Spared Motor Neurons Enable the Control of a Robotic Sixth-Finger for Assistive Grasping in Tetraplegia

Recovering hand function is among the highest priorities for individuals with tetraplegia. Yet, current treatments restoring basic hand movements remain limited for individuals with motor complete spinal cord injury. In this study, we present a non-invasive neuromechatronic interface that directly translates spared motor neuron activities that once encoded the opening and closing of the hand to a supernumerary robotic sixth finger. We re-enabled three individuals with chronic (> 8 years) motor complete cervical spinal cord injury to grasp objects important for daily living with the same neural input that controlled the flexion and extension of the fingers. After a few minutes of training, the participants intuitively modulated the discharge activity of their motor units, controlling hand opening and closing. These motor units were then used to control the robotic sixth finger proportionally. All participants successfully executed various grasping tasks that required considerable force from the digits, e.g., opening a bottle by unscrewing the cap. Our findings present a transformative step in restoring hand function, offering an intuitive and non-invasive neuromechatronic interface without the need to learn new motor skills, as the participants use the same motor commands as before the injury. This can significantly improve the quality of life of individuals with paralysis.