Nucleus Regeneration (ADCT) and Replacement (PDN)

[sharman]

I am very excited about the new technologies for treating DDD by replacing or regenerating the degenerated nucleus. There is a lot to say (and as you've figured out, I talk a lot), so I am splitting this up into four parts: First, in this post, a general discussion of the function and physiology of the nucleus; second, a description of the two competing technologies, ADCT (autologous disc chondrocyte transplantation), a procedure where the patient's cells are harvested, cultured in a lab and re-inserted into the disc, and PDN (prosthetic disc nucleus), where the diseased nucleus is replaced with a prosthethic device; and the third post will compare the risks and benefits of the two procedures. Last (and still a work in progress), a post about complementary therapies/procedures that might be done together with ADCT or PDN, to further enhance the benefits. (Those other therapies were discussed a bit in my post "Can You Heal a Disc?", and the fourth post on ADCT/PDN will follow up on the comments Mark made to "Can You Heal a Disc"?)

Before launching into this, I point out (as discussed on other threads) that these therapies are not going to work with advanced degeneration. The annulus must be relatively intact ("competent"), and there must be decent remaining height (I've seen a minimum of 5 mm cited).

PHYSIOLOGY AND FUNCTION OF THE NUCLEUS

Unlike most organs, the nucleus has very few living cells. Only about 0.25% to 0.5% percent of the nucleus is cells. The rest is water (90% I think?), a substance called proteoglycans, which is the stuff that attracts and holds water in the nucleus, and collagen. The function of the cells is to produce the proteoglycans that attract the water that gives the nucleus its cushioning turgidity. As discs age and degenerate, the amount of water, cells and proteoglycans goes down, the amount of collagen up.

A disc is essentially a ligament: it holds together two bones (vertebrae), in a manner that is strong enough to hold the bones in place, and flexible enough to allow the bones to move properly. The annulus of the disc is virtually the same as other ligaments in the body, composed of the same materials (collagen). But a disc, of course, is a unique ligament, in that it has at its core the jelly-like nucleus. I envision the nucleus as a swollen water balloon, its turgidity giving support and cushion to the adjoining vertebrae. The nucleus decompresses when loaded by expelling water, and re-hydrates at rest.

Besides the mechanical function of support and cushion, the nucleus has a second function, to transmit nutrients and waste to and from the annulus (and itself). This is effected by the 'pumping' action of the nucleus decompressing through the course of the day, and then re-hydrating at night.

So what is going on in a degenerated disc, that causes back pain? The nucleus has no nerve supply, so cannot itself be the source of the pain; IMO, the locus of discogenic low back pain is the innervated outer one-third of the annulus, and specifically the tears in the annulus we see in a degenerated disc.

But we know that a degenerated nucleus is usually associated with DDD--we see the black disc on the MRI. So what role, if any, does a degenerated nucleus play in discogenic low back pain? I see four possibilities:

1) First is the mechanical, the degenerated nucleus can no longer attract and hold water to make the water-balloon-like shock absorber cushion for the spine. This may translate into increasing stress on the annulus--the nucleus no longer absorbs weight and shock, resulting in this burden landing more heavily on the annulus.

2) Second, if the nucleus no longer serves its function of transporting nutrients, water and waste to the annulus, the annulus breaks down, tears, and is less able to heal itself.

3) Third, the degenerated nuclear material may be an irritant to the damaged annulus. Dr Yeung created a procedure he calls Selective Endoscopic Discectomy (SED), in which the endoscope used in the surgery allows him to see the inside of the disc, and to address observed pathologies. Among Dr Yeung's many interesting observations from the SEDs he's performed, two are significant here: First, Dr Yeung stains the nucleus before the operation, and claims that he can tell the difference between healthy nucleus tissue and unhealthy (his SED includes removal of the unhealthy tissue). Second, Dr. Yeung claims to observe "interpositional tissue" in the annular tears--I understand this to mean bits of matter that have become stuck in an annular tear. Possibly then,degenerated nuclear material and/or 'interpositional disc tissue' is an irritant, and/or retards healing of a tear.

4) Fourth, the material of even a healthy nucleus is caustic. If this material leaks onto a nerve root, it can cause sciatica (and maybe also back pain?) If it leaks into an annular tear, this too may be a source of pain. This last may be irrelevant to the ADCT/PDN discussion--if your disc is leaking this badly, the degeneration is probably too far gone to benefit from either of these procedures.