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Restoration of Proprioceptive and Cutaneous Sensation Using Regenerative Peripheral Nerve Interfaces (RPNIs) in Humans with Upper-Limb Amputations
Philip P Vu, PhD1, Charles Lu, MSE1, Alex Vaskov, MSE1, Deanna Gates, PhD1, Richard B. Gillespie, PhD2, Theodore A Kung, MD3, Paul S. Cederna, MD4, Cynthia Chestek, Ph.D.1 and Stephen WP Kemp, PhD1, (1)University of Michigan, Ann Arbor, MI, (2)Mechanical Engineering, University of Michigan, Ann Arbor, MI, (3)Section of Plastic & Reconstructive Surgery, University of Michigan, Ann Arbor, MI, (4)Department of Surgery, Section of Plastic Surgery, University of Michigan, Ann Arbor, MI

Introduction: The sense of touch and proprioception permit meaningful interaction with the environment. Without meaningful and intuitive sensory feedback, even the most advanced prosthetic devices remain insensate, burdensome, and are associated with enormous cognitive demand and mental fatigue. We hypothesize that restoring sensory feedback to the user will not only improve functional prosthetic performance, but also improve prosthesis embodiment, or integration of the prosthesis to the user's body imagery. Currently, direct peripheral nerve electrodes have been able to increase prosthesis embodiment, and reduce phantom limb pain through electrical stimulation. However, they have not been able to also produce high fidelity motor signals for prosthesis control. To provide a bidirectional motor and sensory neural interface, we have developed the Regenerative Peripheral Nerve Interface (RPNI). A muscle graft reinnervated by a transected peripheral nerve, RPNI's have previously demonstrated stable high amplitude motor EMG signals with high signal to noise ratio in human studies.

Materials and Methods: In the current study, we tested if electrical stimulation of RPNI grafts produces proprioceptive and/or tactile sensations. Two distal transradial participants underwent surgical implantation of RPNIs for treatment of neuroma pain. The RPNIs were stimulated with a monopolar, charged balanced, biphasic square wave using intramuscular bipolar electrodes. Participants reported the location, and a description of the invoked sensation. A stepwise increment of 0.1 mA was used at a constant 20Hz frequency and 200 µS pulse-width to determine the sensory perception threshold.

Results: In both participants, proprioceptive sensation was reported in their referred phantom limb. In particular, stimulation of the median RPNI activated a “bending” sensation in the thumb or index finger of participant 1, while stimulation of the ulnar RPNI invoked a “bending” sensation of the ring or small finger. Likewise during stimulation of ulnar RPNI 1, participant 2 felt a “tugging” sensation at the ring distal interphalangeal joint, while stimulation of ulnar RPNI 2 and the median RPNI produced cutaneous sensations on the lateral side of the small finger and in the palm area below the thumb, respectively. Sensory stimulation thresholds remained stable across 7 and 11 months with an average amplitude of 1.27 ± 0.52 mA for participant 2, and 0.98 ± 0.04 mA for participant 1, respectively.

Conclusions: These results suggest that RPNIs have the potential to restore proprioceptive and cutaneous sensory feedback that could increase prosthesis embodiment and motor performance simultaneously, producing a stable bidirectional interface for advanced prosthesis control.

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