Sagittal MRI of the lumbar spine. Normal appearance of the intervertebral disc on T2-weighted image.

This may bring about anatomical changes in adjacent structures— vertebral bodies, facets (sclerosis), and ligaments—and result in segmental instability, spinal stenosis, and neural foraminal narrowing, which eventually lead to axial and radicular pain (Figure 3-1). At the same time, the integrity of the annulus fibrosus is compromised, and radial fissures appear that later develop into tears. Changes in the endplate region such as sclerosis and ossification may further compromise discal nutrition and oxygen supply, resulting in biochemical changes such as the release of inflammatory products into the disc, which can result in discogenic pain.

Clinical Presentation

The clinical significance of a "black disc" is still disputed in the literature. Severe multilevel degenerative changes are frequently observed in asymptomatic subjects. Often, however, patients present with axial pain without radicular symptoms, and with no instability, deformity, and tension signs such as straight leg raising and femoral stretch test. In these patients the degenerated disc is the likely cause of the symptoms. Imaging studies in these patients frequently demonstrate multilevel degenerative disc disease. The question then arises: which of the degenerative discs is responsible for the patient's symptoms? Discography can be obtained in order to determine which of the degenerated discs is symptomatic. This practice, however, has not been fully endorsed by all spine physicians and is still challenged as unreliable and, at times, potentially harmful.

When disc degeneration leads to spondylotic changes and/or instability, radicular symptoms frequently appear.

Imaging Studies

MRI: On T1-weighted images the signal of the nucleus and the annulus may look alike and also resemble the vertebral body signal. In healthy discs the nucleus pulposus and the annulus fibrosis can be easily differentiated using T2-weighted images: the nucleus pulposus appears brighter than the annulus fibrosus (Figure 3-2).

Fatty bone marrow infiltration occurs during the aging process, resulting in brighter vertebral signal whereas the disc signal becomes darker and more homogenous. The end result of this degenerative process is a "black disc," where the annulus and nucleus have the same dark signal (Figure 3-3). As the disc degenerates, nitrogen gas may accumulate within the disc and can be seen on X-rays, CT, and MRI (Figures 3-4A and 3-4B). The accumulation of gas within the substance of the disc is referred to as the "vacuum phenomenon" and is most prevalent in the lower lumbar discs (L4-L5-S1) and occasionally in the lumbar facet joints. The vacuum phenomenon is pathognomonic of the degenerative process. Occasionally gas may be seen within the spinal canal when gas-containing discal material herniates. On CT one can easily identify the gas within the canal, which attests to the presence of a disc fragment.

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