Topographic-anatomical basis of sacral neurostimulation: neuroanatomical variations.

In 4 male and 5 female cadavers we undertook a detailed anatomical dissection of the caudal segment of the spinal cord with special emphasis on the conus medullaris, length of the spinal segment and pattern of nerve bridging between adjacent sacral roots. We found an unexpectedly high incidence of intradural bridging--some of simple formation and others more complex. These interconnections were more frequent between dorsal than ventral roots but never joined dorsal to ventral roots or vice versa. Ventral S5 was missing in some cases and we found no coccygeal roots. The dural sac was seen to extend to the S1 to S2 intervertebral disk or the lower margin of S2, permitting a limited sacral laminectomy to expose the intradural sacral roots in their entirety. Towards the end of the dural sac the ventral and dorsal roots were joined by an extremely thin epineurium, and each root was composed of several lightly fused rootlets that could be separated under magnification but that combined to constitute the root nerve bundle. Our study suggests that the sacral spinal cord is neither symmetrical nor metameric. It exhibits more numerous interconnections and variations than the rest of the spinal roots, which is consistent with the greater incidence of congenital malformations affecting this segment and the varying responses found with neurostimulation.