A study of mechanoreceptors in fibrocartilage masses in the defect of pars interarticularis.

We investigated the origin of low back pain associated with lumbar spondylolysis and spondylolytic spondylolisthesis by removing fibrocartilage masses from the lytic sites in symptomatic patients and staining the masses by the Gairns gold chloride method to examine mechanoreceptors. The fibrocartilage masses were found to contain four types of mechanoreceptors: Pacinian corpuscles, Ruffini receptors, Golgi tendon organ-like receptors, and free nerve endings. All of these mechanoreceptors were present at the periphery of the specimens, and Ruffini receptors and free nerve endings were abundant. Some mechanoreceptors had a slightly atypical structure, in addition to those with typical morphology. Comparison with mechanoreceptors in normal lumbar facet joint capsules showed that there were more mechanoreceptors in the fibrocartilage masses and a greater proportion of atypical structures at lytic sites. The presence of mechanoreceptors at lytic sites suggests that the fibrocartilage masses are not simply scar tissue filling the defect. Rather, these masses also appear to play a protective role by sensing instability via mechanoreceptors and transmitting this information as pain, while at the same time acting as ligament-like tissue that connects and stabilizes the separated vertebral arches.