Posterior epidural migration of a sequestrated disk fragment is a rare condition with few cases reported. A disk sequestration is defined as a perforation of the fibrous ring with migration of the disk to the epidural space. Usually the disk fragment is contained in the anterior epidural space which is anatomically well delimited so movement of the fragment usually occurs cranial, caudal, or lateral but seldom to the posterior space.
Most cases involve the lumbar spine and present with neurological deficits like paresis, paresthesia. Sometimes cauda equina syndrome has been described .
MR imaging preferably with gadolinium administration is the method of choice for diagnosis. Disk fragments images can mimic other causes of posterior epidural space occupying lesions. Usually disk fragments exhibit low signal on T1 weighted images and high signal on T2 weighted images. Most of the disk sequestration fragments show peripheral ring contrast enhancement attributed to inflammatory changes around the sequestrated tissue. [1, 2, 3]. Due to the overlap in the image of these pathologies it is often difficult the diagnosis. We have to make the differential diagnosis with tumor, abscess or synovial cyst. Tumors usually enhance in a homogeneous way on gadolinium images. Synovial cysts have typical signal intensity and are related to the facet joint. The epidural abscess usually associates inflammatory changes and clinical signs of infection .
Signs of a ruptured outermost annulus of the intervertebral disk can help guide the diagnosis. Also a tract like structure with enhancement from the site of the ruptured disk to the posterior epidural space is identified  as in our case (fig 5). It suggests the route of the migration.
The patient long term outcome main factor is the presence of cauda equina syndrome and the period of time from the onset symptoms and surgical treatment. Early surgery is the treatment of choice and helps prevent severe neurological deficits. In asymptomatic or mildly symptomatic patients conservative treatment is an option as the fragment may migrate to another position or be reabsorbed .
Our patient was treated with hemilaminectomy. At surgery a ruptured disk fragment was identified in the posterior epidural space. In the postoperative the patient symptoms were relieved.
It is important to consider sequestrated disk fragment migration in masses of the posterior epidural space. Ring enhancement, tract-like enhancing image connecting to the site of the rupture disk and annulus rupture signs can be the key facts to guide the diagnosis in MRI.