An experimental study of the effects of nerve root retraction on the posterior ramus

Study Design. The histologic and ultrastructural changes in the posterior r amus after posterior lumbar surgery were studied in rabbits. Objective. To investigate the structural changes in the posterior ramus aft er posterior lumbar surgery that may cause injury to the posterior ramus af ter the procedure. Summary of Background Data. Investigators in previous studies have pointed out that low back discomfort after lumbar discectomy relates to neurogenic changes and/or myogenic changes of paravertebral muscle. However, no previo us study has demonstrated the effects of excessive nerve root retraction on spinal posterior rami. Methods. Eighteen male Japanese White rabbits were used. The posterior ramu s arising from the S1 nerve root was examined after exposure of the lamina only, fenes tration, or retraction of the S1 nerve root with light microsco py and transmission electron microscopy at 2, 4, and 6 weeks after the proc edure. Results were compared with a those in control specimens that did not undergo the procedure. Results. In the exposed group, no distinct difference was found compared wi th the control specimen. In the fenestration group, especially at 6 weeks, some attenuation and splitting of myelin sheaths was observed. In the retra ction group, the structural alteration was most severe. Even at 2 weeks, fr agmentation of many myelin sheaths was detected. Examination of specimens b y electron microscopy indicated phagocytosis of myelinated fibers at 4 and 6 weeks. Conclusion. Findings showed that posterior lumbar,procedures, including ret raction of paravertebral muscle, fenestration of the lamina, and retraction of the nerve root affect the posterior ramus. Excessive retraction of the nerve root has an especially disastrous effect on the posterior ramus. Such a violent maneuver within the spinal canal must be avoided.