The correlation of lumbar disc herniation with a patient’s complaints of low back pain or sciatica is not always clearly established. It has been estimated that as many as 20% of patients with radiologic findings of disc herniation are asymptomatic (67). Furthermore, when disc herniation occurs in symptomatic patients, other findings are often present that also could explain the clinical findings.
Lumbar disc herniations occur most frequently posterolaterally because the annulus is thinnest in the posterior quadrants but reinforced in the midline by the posterior longitudinal ligament. Also, flexion is the most prevalent lumbar spine motion, which places greatest stress on the posterior part of the disc. When the disc herniates to the posterolateral direction, it frequently does not impinge on the spinal nerve roots emerging from the neural foramen to which the disc is related, because the nerve roots occupy the upper portion of the foramen, whereas the disc is situated in the anterior wall of the lower part of the foramen. Therefore, when the L5-S1 disc herniates posterolaterally, it frequently spares the L5 nerve roots, exiting through the upper portion of the L5-S1 neural foramen. Instead, it more commonly involves the S1 nerve roots that descend across the posterolateral aspect of the L5-S1 disc before their exit from the S1 sacral foramina. Less common lumbar disc herniations are placed centrally or far laterally. Central herniations can involve any or all of the rootlets of the cauda equina. The infrequent far lateral herniations occur outside of the neural foramina. When present, they usually impinge on the ventral ramus that has just emerged from that foramen.
FIGURE 6-57. Axial T2-weighted images in the same patient as in Figure 6-54. Note deformity to the cord that has assumed a bean shape in the transaxial plane. There is compression and increased signal intensity to the cord compatible with myelopathy (arrow ).
Noncontrast CT has been described as being accurate in diagnosing disc herniations. On CT examination, the herniated disc appears as a focal protrusion of the disc that displaces the epidural fat (Fig. 6-58). The herniated disc material is typically slightly hyperdense relative to the non–contrast-enhanced dural sac and its adjacent nerve roots. The dural sac or adjacent nerve roots may be seen to be indented, displaced, or compressed. In more lateral herniations, the soft-tissue material of the disc can encroach on the neural foramen or the extraforaminal soft tissues, where it also displaces fat, and here it may encroach on the dorsal root ganglion, spinal nerve, or its ventral ramus. Herniated lumbar discs may calcify or contain gas. Extruded disc fragments can become separated from the disc and are thus able to migrate superiorly, inferiorly, or laterally. A herniated disc should be distinguished from a bulging annulus. A bulging annulus is produced by dehydration and volume loss within the nucleus pulposus. In contrast to the focal protrusion of a herniated disc, the bulging annulus typically has a symmetrical smooth contour, bulging beyond all margins of the vertebral body.
FIGURE 6-58. Sagittal reconstructed image of a MDCT acquisition of the lumbar spine demonstrates a large disc protrusion.
FIGURE 6-59. A: Left parasagittal T2WI of the lumbar spine with an extruded disk at the L5-S1 segment. B: Axial T2WI demonstrates elevation of the thecal sac (TS) by the extrusion (E). There is mass effect and posterior displacement of the first sacral root (S1).
On T1-weighted sagittal and axial MR images, the herniated lumbar disc appears as a moderate–signal-intensity intrusion into the high–signal-intensity epidural fat or on the moderate– to low–signal-intensity thecal sac or the lumbar nerve roots within their dural sleeves (Fig. 6-59A,B). Similarly, disc herniation into the neural foramen is visualized by a moderate– signal-intensity mass displacing the foraminal fat and encroaching on the dorsal root ganglion or nerve roots.
On T2-weighted sagittal MR images, the low signal intensity of a degenerated disc contrasts sharply with the high signal intensity of the nucleus pulposus of adjacent well- hydrated discs (Fig. 6-60). Any intrusion of the low–signalintensity disc herniation on the thecal sac is well seen because of the high–signal-intensity myelographic effect of the CSF on T2-weighted images.
FIGURE 6-60. Small central extrusion (E) at the L5-S1 segment. Note decreased signal to the intervertebral disk when compared to the remaining intervertebral segments, a sign of degeneration.
Discography remains a controversial diagnostic imaging modality. It appears that its major diagnostic value lies in the reproduction of the patient’s specific pain on contrast injection of a given disc, with controls demonstrating that injection of adjacent discs produces either no pain or foreign pain (68). Discography, especially when combined with CT, may provide information about degeneration and the extent of fissures and rupture.