Disc Herniation

Findings of disc disease on MRI examination must be carefully correlated with clinical symptoms (6). Jensen et al. performed lumbar spine MRI on 98 patients without back pain and found a 64% prevalence of either a bulge, protrusion, or extrusion (7). Thirty-eight percent had abnormalities at more than one level. A similar study noted extrusions in 24% of asymptomatic people (8). Furthermore, discogenic pain, caused by a small annular tear with an inflammatory reaction, can often produce severe discomfort. This pain may be present even in patients without significant morphologic changes on imaging (9).

The word "herniation" is a generic term for a focal and abnormal extension of the intervertebral disc beyond the margins of the endplates of neighboring vertebral bodies. Protrusions, extrusions, and sequestered discs represent the three types of herniations. A bulge is also an abnormal extension but is by definition not focal. The lumbar levels account for the highest number of herniations, followed by the cervical spine, and more rarely the thoracic spine. Roughly 92% of lumbar disc herniations are found at the L4-5 and L5-S1 levels (10).

A bulge is a symmetric distention of the annulus fibrosus beyond the vertebral body endplates around its entire circumference (3,10). These may exist in either

Annulus Vertebrae
Fig 1. Normal lumbar spine. Sagittal T2-weighted (A) and axial T2-weighted. (B) images show the high signal central nucleus pulposus of the disc surrounded by the low signal outer fibers of the annulus. In addition, the high signal CSF can be distinguished from the low signal nerves.

degenerated or nondegenerated discs (3). Although some believe that the annu-lus fibrosus is intact with disc bulges, a cadaver study reported by Yu et al. showed that 84% of adult discs with radial tears had bulges >2.5 mm beyond the vertebral endplates (11). Similarly, in all but a single case, discs with bulges >2.5 mm contained either a radial tear or complete disruption of the annulus. In a separate MRI investigation of annular tears, 14 of 16 discs with radial tears seen on T1-weighted contrast enhanced images had a disc bulge (12). Masaryk et al., however, state that bulging discs are the result of disc degeneration with an intact annulus, and the T2-weighted images show the low intensity Sharpey's fibers to be intact (13).

In either case, loss of disc height is present owing to the peripheral extension of the disc substance. Bulges may encroach on the spinal canal and clinically mimic a central herniation (see later for definition of central hernia-tion) or, alternatively, may affect the contents of intervertebral foramina and result in foraminal nerve impingement (10). These are best imaged using both sagittal and axial images to compare the diameter of the disc with those of the neighboring levels. Tears of the annulus may or may not be discernible on imaging.

A protrusion is a focal herniation of the nucleus pulposus through an incomplete rent in the annulus fibrosus. Nuclear material passes between layers of annu-lus fibrosus and forces the outer layers of the annulus to extend outward (3). Because of the intact annulus, protrusions cannot migrate cranially or caudally to a significant extent.

On MRI, the protrusion is connected to the body of the disc by a pedicle of high signal on T2-weighted imaging. The actual material in the protrusion may have variable signal intensity with respect to the main body of the disc. The outer fibers of the annulus may be visible as a low signal line abutting the thecal sac or epidural fat (13).

An extrusion consists of nucleus pulposus that has escaped the confines of the annulus (6,10). These can be classified further as subligamentous or transligamentous, based on whether they remain confined anterior or have penetrated posterior to the posterior longitudinal ligament (PLL) (3). Extrusions may also migrate cranially and/or caudally to variable extents (Fig. 2).

On T2-weighted imaging, the extrusion is also connected to the disc by a hyperintense stalk. The extrusion itself is usually of high signal intensity as well and can be readily distinguished from the low intensity signal of the outer fibers of the annulus (13). The distinction between a protrusion and extrusion on MRI is less important than possible symptoms attributable to the particular lesion.

A sequestered disc, or free fragment, is an extrusion that has lost its attachment to its parent disc (3,6,10) (Fig. 3). The free fragment may be found on either side of the PLL and at a range of distances from the disc of origin. Most often, they are situated anterior to the dural sac, but more rarely may be found posterior to the dura or even inside.

