Paralysis of the arm after posterior decompression of the cervical spinal cord. I. Anatomical investigation of the mechanism of paralysis.

An anatomical study of the cervical spinal cord and root to investigate the mechanism of paralysis of the arm after posterior decompression was performed using 14 cadavers of Japanese adults. It revealed that in the intervertebral foramen, extradural portions of the anterior and posterior roots of the cervical spinal cord lay separately in caudal-rostral relation, and the anterior root passed through the narrowest portion of the foramen isolatedly, i.e. the superior notch of the superior articular process. After laminectomy, a posteromedial shift of the dura-root junction occurred in combination with the posterior enlargement of the dual tube, and it showed two effects on the roots, one a relaxing effect on rootlets and the other, a traction effect on the extradural portion of a root. Through these effects, the anterior and posterior rootlets did not lose their laxities even when there was maximal posterior shift of the spinal cord in the maximally enlarged dural tube, and conversely, extradural portions of both roots were laid under traction by the posteriorly expanded dural tube. This traction force to the extradural portion of the roots disappeared with a longitudinal incision to the posterior aspect of the bulging dural membrane, suggesting that it was the posteriorly expanded dural membrane that exerted the traction force on the extradural components of the anterior and posterior roots. From these anatomical findings, mechanisms of paralysis were deduced as follows. The posteriorly expanded dural tube exerts traction force on the extradural portions of the anterior and posterior roots. With the impingement of the medial facet edge on the posterior root, a traction-compression injury of the posterior rootlets or extradural portion of the posterior root occurs. With anchoring of the anterior root inside the foramen, a traction injury of the anterior root develops. The predominance of paralysis at the middle cervical level could be explained by the higher degree of anterior protrusion of the superior articular process and the more frequent degenerative changes here than at other levels. These factors might inhibit the gliding abilities of the roots inside the foramina, with the formation of perineural fibrosis, predisposing the roots to damage by the traction force.