Disc protrusion (or herniation) occurs almost always in the context of pre-existing degenerative change in the affected disc. The degenerative change may be clinically silent to that point, although there is often a history of episodes of back discomfort.
Disc degeneration is currently believed to be the consequence of impaired nutrition of the chondrocyte cells within the disc, which, in health, are nourished by diffusion of nutrients from capillaries in the endplate regions of the adjacent vertebrae.
It is believed that changes occur in the endplates, which impair the diffusion process, possibly as the result of repeated micro injuries throughout life. As a result, chondrocyte function deteriorates and the repair and maintenance function they perform declines. This leads to changes in the biochemical makeup of the disc matrix, one consequence of which is that the matrix material binds less water and becomes less gelatinous and resilient.
As a second consequence, the volume of disc matrix declines and the pressure within the disc reduces, at the same time reducing the tension in the annulus fibres that encapsulate the nucleus.
These changes can be detected on an MRI scan, where they are visible as darkening of the disc, and loss of disc height, often with some ‘deflation’ bulging of the disc annulus.
Reduced intradiscal pressure, combined with reduced annular tension, allows increased non physiological movement at the affected disc level, which alters the stresses on the facet joints, and increases mechanical stresses on the annulus and disc nucleus.
In certain circumstances, this will lead to annular tears or defects, which potentially create a pathway for the degenerated disc material to be pushed out of the disc like toothpaste out of a tube. If this protruded disc material compresses a nerve root, a clinical radiculopathy develops, in its most common form, sciatica.
In some circumstances, the disc material elicits a marked inflammatory response, compounding the effect of compression on the nerve.
Some disc protrusions ultimately shrink and are broken down and absorbed. Others remain chronically compressive. Unfortunately, no external influence, such as physiotherapy, chiropractic or osteopathic treatment, can cause disc material to return to its place within the disc.
If it does not occur naturally, structural decompression of a compressed nerve root requires surgical intervention in the form of a discectomy.