Functional and morphological changes of lumbar nerve roots induced by mechanical compression or the nucleus pulposus in contact with the root: analysis of fiber size-dependent vulnerability in rabbits.

The variation in symptoms of spinal nerve root damage may be caused by the difference of the size of damaged nerve fiber or difference between the mechanical influence and some chemical factors. We studied the relative vulnerability of small versus large myelinated fibers to a focal compression and/or an application of nucleus pulposus. We prepared the mechanical compression model by clipping the nerve root and the nucleus pulposus model by applying autologous nucleus pulposus on the nerve root in a rabbit and evaluated nerve root damage with respect to its fiber diameter. Nerve conduction velocity (NCV) of large myelinated nerve fiber was reduced significantly in every group compared with a sham operation group (sham) after 1 day in compression, but in the nucleus pulposus group (NP) the velocity was reduced significantly compared with sham only after 7 days. Histologic examination revealed that the number of damaged large fibers increased significantly in the compression group compared with sham after 3 days, but there was no difference in NP. NCV of small myelinated nerve fibers was reduced significantly in both the compression and NP groups compared with sham after 7 days. Histologic study showed the number of damaged small fibers increased significantly in the compression group compared with sham after 7 days. However, in the NP group the number of damaged fibers did not increase compared with sham. In conclusion, large myelinated nerve fiber was damaged earlier than small myelinated nerve fiber by compression, while nerve fiber damage caused by only the nucleus pulposus had no relation to nerve fiber size.