ABSTRACT
Study type: Basic science research report
Introduction: Spinal nerve-injury management and prevention constitute a substantial proportion
of a spinal surgeon’s practice. Functional recovery after peripheral nerve injuries
is often unsatisfactory and to optimize the outcomes, an intimate understanding of
these injuries is required. Sunderland classified peripheral nerve injuries into five
grades [1]. Grade 1 (neurapraxia) and grade 2 (axonal disruption) injuries usually recover
with no or insignificant functional deficits within weeks to a few months, respectively.
Injuries that are most difficult to manage clinically are the often mixed grade 3
(endoneurial disruption) and grade 4 (perineurial disruption) lesions where spontaneous
functional recovery is limited or absent, resulting in neuroma in continuity (NIC).
Traumatic NIC is characterized by aberrant intra- and extra- fascicular axonal regeneration
and scar formation within an unsevered injured nerve, resulting in impaired and erroneous
end-organ reinnervation [2]
[3]. Animal models reproducing grade 1, 2, 3, and 5 lesions have been developed, but
to our knowledge a clinically relevant rodent model of NIC has not been developed
[4]
[5]
[6]
[7]
[8]. The effective peripheral nerve regeneration and resilience of rodents make it challenging
to recreate the NIC scenario.
Objective: Our goal was to develop a practical rodent model for focal traumatic NIC, demonstrating
the characteristic histological features, supported by concordant functional deficits.
Such a model may help us to identify this injury pattern earlier and allow development
of intervention strategies to reduce neuronal misdirection, scar formation, and enhance
regeneration for improved functional recovery.
Methods: Various injury techniques were tested on freshly harvested Lewis rat sciatic nerves
ex vivo, and examined histologically before inflicting more refined injuries in vivo.
The optimal experimental injuries combined a 50 g traction force applied with a spring
scale hooked around the sciatic nerve, and focal three second maximal compression
using a malleus nipper (Figure [1]). Nerves were harvested at 0, 5, 13, 21, and 65 days, and processed for longitudinal
8 micron cryostat sectioning, H&E, laminin, neurofilament, and Masson’s trichrome
staining. Skilled locomotion (tapered beam, ladder rung) and flat plane locomotion
for ground reaction force (GRF) analysis were performed serially up to 9 weeks with
the experimental (n = 4) and simple (control) crush (n = 1) injuries by blinded animal
behavior experts, using methods as recently described [9].
Results: Disruption of the endoneurium and perineurium with aberrant intra- and extrafascicular
axonal regeneration and progressive fibrosis was consistently demonstrated histologically
in ten out of ten nerves with experimental injuries. In contrast, crush injuries showed
only signs of Wallerian degeneration (Figure [2]). At 8 weeks, experimental animals made more errors during skilled locomotion as
compared to nerve crush animals. GRFs revealed impaired vertical and fore-aft force
generation by the injured limbs at week 9 in the experimental group, whereas GRFs
from the simple crush animal revealed recovery at the same time point (Figure [3]).
Conclusions: We have demonstrated histological features and poor functional recovery consistent
with NIC formation in a rodent model. The injury mechanism employed combines traction
and compression forces akin to the physical forces at play in clinical nerve injuries.
Additional validating experiments are in progress.
Keywords: Locomotion, nerve regeneration, Sunderland grade 4 nerve injury.
