The effects of normal, frozen, and hyaluronidase-digested nucleus pulposus on nerve root structure and function

Autologous nucleus pulposus was modified and applied to the cauda equina in pigs. Histology and neurophysiology were assessed after 7 days.

To assess if alterations of the nucleus pulposus would change the degree and distribution of the nerve injury induced by autologous nucleus pulposus.

It was reported recently that nucleus pulposus may induce structural and functional changes in nerve roots after epidural application. The basic mechanisms causing these changes are not fully understood.

Nucleus pulposus was harvested from lumbar discs and submitted to either of three treatments; 37 C for 24 hours (n = 5), -20 C for 24 hours (n = 5), or digestion by hyaluronidase for 24 hours (n = 6). In two additional pigs, nucleus pulposus was applied just after harvest as a control to verify previous observations. After 7 days, nerve conduction velocity was recorded, and specimens were processed for blinded light microscopic assessment.

When nucleus pulposus was applied just after harvest, or when it had been kept at 37 C or digested by hyaluronidase for 24 hours, there was a significant reduction in nerve conduction velocity similar to previous observations. When nucleus pulposus had been kept at -20 C for 24 hours, however, there was no reduction in conduction velocity. There were no apparent differences between the groups at the histologic assessment. Staining of the nucleus pulposus showed that the cells in the nucleus pulposus exposed to -20 C were lysed, whereas the cells in the nucleus pulposus treated by the two other methods were mainly unaffected.

Because freezing of the nucleus pulposus probably kills the cells but does not affect other components, one may assume that the biologic effects induced by the nucleus pulposus may be related to its cell population.