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Biomechanical Dynamics of Rapid-Stretch Nerve Injury Model
Mark A. Mahan, MD; Wesley Warner, BS; Stewart Yeoh, 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. Furthermore, prior research on nerve biomechanics is conflicted. Understanding the biomechanical response of nerves to rapid stretch, including failure location, is essential to developing models that mimic the clinical scenario.
Methods: The sciatic nerves of 103 freshly euthanized Sprague-Dawley rats were dissected and subjected to rapid- and slow-stretch methods. Rapid-stretch injury involved fixed direction strain produced via constrained weight drop applied to an intact nerve. Slow-stretch techniques included fixed direction strain produced by slowly progressive force and formal material testing instrumentation, both to an intact nerve. Maximal nerve strain, persistent length deformation, regional strain variation and location of nerve failure were recorded.
Results: Both rapidly- and slowly-stretch sciatic nerves failed at similar strain deformation values, which depended upon the vector of force (antero-posterior vector: rapid = 61.6%, slow = 56.6%; longitudinal vector: rapid = 14.4%, slow = 20.1%). The regional strain variation varied in association with periarticular regions, with maximal compliance at the knee (69.1%,) and at the hip (64.6%), and least at the mid portion of the sciatic nerve (10.1%). Rupture location of stretched sciatic nerves was dependent upon vector more than the rate of stretch and was associated with the differing regional compliance or branch location (antero-posterior vector associated with rupture at the hip/hamstrings branch in 70.1%; longitudinal vector associated with rupture at spinal (17%), middle (43%) and at trifurcation (30%). Velocity of stretch was positively associated with maximal strain and odds of rupture increased when sciatic nerves were stretched above 6 m/s. Elastic deformation (with return to within 15% of baseline value) occurred below 50% maximal strain (37% ±12% stdev), whereas plastic deformation (persistent length change) occurred above 50% maximal strain (59% ±17% stdev).
Conclusions: Effects of rapid-stretch nerve injury can reliably be predicted upon the vector and the rate of injury. Rupture of the sciatic nerve was predominantly predicted by rate of stretch. The location of nerve rupture was correlated with loading, with longitudinal tension on the nerve associated with avulsions and strain accumulation at non-compliant regions. In contrast, antero-posterior tension led to rupture at proximal branch points. Elastic-to-plastic deformation predictably followed the maximal strain occurred during stretching
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