches, enabling fatigue-resistant muscle activation in the chronic setting.

Methods: Motor points were found anatomically in 2 rhesus macaque arms, and confirmed intraoperatively during live surgery. 16 muscles were identified (8 flexors, 8 extensors). Upper extremities were then acutely tested for confirmation, stimulation parameters for muscle belly were 5 mA,10-25 microsecond pulse width, with a 32 ms interpulse interval. Bipolar electrodes were implanted into arm at 16 points intraoperatively, and implanted for chronic testing. Over the course of 1 year, electrophysiologic parameters of the implanted muscles were evaluated for features of activation, range of motion, fatigue reduction, and stability of signal parameters.

Results: Motor points of the extrinsic hand muscles were identified via in ex-vivo exploration. Motor points on average were around 1.25 cm away from each other, which was favorable for implantation of bipolar microelectrodes. Wrist Function demonstrated a large and functional range of motion during chronic stimulation in dmpFES. Finger movement demonstrates paired movement function and high range of motion during chronic stimulation in dmpFES, with an average extension of 191.5 degrees and average flexion of 97 degrees. Wrist movements demonstrated high specificity for movement, while finger movement demonstrated current dependent specificity in activation. Finally, chronic dmpFES stimulation utilizing a fatigue stimulation protocol demonstrated improvement in fatigue in comparison to current state of the art.

Conclusion : The major limitation of current FES systems is specificity of movement and fatigue. We demonstrate that individuated finger movements are possible and that fatigue mitigation can be achieved through directed stimulation of the nerve motor point. Further studies will be needed to determine the feasibility of a complete upper extremity construct for tetraplegia