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 Functional Anatomy

Intervertebral discs are pads of fibrocartilage that resist spinal compression while permitting limited movements. They spread loading evenly on the vertebral bodies, even when the spine is flexed or extended. Individual lamellae of the anulus fibrosus consist primarily of collagen type I fibers passing obliquely between vertebral bodies, with orientation of the fibers being reversed in successive lamellae (Figure 1). The nucleus pulposus consists of a proteoglycan and water gel held together loosely by an irregular network of fine collagen type II and elastin fibers. The major proteoglycan of the disc is aggrecan,[5,6] which, because of its high anionic glycosaminoglycan content (i.e., chondroitin sulfate and keratan sulfate), provides the osmotic properties needed to resist compression (Figure 2).

Intervertebral disc structure and function. Upper, spinal compression (C) generates a hydrostatic pressure (P) in the nucleus, and tensile stresses (T) in the anulus. Lower, Lamellae of the anulus with oblique collagen fibers in alternating directions (approximately, a = 30º). Typical values are given of the number of lamellae in the anulus and the number of collagen fiber bundles in a lamella. Reprinted with permission from Churchill Livingstone; 2002.[4]

The role of aggrecan and collagen in the ability of disc to resist compression. The nucleus pulposus is depicted as containing proteoglycan aggregates entrapped in a collagen fiber network. The proteoglycan aggregates are depicted as a central hyaluronan molecule (dashed line) substituted with aggrecan molecules possessing a central core protein (open line) and sulfated glycosaminoglycan side chains (solid lines). The hydration properties of the glycosaminoglycan chains of aggrecan cause the tissue to swell until an equilibrium is reached, in which the swelling potential is balanced by tensile forces in the collagen network. Compressive loading of the spine forces some water from the disc effectively increasing the aggrecan concentration and its swelling potential, and resisting further compression. On removal of the compressive load, disc height is restored as water is drawn back into the tissue to restore the original equilibrium conditions. Any parameter that decreases proteoglycan concentration or weakens the collagen network will be detrimental to disc function.

The internal mechanical functioning of an intervertebral disc can be studied by pulling a miniature pressure transducer through it. A young healthy disc behaves like a water bed, with the high water content of the nucleus and inner anulus enabling the tissue to act like a fluid (Figure 3A). Only the outermost anulus acts as a tensile skin to restrain the nucleus. With increasing age, disc water content decreases, especially in the nucleus, and most of the anulus then acts like a fibrous solid to resist compression directly (Figure 3B). In physically disrupted discs (Figure 3C), regions of fibrous tissue resist mechanical loading in a haphazard manner, and the hydrostatic nucleus is reduced or absent.

Stress profiles showing the distribution of compressive stress across the midsagittal diameter of lumbar intervertebral discs subjected to 2 kN of compression. Vertical and horizontal stresses are indicated in solid and broken lines, respectively. (A) Young Grade 1 disc. (B) Middle-aged Grade 2 disc, showing a stress concentration of magnitude h in the posterior anulus. The hydrostatic functional nucleus lies between the 2 vertical dashed lines. (C) Degenerated Grade 3 disc with multiple stress concentrations in the anulus (arrow). Compare with Figure 6. Reprinted with permission from Churchill Livingstone; 2002.[4]

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