disc sequestration

complete separation of disc material with rupture through posterior longitudinal ligament into the epidural space; free fragment herniation

findings:

migration superiorly/inferiorly with compression of nerve roots above/below the level of herniation
disc material noted >9mm from disc space

DDx:

postoperative scarring (retraction of thecal sac to the site of surgery)
epidural abscess/tumor
conjoined nerve root: 2 nerve roots arising simultaneously from the thecal sac; normal variant in 1-3%