Intervertebral discs

Fig. 3. 13 Ligaments of the vertebral column.

Radiological features of the ligaments of the vertebral column

Lumbar puncture for myelography A needle is passed between and parallel to the spinous processes (i. e. w i th slight cranial angulation): (i) through the skin and subcutaneous tissues; (ii) through the supraspinous and interspinous ligaments; (iii) through the ligamenta flava if these join in the midline (a gap between these to admit midline vessels is variable in size); and (iv) through the dura to the subarachnoid space.

Myelogram The posterior longitudinal ligament as it passes posterior to the disc may bulge slightly and impinge upon the thecal sac in myelography. This effect is more marked in extension and disappears in flexion.

CT and MRI (see Figs 3. 11 and 3. 12) The ligaments can be viewed directly on both CT and MRI. The ligamentum flavum lies within the spinal canal arising from the lamina. Because the lamina slopes postero¬ inferiorly, the ligamentum flavum appears thicker in successively lower axial slices.

THE INTERVERTEBRAL DISCS

The vertebral bodies are joined by fibrocartilaginous discs, which are adherent to thin cartilaginous plates on the vertebral bodies above and below them. The discs are wedge-shaped in the cervical and lumbar regions and consequently contribute to lordosis in these regions; however, in the thoracic region they are flat. The intervertebral discs contribute one-fifth of the total height of the vertebral column.

Each disc has a central nucleus pulposus, which is gelatinous in the young subject and becomes more fibrous w i th age. This is surrounded by an annulus fibrosus of tough fibrous tissue. The annulus is relatively thin posteriorly and this is the usual site of rupture in the degenerate disc.

Radiological features of the intervertebral discs

Plain radiographs The disc space rather than the disc itself is visible. The disc space at L5/S1 level is narrow and this should not be taken as a sign of disease.

Discography Discography is performed by injection of contrast medium through the annulus fibrosus into the nucleus pulposus of the intervertebral disc. It is usually performed in the lumbar region to establish whether a disc above a proposed fusion is normal. Discography is also performed prior to chemonucleolysis. It can be performed by a posterior approach through the dura or by a posterolateral approach without going through the dura.

A normal disc is seen to have a smooth, central, unilocular or bilocular nucleus pulposus and contrast medium remains within the confines of the annulus fibrosus.