Pathomechanisms of nerve root injury caused by disc herniation - An experimental study of mechanical compression and chemical irritation

Study Design. An electrophysiologic and histologic study on nerve roots after mechanical compression and/or local application of nucleus pulposus (NP) was performed. Objective. To assess the effects of mechanical compression and/or chemical irritation caused by NP. Summary of Background Data. It has been shown that application of NP to nerve roots without compression induces histologic and functional changes in nerve roots and the dorsal root ganglia. In clinical situations, however, mechanical compression has also been considered an important factor in disc herniation. Methods. Eighteen dogs (9-15 kg) were used in this study. Four groups were used to assess the effect of each factor. 1) sham group (n = 3); 2) NP group (NP applied under the S1 lamina) (n = 5); 3) comp group (a plastic balloon placed under the S1 lamina) (n = 5); and 4) comp+NP group (a balloon and NP placed under the S1 lamina) (n = 5). Ascending cauda equina action potentials (CEAPs) and cauda equina-sensory nerve conduction velocity (SCV) were recorded before, immediately after, and 1 week after treatment. Histologic changes were also assessed by light microscopy. Results. There were no significant differences in CEAP and SCV among the four groups immediately after the treatment. However, 1 week after treatment, the amplitudes in the NP group, comp group, and comp + NP group were statistically significantly lower compared with those in the sham group. The comp + NP group showed significantly lower amplitude than did the NP group and comp group. Immediately after treatment., SCV in the NP group and comp group did not show significant differences compared with that in the sham group. The comp + NP group showed significantly lower amplitude than did the NP group and comp group. Immediately after treatment, SCV in the NP group and comp group did not show significant differences compared with that in the sham group. However, 1 week after treatment, SCV in the comp+NP group was significantly lower compared with that in the sham group. Histologic changes such as intraneural edema, Schwann cell edema, and nerve fiber injury seemed to be more pronounced in the comp+NP group than in the other groups. Conclusions. It was shown that each of the assessed factors induces nerve dysfunction. However, the combination of mechanical compression (mass effect of herniated NP) and chemical irritation (inflammation around nerve root) may induce more nerve root injury than each factor per se.