It is a common and puzzling finding on MRI: one intervertebral disc shows clear signs of degeneration, while the disc just above or below looks relatively healthy. If they share the same spine, the same age and the same daily life, why such a difference?
The answer lies in the local biology and loading history of each individual disc. No two discs have exactly the same environment.
A disc is more than a disc
To understand this, we need to look at three structures that work together:
- Nucleus pulposus: the soft, gel-like center that distributes pressure evenly.
- Annulus fibrosus: the tough, layered outer ring that contains the nucleus and provides tensile strength.
- Vertebral endplate: the thin layer of cartilage and bone that separates the disc from the vertebral body above and below. It is the main gateway for nutrition and waste exchange.
Nutrition depends on diffusion
Intervertebral discs are the largest avascular structures in the human body. In adulthood they have no direct blood supply.
Nutrients such as oxygen and glucose reach the disc primarily through diffusion, mainly via the vertebral endplates and to a lesser extent through the outer annulus. Waste products take the same route out.
This makes the endplate critical. If an endplate at one level is damaged, more calcified, or less permeable, transport to that specific disc is reduced. The neighboring disc with a healthier endplate may remain well-nourished.
Limited capacity for repair
Disc cells can maintain tissue and produce matrix molecules, but their ability to repair structural damage is limited. This is especially true in the inner disc where cell density is low and nutrition is poor.
A fissure in the annulus fibrosus or loss of nucleus material does not heal like muscle or skin. Once structural changes occur, they tend to accumulate rather than fully reverse.
Loading affects fluid movement – but it is more complex than a pump
Loading and unloading changes fluid movement in the tissue and can influence transport. When you load the spine, fluid is pressed out; when you unload, fluid is drawn back in.
This is more complex than a simple pump effect. More loading does not automatically mean better nutrition. The type, duration and magnitude of load all matter, as does the health of the endplate. Sustained high compression can hinder transport, while variation in load and position appears more favorable for fluid exchange.
Why are there differences between levels?
Each spinal level lives a slightly different mechanical life:
- Lower lumbar discs (L4/L5, L5/S1) carry more axial load and shear forces.
- A previous injury may have affected one endplate and not another.
- Spinal shape – lordosis, pelvic incidence, minor asymmetries – distributes forces unevenly.
- Genetic factors influence endplate strength, disc composition and susceptibility to degeneration.
Degeneration is therefore often level-specific, not a uniform ageing process of the whole spine.
Movement is not the enemy of the disc
It can be reassuring to know that a degenerated disc on an image does not mean you must avoid movement. Discs benefit from varied loading and regular changes in position to facilitate fluid exchange. Prolonged static postures and general deconditioning are typically less helpful than gradual, varied activity adapted to symptoms.
These are not consecutive stages of the same disease process, but different examples of disc appearance that can be found at different levels or in different individuals. A: Relatively healthy disc with preserved height and bright T2 signal in the nucleus. B: Moderately degenerated disc with reduced signal intensity. C: Advanced degeneration with loss of height, low signal and associated endplate changes. The same person can have an A at one level and a C at the next.
What does this mean for you?
Finding degeneration at one level does not mean your entire spine is degenerating rapidly. It reflects local conditions at that level. Management focuses on optimizing the environment around the affected segment – improving load distribution, building capacity for activity, and supporting general spinal health – rather than trying to reverse the MRI finding itself.
Imaging is one piece of the puzzle. Symptoms and function are guided by many factors beyond the image.
References
- Adams MA, Roughley PJ. What is intervertebral disc degeneration, and what causes it? Spine. 2006;31(18):2151-2161.
- Urban JPG, Smith S, Fairbank JCT. Nutrition of the intervertebral disc. Spine. 2004;29(23):2700-2709.
- Raj PP. Intervertebral disc: anatomy-physiology-pathophysiology-treatment. Pain Pract. 2008;8(1):18-44.
- Battié MC, Videman T, Parent E. Lumbar disc degeneration: epidemiology and genetic influences. Spine. 2004;29(23):2679-2690.
- Brinjikji W, et al. Systematic literature review of imaging features of spinal degeneration in asymptomatic populations. AJNR. 2015;36(4):811-816.