Physiologic Signaling of the Muscle Cuff Regenerative Peripheral Nerve Interface (MC-RPNI) during Volitional Behavior
Shelby Svientek, MD1, Jarred Bratley, BS2, Amir Dehdashtian, MD, MPH2, Carrie A Kubiak, MD2, Paul S Cederna, MD3 and Stephen WP Kemp, PhD2, (1)The University of Michigan, Ann Arbor, MI, (2)University of Michigan, Ann Arbor, MI, (3)Plastic Surgery, University of Michigan, Ann Arbor, MI
Introduction:
Exoskeletons have become a promising device to restore extremity function to those with limb weakness. However, these devices have not become widely adopted due to the inadequacy of current nerve-interfacing methods. The Muscle Cuff Regenerative Peripheral Nerve Interface (MC-RPNI) was developed as a potential solution to this problem. Consisting of a segment of autologous free muscle secured around an intact peripheral nerve, the MC-RPNI is able to regenerate and reinnervate, amplifying its contained nerve's signals through generation of EMG signals. These amplified, high-fidelity signals can then be used to intuitively and accurately control exoskeletons. The purpose of this study was to characterize MC-RPNI physiologic signaling during volitional behavior and determine long-term effects on the associated nerve.
Methods:
Eighteen rats were randomly assigned to one of three groups: (1) sham surgery/control; (2) nerve transection; and (3) MC-RPNI. MC-RPNIs were surgically fabricated by wrapping isogenic donor muscle graft circumferentially around the common peroneal (CP) nerve. At six months, CP nerve cuff electrodes were implanted in Groups 1 and 2, and patch electrodes were placed on all MC-RPNIs. Gait analysis and electrophysiological evaluations were performed the following day. Rats were trained to walk on a treadmill, and electrode recordings were obtained and correlated with gait videocapture. All rats had their proximal CP nerve stimulated, with efferent signals obtained at (1) downstream nerve (CSNAP), (2) MC-RPNI (CMAP), and (3) downstream-innervated extensor digitorum longus (EDL) muscle (CMAP). EDL muscle force testing was also performed following stimulation of the CP nerve.
Results:
All MC-RPNIs remained viable and demonstrated appropriate regeneration, revascularization, and reinnervation on histology. The MC-RPNI was found to generate large-amplitude CMAPs (2.77+/-0.926mV), amplifying its associated nerve's signal (50.5+/-8.18µV) over 50-fold on average. The MC-RPNI was not found to affect muscle function when evaluating downstream-innervated EDL CMAPs (control: 13.4+/-2.33mV vs MC-RPNI: 14.1+/-1.44mV) or maximal twitch force (control: 562+/-70.8mN vs MC-RPNI: 653+/-34.6mN). On gait analysis, recordings from the MC-RPNI correlated with the toe-off phase of gait; for the control groups, nerve signaling could not be differentiated from background noise. When comparing MC-RPNI to control animal gait, no significant differences were noted on qualitative or joint-angle analysis.
Conclusion:
The MC-RPNI has the ability to chronically amplify physiologic nerve signals from intact peripheral nerves by several magnitudes while avoiding functional impairment of downstream-innervated muscle. This amplification has the capability to facilitate high accuracy detection of motor intent in order to intuitively and reliably control advanced exoskeleton devices.
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