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Lately I have been buried in the science of intervertebral discs, their herniation hijinks, and sciatica — inspired by Tom Jesson’s excellent work on this topic. For instance, see this terrific read for pros: What do we actually know about herniations and radicular pain? I’ve been meaning to tell you about some of the several dozen papers I’ve been reading … but how to choose? I have about 150 relevant papers in the PainSci bibliography, half of them added last month. It’s a bit overwhelming. I narrowed it down by picking three that were published in the last year or so. Fresh science.
- the one about how intervertebral discs can be trained
- the one about bacteria in discs
- the one about size not mattering (prolapse size, that is)
A review of the mechanobiology of the intervertebral disc, mostly just affirming that it does react to loading
Ruffilli et al is a comprehensive review of how human intervertebral discs respond to mechanical stimuli — AKA exercise, squishing, flexing, etc — with the main takeaway being that, yep, they do respond. (But also: the truth is complicated, and we still have a lot to learn.) The responsiveness isn’t necessarily obvious, because the discs are pretty gristly, and there’s an old-school view that all they really do is slowly get junky (like knee cartilage).
But the review makes it clear that it’s probably mostly safe to assume that mechanical loading affects discs in the same general way it affects most tissues: it does best with a “just right” amount of stimulation, and gets into trouble with too much or too little. For instance, “moderate mechanical loading may be important in preventing disc matrix degradation” and therefore “moderate exercise may have a protective role against disc degeneration.” But for a disc that’s already degenerating? That “may have detrimental consequences, although this requires further research.”
And, hoo boy, we really do need more in this case. Despite the profusion of back pain and spinal research in general, this review reports on surprisingly little useful research to date on this sub-topic: 15 papers since 1990 that met their standards, just one every couple years.
Click to embiggen. All the main kinds of trouble that intervertebral discs can get into. Mostly less of a problem than people fear… but not nothing either! The images are slightly exaggerated for dramatic effect & it’s “scary” to show them… but these are things that really do happen to intervertebral discs. The good news? It looks worse than it usually is & there are many surprising reassurances.
There’s lots of intriguing evidence about bacterial infection in intervertebral discs, but we can’t do much with it yet
Can a disc be infected? Low-grade infection of intervertebral discs with C. acnes might cause them to degenerate, and therefore eventually cause some back pain. This possibility has been raised by a bunch of small studies over the years. Granville-Smith et al. is a good review of those, but very technical. Most readers will want to stick to this simple summary.
The reviewers stuck to papers about patients who had undergone surgery, the better to shed light on the question of whether C. Acnes was introduced by the surgery (contamination), or was present in the disc before surgery. The C. acnes bacteria has definitely been found in degenerating intervertebral discs — no doubt about that — and contamination probably can’t explain all of it. However, much uncertainty remains, and specifically “culturing bias” might be muddying the waters. That is, it might seem like C. acnes is a culprit when the reality is that it’s just one of many bacteria that normally colonize discs, and it was only found because people looked for it … not because it’s a meaningful “infection.”
All this is worth more study, but so far “insufficient evidence exists to suggest changes to current clinical treatment.” In other words, it’s intriguing but uncertain and it is just not ready for clinical prime time yet.
Size doesn’t matter? Near zero correlation between symptoms and size/position of disc prolapse
Does it matter how far an intervertebral disc sticks out from between vertebra? Or which direction it’s going? You’d really think so, wouldn’t you? I did, despite reading so many papers over the years about the loosey-goosey links between back pain signs and symptoms. I still thought, “But surely the direction and size of herniations is significant. Right? I mean, c'mon!”
Dunsmuir et al checked. They carefully assessed fifty-six patients with uncomplicated lumbar disc prolapse, and found no detectable correlation between symptoms and the size and position of the bulge — technically, the Pearson correlation coefficients were near zero for both pain and disability (.05 and .07), which is stats-speak for “these two things we measured have basically no relationship.”
What does this tell us? Along with a mountain of similar evidence, it tells us that what a patient feels matters more than what we see on scans. 🤯 And back pain is very biological, and not so much “mechanical.”