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Regenerative Peripheral Nerve Interface (RPNI) Muscle Graft Size Influences Optimal Signal Transduction
Yaxi Hu, MD, Ian C. Sando, MD, Racquel Sohasky, BS, Jana D. Moon, BS, Paul S. Cederna, MD, Melanie G. Urbanchek, PhD.
University of Michigan Medical Center, Ann Arbor, MI, USA.

PURPOSE - Regenerative Peripheral Nerve Interfaces (RPNIs) successfully form bio-artificial interfaces between severed nerves and electrical neuroprostheses. Accepted surgeries use muscle grafts of ~130 mg, however, the optimal grafted volume for electrical signaling has not yet been determined. This study evaluates RPNI electrical signaling capacity with respect to increasing grafted skeletal muscle volume.
METHODS - F344 rats (N=24) were assigned to 1 of 4 groups defined on implanted muscle volume: 150mg (RPNI 150), 300mg (RPN 300), 600mg (RPNI 600) or 1200mg (RPNI 1200). Each RPNI consisted of a semimembranosus allograft neurotized by the transected peroneal nerve. RPNIs were analyzed 3 months post-surgery for compound muscle action potential (CMAP), force, and histology.
RESULTS - RPNI 150 and RPNI 300 retained significantly greater muscle tissue volume than RPNI 600 and 1200 (Table 1). RPNI 150 showed a significantly greater CMAP compared to RPNI 1200. Percentage of viable tissue was highest for RPNI 150 and declined with larger RPNIs (Figure 1).
RPNI 150RPNI 300RPNI 600RPNI 1200p-value*
Number with complete data collection6/66/65/64/6-
Mass implanted (mg)171±6329±7**614±9**1213±18**<0.001
Muscle weight (mg)72±798±4134±10**183±13**<0.001
Muscle loss (%)58.3±3.770.2±1.1**78.1±1.7**85.0±1.1**<0.001
Viable tissue (% of total area)84.9 ±0.665.5±1.750.2±3.1**43.1±10.7**<0.001
CSA (mm2)7.04±0.779.54±0.7012.56±0.98**13.97±1.21**<0.001
CMAP (mV)6.6±1.34.7±0.83.1±0.62.3±0.7**<0.020


CONCLUSIONS - Small RPNIs of 150 to 300 mg are optimal for RPNI electrical signaling in a rat model. This is favorable since small muscle grafts will allow for implantation of multiple RPNIs to increase the number of outputs for voluntary control.


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