Impact of Muscle Graft Volume on Signaling Capacity in the Regenerative Peripheral Nerve Interface for Neuroprosthetic Control

Introduction: The Regenerative Peripheral Nerve Interface (RPNI) is an interface for high-fidelity control of neuroprostheses in amputees. In standard RPNIs, a small (130mg) graft of free skeletal muscle becomes quickly revascularized by collateral blood flow. Larger muscle grafts may increase signaling capacity by greater amplification of nerve signals if they remain viable. The purpose of this study is to evaluate the impact of muscle volume on RPNI survival and signaling capacity by comparing RPNIs with varying muscle graft size.

Methods: Thirty F344 rats were assigned into one of five groups (n=6 per group). In RPNI groups, each RPNI was implanted in the same fashion: the semimembranosus muscle was harvested from a donor rat, longitudinally trimmed to its predetermined weight of approximately 150mg (RPNI-Control), 300mg (RPNI-300), 600mg (RPNI-600) or 1200mg (RPNI-1200), transferred into the recipient rat and then neurotized by the transected common peroneal nerve. In the negative control group, the peroneal nerve was transected and no muscle graft was implanted. After 3 months of recovery, in situ nerve conduction studies, muscle force testing and histology analysis were performed on the RPNI.

Results: Groups with greater mass retained less tissue volume (Table 1). Also, mean compound muscle action potential amplitude was significantly smaller for RPNI-1200. Histology examination demonstrated central necrosis within the large RPNI-600 and RPNI-1200 muscle grafts, indicating they were too big for adequate regeneration in the rat model (Fig1).

Conclusion: Muscle grafts eight times greater in mass than our standard rat RPNI demonstrated significantly reduced signaling capacity attributable to limitations in muscle regeneration, reinnervation and revascularization. We suggest an upper limit of 300mg for survival of free muscle grafts and optimal performance of the RPNI in the rat model. 

 

	RPNI-Control	RPNI-300	RPNI-600	RPNI-1200	p-value*
Number of rats evaluated	6/6	6/6	5/6	4/6	-
Mass implanted (mg)	171±6	329±7**	614±9**	1213±18**	<0.001
Muscle weight (mg)	72±7	98±4	134±10**	183±13**	<0.001
Muscle loss (%)	58.3±3.7	70.2±1.1**	78.1±1.7**	85.0±1.1**	<0.001
CSA (mm2)	7.04±0.77	9.54±0.70	12.56±0.98**	13.97±1.21**	<0.001
CMAP (mV)	6.6±1.3	4.7±0.8	3.1±0.6	2.3±0.7**	0.020
Tetanic Force (mN)	289.0±43.3	259.2±49.8	235.0±75.8	116.4±31.0	0.204

     

Legends

Table 1. Summary data of major outcomes (mean±SEM), *indicates significance level of main effects for one-way ANOVA. Multiple comparisons, **indicates different from RPNI-Control (p<0.05).

Figure 1. Representative images of HE-stained cross-sections of RPNI muscles.