Free Fragments

      When an extruded disk loses its attachment to the parent disk, it becomes a free fragment or sequestration. If the disk fragment is near an interspace, sometimes it can be difficult to discern whether or not a pedicle of attachment remains. Free fragments can migrate some distance cephalad or rostral to the disk space, and it is important to alert the surgeon to their precise location. Rarely, a disk fragment may rupture through the thecal sac into the intradural compartment.

      Most sequestered disks are higher signal than their disk of origin on T2-weighted images. The cause for this is unclear, but it may be due to increased water from granulation tissue, immune response, and inflammation. Endnote Chronic disk herniations tend to be hypointense due to loss of water content.

      Subligamentous disk fragments are contained by the posterior longitudinal ligament (PLL). Schellinger and colleagues Endnote reviewed the anatomy of the PLL and anterior epidural space. Most contained disk fragments lateralize to either side of the anterior epidural space. An equal number migrate superiorly and inferiorly. The PLL has a high collagen content and is hypointense on MR. It can be seen as a thick dark line covering a contained herniated disk, usually seen best on sagittal images. The posterior margin of contained disk fragments usually maintain a smooth contour.

      Non-contained disk fragments have gone through the PLL. Either interruption or absence of the peripheral dark line suggests disruption of the PLL. Endnote Once through the PLL, the disk fragments are not bounded by any membranes, and they tend to have more irregular contours. Endnote  

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