Micro-CT Analysis of a Needle to Nerve Approach for Interfascicular Peripheral Nerve Stimulation in Porcine Nerves

MotivationPeripheral nerve interfaces typically have a tradeoff between selectivity and invasiveness. Interfascicular interfaces placed within the peripheral nerve, but outside of the fascicles preserving the perineurium are a promising avenue in balancing this tradeoff. This study quantifies electrode placement using novel tools designed to deliver flexible microwires interfascicularly in a porcine model. MethodsSeven Yorkshire pigs were implanted with Minimally invasive interfascicular Nerve Stimulation (MiiNS) arrays in the nerves of the brachial plexus resulting in a total of ten implanted nerve samples. High-resolution micro-computed tomography (micro-CT) of implanted nerves stained with phosphotungistic acid (PTA) showed contact placement relative to fascicular anatomy and all other contacts. The analysis also examined the relationship between microwire trajectory angle and fascicle traversal. Further, in a subset of samples (n=4), hematoxylin and eosin (H&E) histological analysis was performed after micro-CT imaging. These histological images were coregistered with the micro-CT to validate the positional information of the micro-CT. ResultsAcross the 56 total MiiNS microwire placements, 84% were interfascicular, while 16% were intrafascicular, characterized by traversal through a fascicle. Contacts were broadly distributed throughout the nerves cross section and the majority (79%) were in the central half of the nerves cross section (R>0.707), with no evidence of angular clustering in any single direction. On average, nearest neighbor distances between contacts measured 2256.26{+/-}1760.28 {micro}m in 3D and 730.71 {+/-}564.83 {micro}m transversely, with implants spanning 10.3 {+/-} 4.3 mm along length of nerve. The angle of wire trajectory into the nerve was correlated with fascicle traversal, with steeper angles of insertion associated with more instances of fascicle traversal. Histological analysis corroborated the fascicular borders and wire placements found on micro-CT and demonstrated perineurium integrity. SignificanceInterfascicular implantation can reliably access both central and distributed regions of the nerve, while not being confined to a single plane, enabling access to regions which have been historically challenging to stimulate effectively. The observed nearest-neighbor transverse spacing between contacts is within the range of reported ideal values for selective activation. The MiiNS placement characteristics show its potential as an effective peripheral nerve interface alternative which achieves distributed contact placement, including in the center of the nerve volume, while remaining predominantly outside of the perineurium.