Neurological symptoms of herniated nerve fibers resulting from limited perineurial injury from sharp materials such as needles have become a recent topic in clinical practice. However, the mechanism of this disorder, which is known as a perineurial window, has not been clarified. To investigate the mechanism of nerve damage in the perineurial window, we designed small (1-mm length) and large (5-mm length) perineurial windows using tibial nerves of Wistar rats. In the 1-mm group, a marked hernia of the endoneurial contents developed soon and decreased in size with time, but protrusion of nerve fibers was still observed after 12 weeks. Nerve fibers in both the herniated portion and under the edge of the window were damaged. Even after 12 weeks, regeneration of the nerve fibers and the perineurium was incomplete. In contrast, in the 5-mm group, the initial endoneurial edema was remarkable, but herniated nerve fibers were not seen after 12 weeks. Neurological impairment in the 5-mm group was marked in the early stage but rapidly recovered. The repair of the perineurium and nerve fibers in the 1-mm group was slower than in the 5-mm group. Persistent neurological symptoms in the perineurial window appeared to be more closely associated with entrapment of nerve fibers at the window edge rather than with disruption of endoneurial homeostasis.
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