Age Related Degenerative Disc Disease

As we get older, one of the common effects of aging is that the intervertebral discs within the spine lose water content and undergo biochemical changes and become more susceptible to discontinuity and resultant herniation.  Degenerative disc changes often precede bony changes in the spine associated with the degenerative process.  Degeneration within the vertebral disc is characterized by a loss of disc volume characterized on X-rays studies by a reduction of vertical disc height represented by the space located between the intervertebral bodies.  A significant loss of vertebral disc height leads to buckling of ligaments that can subsequently encroach upon the spinal canal at the point of exit of spinal nerve roots.  These changes are accompanied by increased weight-bearing stresses upon the facet joints leading to further degenerative involvement with thickening of the joint capsule, as well as thickening of adjacent ligaments.  This combination of tissue changes during the degenerative process can lead to narrowing of the central spinal canal or the lateral openings along the spine where the spinal nerve roots exit. Narrowing of spaces within the spine is referred to as stenosis. MRI evaluation of the degenerated spine reveals dehydration of the intervertebral disc.  This is characterized by a dark disc on certain T2-weighted MR images.  An MRI of a healthy disc is characterized by an area within the center of the disc that is more hydrated than the area around the disc, not unlike the center of a jelly donut.  This line of demarcation between the harder more supportive surrounding elements of the disc with the gel-like center becomes blurred secondary to dehydration and chemical changes associated with the aging process.  Rostrocaudal subluxation is a term sometimes used to describe the degenerative process leading to a loss of vertical disc height and the resultant approximation of adjacent vertebra.  The term subluxation refers to a disrelationship between two adjacent bones forming a union or joint. 

Degenerative changes involving the intervertebral disc render the disc more susceptible to injury and subsequent herniation.  Loss of support within the intradiscal tissue also leads to greater propensity for physical compression of the disc, resulting in broad-based or focal bulging of the disc beyond the outer boundaries of the adjacent vertebrae.  During the degenerative process, discontinuity (separation) of annular fibers occurs, leading to internal fissures within the internal portion of the disc.  The gel-like center of the intervertebral disc will migrate to the path of least resistance, working its way through fissures within the annular fibers. This process can eventually and eventually destabilize the disc.  Discontinuity of the outer annular fibers can result in extrusion of the gel-like material from the center of the disc.  This can lead to release of chemicals that cause inflammation of adjacent neurological tissues.  Extruded disc material can cause compressive compromise of adjacent nerve structures.