Hydrorelease Within the Paraneural Sheath: A Cadaveric Study

Background: Definitive quantification of fluid spread within the paraneural sheath (PNS) but external to the epineurium during hydrorelease (HR)-like procedures is lacking. We aimed to investigate the spread of low-volume HR within the intra-PNS surrounding the sciatic, tibial, and common peroneal nerves using human cadaveric specimens. Methods: HR with 2.5 mL of dye-mixed saline was performed under ultrasound guidance into the intra-PNS of seven lower limbs from four fresh-frozen cadavers. Dye spread was quantified by measuring longitudinal distance and circumferential dispersion, followed by anatomical dissection within 1 min of injection. Results: All injections demonstrated consistent longitudinal spread along the intra-PNS layer without intraneural infiltration. The mean spread distances were 10.63 ± 3.66, 9.97 ± 3.60, and 8.36 ± 3.04 cm in the sciatic, tibial, and common peroneal nerves, respectively, indicating no significant differences. An opposite-side circumferential spread was observed in all cases, with mean scores indicating mild-to-moderate extension. Conclusions: Low-volume HR selectively spreads within the intra-PNS layer, suggesting that this anatomical layer is a structurally valid and reproducible target for perineural injection techniques.