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Presenter: Andrej Nedic, BSE
Co-Authors: Moon JD; Kung TA; Langhals NB; Cederna PS; Urbanchek MG
University of Michigan

Introduction: Regenerative peripheral nerve interfaces (RPNI) are used to transduce motor nerve signals to and electrical sensory signals from a robotic arm. Quantification of sensory feedback is essential to assessment of closed loop neural continuity. Von Frey (VF) filament testing has traditionally been used clinically for this purpose but has not been validated in the rodent. Our purpose was to (a) determine the ideal, 8-filament range for VF sensation measurements in the ankle and thigh of the rat, (b) determine the reliability at both locations and (c) apply the test in experimental studies of Mixed Nerve (MN) and Sensory Nerve (SN) RPNIs.

Methods: VF nylon filaments apply a standardized, reproducible force, based on filament diameter, when pressed on skin. VF reliability testing was applied to normal rats (n=6), at the ankle and thigh (on separate days), in an up-down pattern, 3 times per session, twice every week, for 6 weeks. Withdraw threshold was determined by alternately exceeding and reducing filament pressure. Once the reliability testing was completed, the perfected VF technique was administered to rats with MN-RPNI (n=12) or SN-RPNI (n=4).

Results: The ideal 8-filament range was 0.03 g-3.63 g at the ankle and 0.17 g-8.51 g at the thigh (Fig. 1). At the ankle, during the six week testing period, responses did not vary over time (Fig. 2). At the thigh, normal rats respond similarly during week 1 and 2 but with further test repetitions the rats responded at lower thresholds: weeks 3-7. At weeks 1 and 2, the ankle was significantly more sensitive than the thigh (p<0.05). The MN-PRNI group was significantly less sensitive than both the SN-RPNI and Normal groups (p<0.05); the SN-RPNI and Normal groups did not differ (Fig. 3).

Conclusion: Von Frey filament testing was reliable when standardized procedures were applied. It distinguished between sensation of normal skin with dense sensory endings (ankle) and disperse sensory endings (thigh). It also successfully discriminated between sensory and mixed motor nerve RPNI groups.
This work was supported by the DARPA RPI program under grant N6601-11-C-4190.

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