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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, PhD, Charles Lu, MS, Alex K. Vaskov, MS, Deanna H. Gates, PhD, R. Brent Gillespie, PhD, Theodore A. Kung, MD, Cynthia A. Chestek, PhD, Paul S. Cederna, MD, Stephen W.P. Kemp, PhD.
University of Michigan, Ann Arbor, MI, USA.

Purpose: Without meaningful and intuitive sensory feedback, even the most advanced prosthetic devices remain insensate, burdensome, and are associated with enormous cognitive demand. To provide a bidirectional motor and sensory neural interface, we have developed the Regenerative Peripheral Nerve Interface (RPNI). An RPNI consists of a free muscle graft that is reinnervated by a transected peripheral nerve. In previous human studies, RPNI’s have demonstrated stable high amplitude motor EMG signals with high signal to noise ratio for prosthetic control, and have been used to treat and prevent neuroma pain. In this study, we investigate if applying electrical stimulation through the RPNIs can produce meaningful proprioceptive and/or tactile sensations in participants with upper-limb amputations.
Materials and Methods: Two distal transradial participants underwent surgical implantation of RPNIs for treatment of neuroma pain. One RPNI was created on each of the median and ulnar nerve for participant 1, while participant 2 had two RPNIs created on the ulnar nerve and one on the median nerve. The RPNIs were stimulated with a monopolar, charge balanced, biphasic square wave using intramuscular bipolar electrodes. To determine the sensory perception threshold, a stepwise increment of 0.1 mA was used at a constant 20 Hz frequency and 200 µS pulse-width. Stimulation lasted for 1 to 2 seconds for each increment. After each increment, participants were asked to report the sensations they felt. Once a sensation was felt, the amplitude was decremented until the sensation stopped, and then increased until a sensation was felt again. Amplitudes were then marked as the perception threshold. After a threshold was determined, amplitudes were increased until participants could clearly perceive the sensation. Participants then reported the location of the sensation on their phantom limb using their contralateral intact hand, and a description of the invoked sensation.
Results: Stimulation of the RPNIs resulted in both participants reporting proprioceptive sensations in their phantom limb. In particular, stimulation of the median nerve RPNI activated a flexing sensation in the thumb or index finger of participant 1, while stimulation of the ulnar nerve RPNI invoked a flexing sensation of the ring or small finger. Likewise, during stimulation of ulnar nerve RPNI 1, participant 2 felt a flexed sensation at the ring finger distal interphalangeal (DIP) joint, specifically describing that only the DIP joint was being flexed on the phantom ring finger. Stimulation of the ulnar nerve RPNI 2 and the median nerve RPNI resulted in cutaneous sensations that corresponded to the dermatome of their respective nerve. 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 skin-referred sensory feedback that could improve prosthesis motor performance, producing a stable bidirectional interface for advanced prosthesis control.


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