Identification of spread after deliberated intraneural injection in five mammalian species: not all animal models and needle types are valid for studying the effect of intraneural injections in humans

This research endeavors to investigate the phenomenon of intraneural spread across distinct locations: subcircumneurium, extrafascicular intraneural, intrafascicular intraneural, and intraperineurium after deliberate intraneural injections across five mammalian species. The study also aims to propose determinants influencing this spread. Furthermore, the investigation strives to ascertain the optimal animal species and needle configuration for extrapolating intraneural injection outcomes to human contexts.This study examined 60 sciatic nerves from 30 fresh and untreated cadavers of rats, rabbits, dogs, pigs, and sheep. The specimens were organized into five groups, each comprising an equal number of nerves. Histological assessments were performed on 30 nerves, involving fascicle metrics. The remaining 30 nerves underwent intentional intraneural injections, facilitated by 19G and 23G needles under ultrasound and direct visualization guidance.Heparinized erythrocytes combined with a methylene blue solution were used as a marker to analyze the extent and patterns of intraneural spread. Needle orifice measurements were obtained, and these data were overlaid onto images of both nerves and needles. This enabled a comparative evaluation of sizes and an assessment of marker diffusion.The findings indicated that sciatic nerves in rats, rabbits, and dogs were oligofascicular, characterized by larger fascicles, whereas pigs and sheep exhibited polyfascicular nerves comprised of numerous smaller fascicles. Fascicular diameters were variable across species, with dogs presenting the largest measurements. While intraneural spread was observed and documented, intrafascicular marker spreading was rare, occurring only in one rabbit specimen. Needle orifice attributes were scrutinized and visually depicted.Despite the formidable challenges associated with the practical realization of intrafascicular injection, the utilization of animal models possessing monofascicular or oligofascicular nerves, such as rats, rabbits, and dogs, in conjunction with needles featuring aperture dimensions surpassing those of the fascicles, likely contributes to the compromised reliability of investigations into intraneural injection outcomes.