What’s the Biochemical Basis for the Success of Continuous Passive Motion?

When it comes to dealing with knee pain, I believe strongly in the beneficial effects of motion. I often prefer the word “motion” to “exercise,” as the latter conjures up images of fitness buffs doing strenuous activities that may not be good for bad knees. Move, move, move (in the proper amounts, in the proper ways).

For those recovering from surgery (or maybe even for those who just have really bad knees), I think continuous passive motion can be a great idea. In Saving My Knees, I mention the experiments on rabbits that Robert Salter performed more than three decades ago. Salter discovered that the cartilage fell apart in rabbit knees that were immobilized, while the tissue improved in knees that were constantly flexed using a CPM device.

So continuous passive motion works. But why?

Well, one advantage of having a blog that’s read by lots of smart, well-informed people is that I’m constantly being alerted to interesting studies. Like this one from 2005: "Anti-Inflammatory Effects of Continuous Passive Motion on Meniscal Fibrocartilage." It’s another rabbit study, and one of the researchers is Salter.

Arthritis was induced in all the rabbits’ knees. It appears that the knees of half the animals received CPM for 24 or 48 hours, while the joints of the others were immobilized.

While rabbits certainly aren’t people, one “advantage” of experimenting on them (or disadvantage, if you’re one of the rabbits) is that their cartilage can be “harvested” for close-up, thorough inspection at the study's end because they’re generally euthanized.

What the researchers discovered were changes that may shed light on the biochemical reasons that CPM works (caveat: again, I’m reduced to summarizing an abstract because the full study lies behind a paywall).

Even after a timeframe as short as a day or two, the study noted significant differences between the knees that moved all the time and those that didn’t move at all.

The immobilized knees:

* Showed “marked GAG degradation.” GAGs, of course, are glycosaminoglycans, which contribute to the strength and resiliency of cartilage (Saving My Knees explains in more depth why glycosaminoglycans are important).

* Had higher levels of three different molecules that contribute to inflammation.

Knees that underwent continuous passive motion exhibited the converse of both these trends: there was a “rapid and sustained decrease” in glycosaminoglycan breakdown, and fewer molecules involved in inflammation were found. What’s more, CPM led to synthesis of an anti-inflammatory molecule.

The researchers conclude:

These studies explain the molecular basis of the beneficial effects of CPM observed on articular cartilage and suggest that CPM suppresses the inflammatory process of arthritis more efficiently than immobilization.

Now here’s my hunch:

Those molecular-level benefits of continuous passive motion will be found, in future studies, to apply to motion more broadly, and to knees more broadly (not just the ones that are post-surgery or arthritic). And that would be pretty good news for people battling chronic knee pain who are looking for a way out of the trap they’re in.