MECHANORECEPTOR ENDINGS IN HUMAN THORACIC AND LUMBAR FACET JOINTS

Study Design. Histologic analysis of normal human facet capsules to de termine the density and distribution of encapsulated nerve endings in the thoracic and lumbar spine. Objectives. To quantify the extent of m echanoreceptor innervation in normal facet tissues and determine the r elative distribution of three specific receptor types with respect to thoracic and lumbar segments. Summary of Background Data. Ongoing stud ies of spinal innervation have shown that human facet tissues contain mechanoreceptive endings capable of detecting motion and tissue distor tion. The hypothesis has been advanced that spinal proprioception may play a role in modulating protective muscular reflexes that prevent in jury or facilitate healing. Methods. Whole facet capsules harvested fr om seven healthy adult patients were processed using a gold chloride s taining method and cut into 35-micron sections for histologic analysis . No sampling was performed; all sections were analyzed, Receptor endi ngs were classified by the method of Freeman and Wyke if they met the following three criteria: 1) encapsulation, 2) identifiable morphometr y, and 3) consistent morphometry on serial sections, Results. One Type 1 and four Type 2 endings were identified among 10 thoracic facet cap sules. Five Type 1, six Type 2, and one Type 3 ending were identified among 13 lumbar facet capsules. Occasional atypical receptive endings were noted that did not fit the established classification. Unencapsul ated free nerve endings were seen in every specimen, but were not quan tified. Conclusions. Encapsulated nerve endings are believed to be pri marily mechanosensitive and may provide proprioceptive and protective information to the Central nervous system regarding joint function and position. A consistent, but small population of receptors has-been fo und previously in cervical facets, but innervation of the thoracic and lumbar levels is less consistent. This suggests that proprioceptive f unction in the thoracic and lumbar spine is less refined and, perhaps, less critical than in the cervical spine.