What is Intervertebral Disc Degeneration, and What Causes It?

Michael A. Adams, PhD; Peter J. Roughley, PhD

Disclosures

Spine. 2006;31(18):2151-2161. 

In This Article

Disc Structural Failure

There are 3 types of tears that can be distinguished: circumferential tears or delaminations, peripheral rim tears, and radial fissures (Figure 5). They become increasingly common after the age of 10 years,[39] especially in the lower lumbar spine, and reach a peak in middle age.[45] Circumferential tears may represent the effects of interlaminar shear stresses,[46] possibly occurring from compressive stress concentrations in older discs (Figure 3). Peripheral rim tears are more frequent in the anterior anulus[47] and may be associated with bony outgrowths.[48] Mechanical[4] and histologic[47] considerations suggest that they are related to trauma. Radial fissures progress outward from the nucleus, usually posteriorly or posterolaterally,[47] and this process can be simulated in cadaveric discs by cyclic loading in bending and compression.[4] Radial fissures are associated with nucleus degeneration,[47,49] but it is not clear which comes first. The 3 types of anulus tear probably evolve independently of age and each other.[50]

Three common types of anulus tears. A, Circumferential clefts or delamination. B, Radial fissure. C, Peripheral rim lesion. Disrupted tissue is shown in black, and nucleus pulposus is shaded. Images are in the transverse plane (left) and sagittal plane (right). Reprinted with permission from Churchill Livingstone; 2002.[4]

When radial fissures allow gross migration of nucleus relative to anulus, to the extent that the disc periphery is affected, then the disc can be said to be herniated, or prolapsed. Depending on the extent of nucleus migration, the disc herniation may result in protrusion, extrusion, or sequestration of the nuclear material. Disc prolapse can be simulated in cadaveric discs by combined loading in bending and compression, with either one component exceeding physiologic limits,[24,35] or as a result of intense repetitive loading.[51,52] Mechanically induced prolapse (Figure 6E) occurs most readily in discs aged 30-40 years,[24,53] which presumably still have a fluid nucleus and an anulus starting to become weakened by age. Severely degenerated discs do not prolapse in the laboratory, presumably because the nucleus is no longer able to exert a hydrostatic pressure to tension the anulus. In living people, prolapsed disc tissue consists primarily of nucleus pulposus displaced down a radial fissure.[54]

Cadaveric lumbar intervertebral discs sectioned in the midsagittal plane (anterior on left). (A) Young disc (male, 35 years old). (B) Mature disc (male, 47 years old). (C) Disrupted young disc (male, 31 years old). Note the endplate damage and inward collapse of the inner anulus. (D) Severely disrupted young disc (male, 31 years old). Note the collapse of disc height. (E) Disc induced to prolapse in the laboratory (male, 40 years old). Some nucleus pulposus has herniated through a radial fissure in the posterior anulus (right). Discs (A-D) correspond to the 4-point scales typically used to grade disc degeneration from macroscopic features. Reprinted with permission from Churchill Livingstone; 2002.[4]

Vertebral endplates (Figure 4) are the spine's weak link in compression, and accumulating trabecular microdamage[55] probably explains why the nucleus increasingly bulges into the vertebral bodies in later life.[56] Endplate damage immediately decompresses the adjacent nucleus and transfers load onto the anulus, causing it to bulge into the nucleus cavity (Figure 6C).[35,57] If nucleus pulposus herniates through a damaged endplate, then subsequent calcification can create a Schmorl's node.

[58]

Collapse of the inner anulus into the nucleus is a common feature of elderly discs (Figures 6C, D), with the anterior anulus being affected more than the posterior.[59,60] It could be caused by nucleus decompression following endplate fracture, as described previously. In many elderly discs, the cartilage endplate becomes detached from underlying bone,[60] presumably because the high internal pressure that presses it against the bone in young discs has been lost.

These 3 features are closely associated with one another and with the term spondylosis (Figure 7). With increasing age, the nucleus tends to bulge into the vertebral bodies. Nucleus pressure is reduced,[61,62] and increased vertical loading of the anulus61 causes it to bulge radially outward,[63] and sometimes inward. Severe changes are accompanied by a marked loss of nucleus pressure[62] and collapse of anulus height (Figure 6D). In effect, the disc behaves like a flat tire.[63] It is anulus height that determines the separation of adjacent neural arches, and anulus collapse/bulging in old discs can lead to more than 50% of the compressive force on the lumbar spine being resisted by the neural arch.[64] This effect probably explains why narrowed discs are associated with osteoarthritis in the apophyseal joints and with osteophytes (Figure 7) around the margins of the vertebral bodies.[65]

Radiograph of an old cadaveric lumbar spine (anterior on left). The radiograph depicts how severe disc narrowing can be associated with vertebral osteophytes, sclerosis of the vertebral endplates, and selective loss of horizontal trabeculae from the vertebral body. Reprinted with permission from Churchill Livingstone; 2002.[4]

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