Proteoglycans is the X factor of discs. Early signs of progression towards 'degenerative disc disease' is decreasing concentrations of proteoglycans in the heart of the disc. Proteoglycans provide the compression-resisting qualities that help a disc hold its water and keep buoyant under the compressive forces of muscle activity and gravity.
As the PGs concentration declines a disc gets sicker and flatter. It changes insidiously from being painfully non-compliant to a totally defunct spacer riven with cracks and a disc cell population struggling to stay alive. The lack of internal pressure in the disc makes the disc wall bulge, just like a car tire lacking air.
A disc in early stage degenerative disc disease behaves exactly like a slow leak in a car tyre; harder to pump up and quicker to go down. As the disc dehydrates the nucleus shrivels, sometimes by 50%. Load is then transferred to the disc wall, which thickens by up to 80% as it bunches down like a 3D lattice.
Taking load through its wall instead of the nucleus at the centre makes the disc start to break up. Cracks - both radial and circumferential - appear in the wall and the nucleus is less definable as a ball of fluid; fibres in the wall break and torn bits hang off. Even asymptomatic discs are notoriously grotesque in their appearance as they break down earlier in life then any other structure in the human body.
Shabby old discs are often wrongfully (and misleadingly) nominated as pain sources by MRI. Known as 'false positive' reporting, they can have unfortunate effects in electrifying the imagination of patients. Over-florid reporting of commonplace degenerative features that are asymptomatic can cause great alarm - and hamper recovery of a very fixable back problem.
Only the outer wall of the disc is innervated and senses pain. So it doesn't matter how degenerated the inside of the disc is; if its outer wall is sufficiently healthy and compliant to absorb spinal movement without being strained, the disc will not be painful. Disc degeneration is by no means indicative of back pain.
See Sarah in person explaining spinal breakdown following disc dehydration. The sequence is outlined in more detail in video 4 of the 'Causes of Back Pain Video Package'.
The end-of-the-line degenerated disc is in a league of its own when it comes to pain generation. With severe dehydration and no liquid centre of the disc to disperse pressure (particularly with high impact activities) the vertebral endplates progressively shatter as the walls also develop deeper clefts. The wider cracks allow blood to penetrate to the inside of the disc - and where blood goes nerves can follow. In effect, the nerves grow into the disc from the outside, looking for trouble.
In the terminal stages the disc undergoes its own enzymatic breakdown, leaking its highly toxic contents like a porous hessian bag. In the photograph below (taken at surgery to assess the damaged state of the L4 disc) the methylene blue dye can be seen leaking out through the disc walls within moments of injection into the disc.
Very few discs degenerate to this degree and one of the primary aims with all conservative spinal therapy is to stop any back ever getting this bad. Where there is nerve in-growth in advanced stage degenerative disc disease the best option is surgical resection.
Spinal surgery procedures of yesteryear that involved wholescale removal of the disc had more satisfactory outcomes as the disc space filled more effectively with scar tissue. Lesser scale discectomy often leaves the segment unstable so that artificial disc replacement is necessary.
You can read all about the different surgical options, their post-operative management and some of the causes of failed back surgery in Sarah Key's small 42 page eBook
Read about Stage 2 of segmental breakdown Facet Joint Arthropathy
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