Mechanoreceptor endings in human cervical facet joints.

Normal cervical facet capsules, taken from three human subjects, were examined to determine the density and distribution of three types of mechanoreceptive nerve endings. Clearly identifiable mechanoreceptors were found in 80% of the specimens and were categorized according to the classification of Freeman and Wyke. Eleven Type I, twenty Type II, and five Type III receptors were identified, as well as a number of small, unencapsulated nerve endings. Type I receptors were small, globular structures measuring 25-50 microns in diameter. Type II receptors varied in size and contour, but were characterized by their oblong shape and broad, lamellar capsule. Type III receptors were relatively large, oblong structures with a thin, amorphous capsule, within which a reticular mesh-work of fine neurites was embedded. Free (nociceptive) nerve endings were found in sub-synovial, loose areolar, and dense capsular tissues. The presence of mechanoreceptive and nociceptive nerve endings demonstrates that cervical facet capsules are monitored by the central nervous system, and implies that neural input from the facets is important to the function of the cervical spine. Previous studies have suggested that protective muscular reflexes modulated by these types of mechanoreceptors are important in preventing joint instability and degeneration. The complex neural elements identified in this study may be damaged inadvertently when surgically exposing the cervical spine. Care should be taken, during posterior approaches, to avoid excessive stripping of the facets in portions of the spine not being fused.