Sequestered discs are best demonstrated on sagittal images, especially when the fragment has migrated significantly from the parent disc (13,14). T1-weighted imaging shows isointensity with the parent disc, and post-contrast

Sequestered Disc Herniation

Fig. 2. Disc extrusions. Sagittal T2-weighted image (A) demonstrates a large disc extrusion (star) that extends to the posterior aspect of the canal. Sagittal T2-weighted image (B) in a different patient demonstrates a superiorly migrating neural foraminal disc extrusion (arrowhead) which compresses the exiting nerve root (arrow).

Fig. 2. Disc extrusions. Sagittal T2-weighted image (A) demonstrates a large disc extrusion (star) that extends to the posterior aspect of the canal. Sagittal T2-weighted image (B) in a different patient demonstrates a superiorly migrating neural foraminal disc extrusion (arrowhead) which compresses the exiting nerve root (arrow).

images reveal a peripheral rim of enhancement (3,13). The discs of origin commonly demonstrate loss of signal intensity on T2-weighting, signifying degenerative changes that predispose to disc disease (13). Sequestered

Sequestered Disc Procedure
Fig. 3. Sequestered disc fragment. Sagittal image shows an inferiorly migrated free disc fragment (arrowhead) posterior to the S1 vertebral body and just superior to the exiting S1 nerve root.

discs are a clinically significant finding, as they can have a major impact upon the management of the patient (13).

MRI is fairly reliable at differentiating between the types of disc herniation. In a prospective study comparing surgical findings with MRI, Kim et al. reported an overall accuracy of 80.6% of MRI in distinguishing between protrusions, subligamentous and transligamentous extrusions, and sequestered discs (15). Distinguishing between subligamentous and transligamentous extrusions was best performed with T2-weighted imaging, in which the disruption of the PLL, represented by the hypointense line immediately posterior to the vertebral bodies, could be identified (15). The authors reported an accuracy of 75% when using this criterion. Gadolinium enhancement was particularly helpful for identifying sequestered discs, demonstrating an anterior rim of enhancement and clearly demarcating the lack of connection with the parent disc.

The orientation of a herniation with respect to the central canal is critical in determining which nerve roots will most likely be affected. A central herniation has most of its substance in the midline, with lesser amounts to either side. A central/right paracentral or central/left paracentral herniation has the major portion between the intervertebral foramen and the midline of the central spinal canal but does not extend into the intervertebral foramen. Lateral herniations occur beyond the central spinal canal and usually affect the intervertebral foramina. These herniations may extend more anteriorly, beyond the intervertebral foramen (5,16,17).

A central or paracentral L4-5 disc extrusion will most likely impinge upon the L5 nerve root as it first descends in the central spinal canal in close proximity to the dorsal aspect of the L4-5 disc and then traverses the lateral recess en route to the L5-S1 intervertebral foramen. It does not regularly affect the L4 nerve root, as this descends inferolaterally through the lateral recess at a level superior to that of the L4-5 disc before reaching the L4-5 intervertebral foramen. In addition, the disc is located at the level of the caudal half of the intervertebral foramen. A lateral extrusion into the intervertebral foramen at the same level, however, will likely affect the L4 nerve root, while a paracentral or central extrusion migrating crani-ally may also reach the descending fibers of the L4 nerve root (5). Thus, vertical migration of an extrusion or sequestered disc may have significant clinical consequences (16).

MR examinations evaluate intervertebral disc degeneration. Because degenerated discs demonstrate decreased T2 signal intensity of the nucleus pulposus when compared to normal discs and CSF, this finding on sagittal examination may serve as a guide in choosing the levels for axial imaging (1,18). Indeed, axial imaging is critical for evaluating certain types of pathology, such as a lateral herniation, that are more difficult to detect on sagittal images (5).

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Essentials of Human Physiology

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Responses

  • KAYLA
    What does a 2.5mm central disc protrusion mean?
    8 years ago
  • Frank
    What is superiorly migrating left paracentral disc protrusion?
    7 years ago
  • Diana Walter
    Can detect protrusion by sagittal?
    6 years ago

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