Experimental disc herniation - Evaluation of the natural course

Study Design. Changes in L7 nerve root conduction velocity and changes in appearance on magnetic resonance study of the L6-L7 intervertebral disc in the dog were assessed for 2-6 months after an experimental disc herniation was performed. Objectives. To assess the time-related changes of nerve conduction velocity and magnetic resonance changes in the invertebral discs. Summary and Background Data. It is known that nucleus pulposus may induce nerve root morphologic and functional changes when applied epidurally. However, it is not known whether such changes are reversible. Methods. The spinal canal was opened by laminotomy of the upper part of the L7 lamina and the lower part of the L6 lamina on the left side. The L7 nerve root was gently retracted (sham) or the disc was punctured and injected with saline to produce herniation of the nucleus pulposus during the retraction time (herniation). After 1 day to 2 months, nerve root conduction velocity was determined by local electrical stimulation. Six dogs had the herniation or the sham procedure, and the L6-L7 disc was studied by magnetic resonance imaging at various times up to 6 months after the procedure. Results. Decrease in nerve conduction velocity reached a maximum after 7 days and recovered to baseline level fully within 2 months. Although there was a clear reduction-recovery patten, the difference in conduction velocity compared with that of the sham group was statistically significant after only 7 days. Disc degeneration started in the herniated discs within 7 days after the herniation procedure. However, none of the experimentally induced disc herniations were visualized by magnetic resonance imaging 7 days after the procedure. In no case was there subsequent nerve root compression. In one case, disc protrusion was visible 6 months after the herniation procedure. Conclusions. The results demonstrate for the first time that nucleus pulposus-induced nerve root injury reverses in 2 months and that it may be present without simultaneous nerve root compression, as confirmed by findings in magnetic resonance imaging. The previously described nucleus pulposus-induced nerve root changes may therefore be of clinical importance, and experimental studies of these mechanisms will probably be relevant for expanded understanding of the pathophysiologic mechanism behind sciatica that is caused by disc herniation.