What Is the Sequence of Mechanical and Structural Failure During Stretch Injury in the Rat Median Nerve? The Neuroclasis Classification.

Background: Peripheral nerve injury commonly results in long-term disability and pain for patients. Recovery after nerve traction or crush injury is unpredictable and depends on the degree of injury. Our inability to accurately assess the severity or degree of injury hampers our ability to predict the chances for recovery or need for surgical intervention in the form of neurolysis, nerve repair, or nerve graft. An investigation into the histologic sequence and mechanics of nerve failure under tension may help in the process of accurately assessing the severity of the nerve injury, the prognosis for recovery, and the need for surgical treatment.

Questions/purposes: Using an in vivo rat model, we asked: (1) What is the pattern of mechanical failure during nerve stretch? (2) Is there staggered disintegration of specific anatomic substructures when mechanical failure occurs?

Methods: To answer our first research question about the pattern of mechanical failure during nerve stretch, four 12-month-old male Sprague-Dawley rats were enrolled in a load-to-failure experiment generating load-deformation curves of the rat median nerve. Under anesthesia, the median nerves of both forelimbs were surgically exposed and secured under two blunt metal pins 1 cm apart. A metal hook was attached to a load-cell and raised from beneath the nerve at a speed of 0.2 mm/second until complete rupture occurred. Applied forces were monitored in real time via a force-time curve. All experiments were filmed, and the rats were euthanized afterward. Based on load-to-failure experiments, we identified two distinct events of sudden force reduction during stretching in the load-deformation curve of the rat median nerve. We labeled the first of these two events as epineuroclasis and the second as endoneuroclasis. Neuroclasis derives from the Greek term "neuron" for nerve, and the suffix "-clasis" means breaking or fracture. An additional eight rats were used to investigate whether this staggered mechanical failure was caused by staggered disintegration of specific anatomical substructures. Under anesthesia, eight left median nerves were stretched to the epineuroclasis point and eight right nerves to the endoneuroclasis point. Induction of injury was confirmed by load-time curves, and nerves were held in place for 5 minutes before tension was released. Nerve function was assessed before and after injury using a handheld electrical nerve stimulator. The nerves were harvested for histology (to assess integrity of the epineurium, axons and intraneural vasculature, endoneurial collagen [dis-]organization, as well as molecular collagen damage), and the rats were euthanized immediately after. The uninjured median nerves of two additional rats were harvested for histology as control tissue. Mechanical, functional, and histologic findings were compared between both injury levels and with uninjured nerves.

Results: Load-to-failure experiments revealed a characteristic failure pattern of the rat median nerve with two distinct events of mechanical failure that occurred at a mean ± SD resistance force of 2.3 ± 0.5 N and 1.4 ± 0.2 N (mean difference 0.9 ± 0.6 N [95% confidence interval 0.4 to 1.4]; p = 0.003), respectively. Additional experiments investigating both mechanical failure points revealed that the first failure point (epineuroclasis) was associated with epineurium rupture and plastic deformation of nerve fibers, whereas the second injury point (endoneuroclasis) was associated with failure of endoneurial tubes, axons, and intraneural vasculature. Epineuroclasis severely impaired nerve conductivity (median stimulation of 25 nC [range 25 nC to 50 nC] preinjury and 170 nC [range 25 to 300 nC] after epineuroclasis, difference of medians 145 nC; p < 0.001). Endoneuroclasis induced an even greater functional impairment than epineuroclasis (median stimulation of 400 nC [range 300 to 2000 nC] after endoneuroclasis, difference of medians 230 nC; p = 0.005). Both injury levels could be induced through live analysis of resistance forces during nerve stretch.

Conclusion: Peripheral nerve injuries follow a characteristic sequence of mechanical and structural failure, with two distinct injury levels preceding nerve transection.

Clinical relevance: The order and degree of failure in the rat median nerve seems to follow a predictable "outside-in" pattern. This sequence of mechanical and structural failure may inform a classification system of nerve stretch injuries that may more accurately reflect the pathoanatomy and prognosis. We offer a new classification of nerve injuries based on the sequence of nerve failure in rats. However, future studies are needed to validate the applicability of the neuroclasis classification to human peripheral nerves before clinical implementation and use can be proposed. The characterization of specific injury levels using an animal model provides a framework that could facilitate the development of novel diagnostic tools, potentially identifying the specific structural changes found in this study in the acute clinical setting.