Rob,
It's all of the above, and it's also a chicken and egg question.
Is the disc dehydrated because the chemical structure changed? (You'll hear a lot about proteoglycans in a scientific discussion of healthy vs. unhealthy disc nucleus.) Or, has the chemical structure changed because the disc became dehydrated. Endplates becoming 'clogged' and not passing nutrients will cause it to become dehydrated. Annular tears change the exchange of fluid and nutrients and causes a local anflammatory response... there are many things that work in concert. I believe that there are many processes that may be responsible... some injury... some genetic... some have to do with lifestyle and stresses (like injury, but without the big event... microtrauma over years of pounding.) For most of us it's probably all of the above.
One of the more interesting things about what I do is that I spend so much time in the OR, I've been able to closely examine nucleus tissue in all different stages of degeneration in many dozens of procedures. You would not believe the difference. Also, the nucleus tissue after a variety of procedures (SED, IDET, biologic disc injections, disc prolotherapy and others), all feels different. Very interesting stuff... I'll post more later.... gotta run now.
Mark
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