Recovery Patterns After Epineuroclasis and Endoneuroclasis Stretch Injuries in the Rat Median Nerve

Background/Objectives: This study used the neuroclasis nerve injury model to compare the long-term histomorphological, cellular, and inflammatory consequences of epineuroclasis and endoneuroclasis stretch injuries. Methods: Upon IACUC approval, 45 male Sprague-Dawley rats were allocated to 6 injury groups: an epineuroclasis and an endoneuroclasis group with follow-up at 2 weeks (n = 6 each), 6 weeks (n = 8 each), and 12 weeks (n = 8 and 9). Rats underwent left median nerve stretch to epineuroclasis or endoneuroclasis thresholds using load-deformation curve monitoring. Nerves were harvested at follow-up for qualitative histology and immunohistochemistry (H&E, NF200, S100, CD68, and Glut-1). Results: At 2 weeks, both injury levels demonstrated minimal NF200 staining, consistent with Wallerian degeneration. Epineuroclasis resulted in persistent epineurial rupture with exposed endoneurial tubes at 2, 6, and 12 weeks, indicating no structural restoration of the epineurium. At 12 weeks, axonal regrowth was observed primarily in regions with an intact epineurium and was limited in regions lacking epineurial coverage. Endoneuroclasis was associated with neuroma formation in 16/17 nerves (13/14 in-continuity) at 6 and 12 weeks, characterized by hypervascularity, aberrant fascicle formation, diffuse axonal sprouting, Schwann cell hypercellularity, and sustained macrophage infiltration. Conclusions: Epineurial integrity appears more important for axonal regeneration than previously thought. Persistent epineurial disruption was associated with limited axonal regrowth, whereas endoneurial disorganization was associated with neuroma formation despite macroscopic nerve continuity at time zero. Understanding the regenerative consequences of distinct degrees of connective tissue damage after stretch injury may help guide the development of tools that could diagnose these injuries early and potentially predict recovery after nerve injury.