Organization of intrathecal nerve roots at the level of the conus medullaris.

The three-dimensional organization of the spinal nerve roots at the level of the conus medullaris has not been described previously, to our knowledge. In this study, we used a newly developed technique of in situ fixation and embedding to define the cross-sectional anatomy at the level of the conus medullaris in ten fresh human cadavera. A highly organized overlapping pattern of nerve roots was demonstrated in all specimens. The nerve roots form a peripheral rim around the spinal cord at the levels of the tenth and eleventh and the eleventh and twelfth thoracic intervertebral discs. More caudally, the cord diminishes in size and the nerve roots predominate. The most cephalad roots lie laterally, with the motor roots ventral to their sensory counterparts. The more caudad roots overlap toward the midline, and the motor and sensory portions of each root are separated by spinal cord tissue. An intricate web of arachnoid membrane holds the nerve roots in a fixed relationship to each other.