Pain Generated by the Spinal Disc

disc
Fig. 1: Inner (nucleus) and outer (annulus) portion of the disc
(larger view)

disc
Fig. 2: Disc distributes force without injury
(larger view)

Before discussing how the spinal disc can cause back pain and/or leg pain (sciatica), it is useful to first understand the role of a healthy disc in the spine and the anatomy.

The intervertebral disc has several important functions, including functioning as a spacer, as a shock absorber, and as a motion unit:

  • Spacer. The height of the disc maintains the separation distance between the adjacent boney vertebral bodies. This allows biomechanics of motion to occur, with the cumulative effect of each spinal segment yielding the total range of motion of the spine in any of several directions. Proper spacing is also important because it allows the intervertebral foramen to maintain its height, which in allows the segmental nerve roots room to exit each spinal level without compression (e.g. a pinched nerve).
  • Shock absorber. Shock absorption allows the spine to compress and rebound when the spine is axially loaded during such activities as jumping and running. Importantly, it also resists the downward pull of gravity on the head and trunk during prolonged sitting and standing.
  • Motion unit. The elasticity of the disc allows motion coupling, so that the spinal segment may flex, rotate, and bend to the side all at the same time during a particular activity. This would be impossible if each spinal segment were locked into a single axis of motion.
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The gelatinous central portion of the disc is called the Nucleus Pulposus. It is composed of 80 - 90% water. The solid portion of the nucleus is Type II collagen and non-aggregated proteoglycans.

The outer ligamentous ring around the nucleus pulposus is called the Annulus Fibrosus, which hydraulically seals the nucleus, and allows intradiscal pressures to rise as the disc is loaded. The annulus has overlapping radial bands, not unlike the plies of a radial tire, and this allows torsional stresses to be distributed through the annulus under normal loading without rupture.

The disc functions as a hydraulic cylinder. The annulus interacts with the nucleus. As the nucleus is pressurized, the annular fibers serve a containment function to prevent the nucleus from bulging or herniating. The gelatinous nuclear material directs the forces of axial loading outward, and the hoops of annular fibers help distribute that force without injury.

Figure 1: Inner (nucleus) and outer (annulus) portion of the disc
Figure 2: Disc distributes force without injury

Pain Caused by the Disc

As the inner nucleus dehydrates, the disc space narrows, and redundant annular ligaments bulge. With progressive nuclear dehydration, the annular fibers can crack and tear.

Loss of normal soft tissue tension may allow the spinal segment to sublux (e.g. partial dislocation of the joint), leading to osteophyte formation (bone spurs), foraminal narrowing, mechanical instability, and pain.

Lumbar disc disease can cause pain and other symptoms in two ways:

  • Herniated disc (sciatica). If the annular fibers stretch or rupture, allowing the pressurized nuclear material to bulge or herniate and compress neural tissues, leg pain and weakness may result. This is the condition called a pinched nerve, slipped disc, or herniated disc. This condition will typically cause sciatica, or radiating leg pain as a result of mechanical and/or chemical irritation against the nerve root.

    Although the overwhelming majority of patients with a herniated disc and sciatica heal without surgery, if surgery is indicated it is generally a decompressive removal of the portion of herniated disc material, such as a discectomy or microdiscectomy.
  • Degenerative disc disease. Mechanical dysfunction may also cause disc degeneration and pain (e.g. degenerative disc disease). For example, the disc may be damaged as the result of some trauma that overloads the capacity of the disc to withstand increased forces passing through it, and inner or outer portions of the annular fibers may tear. These torn fibers may be the focus for inflammatory response when they are subjected to increased stress, and may cause pain directly, or through the compensatory protective spasm of the deep paraspinal muscles.

    This mechanical pain syndrome, unresponsive to nonsurgical treatment, and disabling to the individual's way of life, is generally the problem to be addressed by spinal fusion or artificial disc technologies.
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Written by Jack Zigler, MD