Like getting blood from a bone

Flexing your knee can squish the blood out of your kneecap like it’s a sponge — maybe literally, or maybe just a metaphor — suggesting that poor circulation could be a cause of patellofemoral pain syndrome (anterior knee pain). In particular, it might explain the notorious “movie sign,” in which simply sitting with flexed knees makes them ache. A 2008 study (Näslund et al) of 22 patients showed that 19 of them had reduced blood flow while the knee is flexed, but no such sign could be detected in healthy people. The method used to measure blood flow (photoplethysmography!) is new and therefore not exactly a sure thing, but there’s a good chance it works as advertised. Their data was a bit all over the map, but the averages were clear enough. If the results can be believed, it raises more questions than it answers: Is it cause or a symptom? Why would blood flow be reduced in the first place? Could this be why almost any kind of exercise tends to help — normalizing blood flow? And since when can you squeeze blood out of living bones?moreBecause living bones are surprisingly rubbery. The kneecap in particular is subjected to simply astonishing compression forces even in unloaded knee flexion. The squishableness of kneecaps in itself is a fun fact, but not especially surprising. The real curiosity here is: what’s the difference between the patients with knee pain and healthy controls? Why does knee bending impair circulation in some kneecaps and not others? How does that work? Tantalizing data!

“Decreased pulsatile blood flow in the patella in patellofemoral pain syndrome”
Näslund et al. American Journal of Sports Medicine. Volume 35, Number 10, 1668–73. Oct 2007.

Update: this item got my friend Dr. Rob Tarzwell of One-Minute Medical School curious about arterial supply to the patella. He writes: “It looks like normal anatomy involves a circulatory anastomosis. That's where multiple arteries plug into a ring, and the ring then has feeders going to the patella. Presumably, this is to allow for redundancy of supply in the event of flexion of the knee closing off supply. Now, if the anatomy isn't sufficiently redundant, then supply could become temporarily compromised. Variants of normal anatomy are legion.” In other words, not everyone’s arteries may be arranged optimally to cope with flexion, and some may fail to keep the blood flowing during flexion — which would explain these results, and potentially a lot of otherwise mysterious anterior knee pain. This is a fascinating, plausible hypothesis.