Spinal discs required a regular supply of nutrients and oxygen to withstand everyday stresses and strains and repair itself.
Described as a degenerative cascade, the following three stages of disc degeneration have been proposed by researchers and are generally accepted in the medical community to provide insight into how degenerative disc disease gradually develops.
How Does Degenerative Disc Disease Become Painful?
In the 1970s, Kirkaldy-Willis first described the degenerative cascade. This model helped to explain the normal changes that all discs have. He postulated that after a torsional (twisting) injury to the disc, the degenerating disc follows three general stages.1
- Dysfunction. Small circumferential tears in the outer layer of the disc grow inward. After injury, mobility may be limited.
- Relative instability. Inner material of the disc begins to protrude outward through the tears, leading to decreased disc height.
- Restabilization. Osteophytes form at the margins of the disc and the changes in the disc become fixed. The vertebral segment restabilizes and dysfunction subsides.
Other factors such as loss of nerve endings in the muscles near the spine may also play a role in the cascade.2 As the disc restabilizes, mobility recovers and the injury resolves. Disc injuries may occur with increasing frequency, and with increasing pain intensity, without the incorporation of lifestyle changes and exercise.
The stages of degeneration may occur over a period of 20 to 30 years, with pain less likely and less frequent in older age. Adults over 60 years old are less likely to have pain from degenerative discs than adults between 30 and 50 years old. Pain from other degenerative conditions, such as degenerative osteoarthritis or spinal stenosis, is far more likely in elderly adults than pain from degenerative disc disease.
How Does Aging Lead to Degenerative Disc Disease?
On a cellular and tissue level, the aging process of the disc can also be defined as a cascade of biochemical changes in three stages.3
- Damage to DNA and proteins. The balance between protein formation and protein breakdown can become off-set, leading to degradation of the extracellular matrix. A healthy matrix is critical for maintaining the structure of the disc.
- Irregular response to damage. Cell signaling can become poorly regulated and lead to cell death, as well as a state of arrest in the cellular growth cycle.
- Loss of disc structure and function. Changes from biochemical damage and from subsequent immune response can alter the disc, for example by increasing disc stiffness.
Biochemical damage can be induced with the help of certain internal stressors, such as free radicals or low levels of glucose, oxygen, and pH. Other stressors may be from daily activity, such as regularly lifting heavy loads. Stressors may be prevented from perpetuating painful disc degeneration, by exercising and eating healthy foods.
Lumbar Degenerative Disc Disease
The natural history of lumbar or cervical degenerative disc disease, is relatively benign. Disc degeneration is progressive, moving along the stages of the degenerative cascade over time at various speeds depending on multiple variables. Intermittent pain symptoms such as lower back pain may seem to be progressive as well, but the pain generally does not worsen.
Understandably, low back pain at a young age can be a cause for concern. Psychological stress may be caused by the anticipation that pain will worsen and confine the individual to a wheelchair in older age. In actuality, pain management and regular activity can be achieved and can provide a favorable outcome.
As disc degeneration progresses:
- All the inflammatory proteins within the disc space will eventually “burn out.”
- The disc will stiffen from the changes in protein structure and changes to the extracellular matrix.
- Micro-motions will decrease as the disc regains stability.
On an MRI scan, a degenerating disc will appear black or dark, as opposed to a healthy disc that has a lighter appearance. Disc degeneration can be present without causing any symptoms of lower back pain; for example, an MRI of an older person may be expected to show degeneration in all the discs without causing degenerative disc disease.
Degenerative disc disease is typically difficult to diagnose, because other lower back problems, such as a herniated disc, facet joint mediated pain, or spinal stenosis, may cause similar symptoms. Doctors often use a combined approach, by evaluating the medical history, diagnostic tests, and reported signs and symptoms, to arrive at a diagnosis. Discogram, a controversial and infrequent diagnostic tool, may be used during an interventional or surgical treatment to confirm which disc mediates pain.
Extensive research on the degenerating and aging intervertebral disc continues, with the potential of developing regenerative treatments and novel approaches to pain control.