The intervertebral discs provide a cushioning effect between the vertebra. They consist of a firm fibrous outer ring called the annulus fibrosus and a soft inner core called the nucleus pulposus. The nucleus pulposus has a consistency and appearance similar to crabmeat. With wear and tear of the spine, the annulus may become weakened so that the fibers are stretched, leading to bulging of the disc beyond the margin of the disc. This is commonly seen in individuals in their 50’s and older. The bulging of itself is usually not enough to take up so much room that the nerves in the spinal canal are compressed. However, if the area containing the nerves is constricted because of bone spurs or thickened ligaments, the bulging disc could be a contributing factor to nerve compression.
Herniated discs, on the other hand, are more significant projections of disc material into the spinal canal. They usually are due to a defect in the outer annulus allowing nucleus pulposus material to push out of the central core. Because they take up more space in the spinal canal, herniated discs are more apt to cause nerve compression. In addition, when a disc herniates, there is a tear in the annulus through which the nucleus pulposus extrudes. The chemical composition of the nucleus pulposus may have an irritative effect on the nerve exposed to it. This irritation of the nerve may cause pain and is sometimes called a chemical radiculitis. When a piece of the nucleus pulposus completely extrudes from the confines of the annulus it is called a disc extrusion.
An often quoted study on the frequency of disc abnormalities with age defined bulging discs as symmetrical extension of the disc beyond normal level of the annulus; protrusion as an asymmetric extension of the disc with the base against broader than the apex ; and extrusion being a more extreme extension of the disc beyond the disc space with the base against the disc narrower than the apex in the spinal canal.
In summary, the disc bulge is a mild encroachment on the spinal canal space due to a weakened annulus. The disc herniation represents a defect in the annulus and may either be a disc protrusion or a disc extrusion. The exposure of the nerve to the nucleus pulposus may cause irritation, called a chemical radiculitis. Nerve related pain may be due to either nerve compression by the disc herniation or by chemical irritation.
Jensen M, Brant-Zawadzki M et al , Magnetic Resonance Imaging of the Lumbar Spine in People without Back Pain; N Engl J Med 1994; 69-73, July 14, 1994.