American Society for Peripheral Nerve

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Histologic Evaluation of Rapid-Stretch Nerve Injury Model
Mark A. Mahan, MD; Wesley Warner, BS; Stewart Yeoh, PhD2; Jie Zhang, PhD; Alan Light, PhD;
University of Utah, Salt Lake City, UT

Objective: While the majority of adult brachial plexus injuries result from high speed mechanisms, no laboratory model has been created to model rapid-stretch nerve injuries. Importantly, the microanatomical injury of rapid-stretch has not be qualitatively nor quantitatively described. Elucidating the patterns of injury is essential to developing models that mimic the clinical scenario.

Methods: The sciatic nerves of 28 freshly euthanized Sprague-Dawley rats were dissected and subjected to rapid-stretch nerve injury, utilizing fixed direction strain produced via constrained weight drop applied to an intact nerve. Biomechanical parameters, including maximal nerve strain, persistent length deformation, regional strain variation and location of nerve failure were recorded to categorize nerve injury patterns. Sciatic nerves were then histologically evaluated with modified Lillie's trichrome staining, osmium tetroxide, and immunofluorescent techniques. Serial longitudinal sectioning was performed to volumetrically quantify nerve injury patterns.

Results: Five injury grades were produced: control, sham, elastic (stretch with return to within 15% of pre-stretch length), plastic (persistent length change) and rupture. Two patterns of injury were noted on histologic examination: loss of fiber undulation (straight fibers) and microruptures of fibers. Epineurial tissue remained associated with the stretched sciatic nerve, except at rupture, partial rupture sites or segments with severe fiber disruption. No cases of intact epineurium and complete internal disruption were found (Sunderland grade 4). Serial longitudinal sectioning demonstrated a predilection for microruptures at the hamstring branch in elastic and plastic grade injuries, and incidence of microruptures demonstrated distinct levels based upon injury grade (p<0.05, Two-way ANOVA with posthoc tests). Loss of fiber undulation, similarly, was found to correlate with progressively severe injury (p<0.05, Two-way ANOVA with posthoc tests). Laminin immunofluorescence demonstrated that endoneurial tubes were intact in sciatic nerves stretched within elastic limits, but ruptured in plastic grade injuries.

Conclusions: The microarchitecture of peripheral nerves is altered in predictable patterns that primarily affect the fibers of the nerve, not external structural elements. The internal architecture is injured in graded fashion that reflects the tissue biomechanics. Nerve fibers are more resistant to rapid stretch than epineurium. This study suggests a new consideration for producing graded nerve injuries with a rapid-stretch nerve injury model.

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