Histology of Intervertebral Disc Protrusion

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Abstract
Study Design. In vitro experimental intervertebral disc ruptures of aged rats were examined histologically. Objectives. To clarify the mechanism of intervertebral disc herniations by microscopic investigation of ruptured discs. Summary of Background Data. Clinically, disc herniations have been classified into two types: extrusion and protrusion. However, the pathogenesis of protrusion type herniations has not yet been demonstrated by any studies. To clarify this issue, it is essential to establish an appropriate model producing disc herniations, and to examine the sequential changes in the structure of herniated discs. Methods. Lumbar discs of 2-year-old rats were examined histologically and compared with human lumbar discs. To examine structural changes in discs subjected to repetitive motion stress, 400 repetitions of a sequence of flexion (30°) and axial rotation (6°) were applied in vitro to the lumbar discs of the animals. Results. The microstructure of normal lumbar discs in aged rats was similar in many ways to the human lumbar discs in a 20- to 40-year-old adult. Of 10 discs subjected to repetitive stress, 4 were ruptured at the junction between the posterior anulus fibrosus and the sacral cartilage endplate. One had an extruded nucleus pulposus, and three had a protruded anulus fibrosus, which displayed disorganized structure containing widened and flaccid lamellae. Conclusions. The results from this study indicate that disc protrusion can be caused by disorganization of the ruptured anular lamellae, not by focal compression of the nucleus pulposus.