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Validating Regenerative Peripheral Nerve Interface Function in Relationship to Hind Limb Kinematics during Treadmill Locomotion
Daniel C. Ursu, MS, ,Andrej Nedic, MSE, Cheryl A. Hassett, BS, Jana D. Moon, BS, Nicholas B. Langhals, PhD, Richard B. Gillespie, PhD, Paul S. Cederna, MD, Melanie G. Urbanchek, PhD.
University of Michigan, Ann Arbor, MI, USA.
PURPOSE: Regenerative Peripheral Nerve Interfaces (RPNI) are neurotized autologous free muscle grafts equipped with electrodes to record myoelectric signals for prosthetic control. RPNI devices implanted into rats have been shown, using evoked responses, to be stable and viable for up to 2 years. In vivo characterization of RPNI signaling is critical for assessing their utility as a control modality for prosthetic devices. This work quantifies RPNI signal activation and relates it to gait; its ultimate purpose is to define signaling relationships between RPNI and native muscle during volitional control.
METHODS: Three experimental groups of two rats each were created (Fig 1): Control, RPNI, and Denervated. In the Control group, the soleus muscle remained intact; subjects in the Denervated and RPNI groups underwent soleus muscle neurotomy. The RPNI group received a free soleus muscle transfer to the ipsilateral thigh and reinnervation with the proximal end of the transected tibial nerve. In all groups, bipolar wire electrodes were positioned on the soleus surface. Evaluation was performed 4-5 months post-surgery.
Rats were conditioned to walk on a treadmill at constant pace between 8.5 and 9.0 m/min. A synchronized 120 frames per second videography and 3 kHz data acquisition system identified hip, knee, ankle, and toe joint angle trajectories and myoelectric signals. Each gait cycle and corresponding conditioned myoelectric signal was temporally normalized to facilitate comparisons across gaits and subjects. Within each subject group, normalized trajectories and signals were averaged. Integrated myoelectric activity over each phase of gait was then compared between subject groups.
RESULTS: Per the kinematic analysis, ankle movement in the Control group was characterized as normal (according to published data), while the ankle movement of the RPNI and Denervated groups exhibited a marked inability to extend the ankle. Myoelectric activity was highly repeatable within subjects and within subject groups. The Control group’s electromyographic signals were periodic with gait and reflected typical activation patterns (Fig 2). An altered electromyographic signal pattern was found for the RPNI group, but this pattern was also periodic with gait. A comparison of myoelectric activity integrated separately over stance and swing phases for the Control and RPNI groups indicates that RPNI signaling predicts firing of the tibial nerve that is lower, yet proportionally similar to Control. The Denervated group demonstrated low amplitude random myoelectric activity, unrelated to gait.
CONCLUSION: This study demonstrates that in vivo myoelectric RPNI activity is periodic and occurs during stance phase preceding push-off, when the tibial nerve is expected to be most active. Contamination from muscles adjacent to the RPNI is minimal, as demonstrated by the results obtained from the Denervated group. The periodicity recorded during RPNI firing suggests control governed by volitional or reflexive processes appropriate to an altered gait.
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