Effects of anterior lumbar spinal fusion on the distribution of nerve endings and mechanoreceptors in the rabbit facet joint: quantitative histological analysis

The purpose of this study was to investigate the density and distribution of neural endings in rabbit lumbar facet joints after anterior spinal fusion and to evaluate the effects of intervertebral immobilization. An extraperitoneal approach was applied, and L5/6 was fixed with a plate and screws. Bilateral L4/5, L5/6, and L6/7 facet joint capsules were harvested from the rabbits 4, 8, and 16 weeks postoperatively. Capsular tissues were processed using a modified gold chloride staining method, and the specimens were sliced into 15-m sections. All sections were analyzed microscopically, and neural ending numbers per unit volume were calculated. Three types of neural ending were identified in each specimen: Pacinian corpuscles, Ruffini corpuscles, and free nerve endings. In the L5/6 fusion segment there was a significant decrease in the number of Pacinian corpuscles at 4 weeks and of Ruffini corpuscles at 4, 8, and 16 weeks after the fusion compared with the control; and in the L4/5 upper adjacent segment there was a significant increase in the number of free nerve endings. The number of Ruffini endings for the L6/7 lower adjacent segment was significantly lower more than 8 weeks after the fusion. These results suggest that immobilization of the intervertebral segment causes a reduction in the number of mechanoreceptors in the facet joint capsules because of the reduction in mechanical stimulation. Moreover, in the upper adjacent facet joint there may be neural sprouting caused by nociceptive stimulation.