Effects of anulus fibrosus and experimentally degenerated nucleus pulposus on nerve root conduction velocity: relevance of previous experimental investigations using normal nucleus pulposus

STUDY DESIGN: Nerve conduction velocity was measured in the pig cauda equina after local application of anulus fibrosus or in vitro/postmortem degenerated nucleus pulposus from the same pig. OBJECTIVES: To analyze the effects of anulus fibrosus and degenerated nucleus pulposus on nerve conduction velocity. SUMMARY OF BACKGROUND DATA: Previous studies on nucleus pulposus-induced effects on nerve roots have used normal, nondegenerated nucleus pulposus. Because both anulus fibrosus and degenerated nucleus pulposus are commonly seen in the clinical situation of disc herniation, the value of the previous work could be questioned. METHODS: Anulus fibrosus and nucleus pulposus were harvested using a retroperitoneal approach. The nucleus pulposus was degenerated artificially either by addition of sodium lactate with HCl added to form a pH of either 6.0 or 3.5 (in vitro degeneration), or by storing the nucleus pulposus at 4 C until a pH of 6.0 (postmortem degeneration) was reached. After epidural application, the nerve conduction velocity was determined at 7 days (anulus fibrosus) or 3 days (degenerated nucleus pulposus). RESULTS: Application of anulus fibrosus did not induce any reduction of nerve conduction velocity. In vitro and postmortem degenerated nucleus pulposus induced a reduction of nerve conduction velocity similar to that of normal nucleus pulposus. CONCLUSIONS: Although only nerve function and not pain was assessed, it seems likely that previous experiments using normal nucleus pulposus may be relevant for evaluating the pathophysiologic mechanisms behind the nucleus pulposus-induced nerve root injury, also in a clinical perspective.