Blood-starved kneecaps
Flexing your knee can, perhaps, squish the blood out of your kneecap like it’s a sponge, suggesting that poor circulation is a cause of patellofemoral pain, a common form of knee pain that I have written a whole book about. In particular, it might explain the notorious “movie sign,” in which simply sitting with flexed knees gets them aching.
Could that position be strangling some kneecaps?
A 2007 study by Näslund et al. of 22 patients showed that 19 of them had reduced blood flow while the knee is flexed, while no such sign could be detected in healthy people. The method used to measure blood flow — photoplethysmography! — was new then, and not exactly a sure thing. Their data was a bit all over the map, but the averages were clear enough: those people had bloodless kneecaps! Relatively.
Tantalizing data! But very preliminary, and a long time ago now.
Replication after seventeen years
That experiment needed replication and finally got backed up partially in early 2024. It’s not exactly “replication,” because Ophey et al. used near‐infrared spectroscopy to measure changes in concentrations of the oxygen-toting molecule, hemoglobin. But the results have the same implications: “potential impairment in patellar hemodynamics in patellofemoral pain patients, providing support for the homeostasis model.”
(I should also mention a 2018 experiment that found nothing … but quite likely because, weirdly, they didn’t measure blood flow while their subjects were in positions that actually provoke patellofemoral pain.)
| title | Impaired hemodynamics of the patella in patients with patellofemoral pain: A case-control study |
|---|---|
| journal | J Exp Orthop |
| Volume 11, Number 1, Jan 2024, e12009 | |
| authors | Martin J Ophey, Anne Westerweel, Maxime van Oort, Robert van den Berg, Gino M M J Kerkhoffs, and Igor J R Tak |
| links | publisher • PubMed • PainSci bibliography |
More questions than answers
If the results of these experiments can be believed — and they can be a little more now — they raise more questions than they answer (like all the best science).
Is the limited circulation a cause or a symptom? Why would blood flow be reduced in either case? What’s the difference between the patients with knee pain and healthy controls? Why does flexion impair circulation in some kneecaps and not others? Is patellar circulation increased by exercise, and could this be why almost any kind of exercise seems to help — normalizing blood flow?
And since when can you squeeze blood out of living bones? Okay, that one we can answer quite easily: because living bones are surprisingly rubbery! And the kneecap in particular is subjected to astonishing compression even in unloaded knee flexion. The squishableness of kneecaps in itself is a fun fact, but not especially surprising.
A dissected patella. Squishier than it looks!
Anatomical variation strikes again?
The normal anatomy of the patella involves a circulatory anastomosis, an arterial ring that has several “spokes” supplying it, and then more feeders going into the kneecap. This arrangement specifically exists to make it more difficult to block circulation — more entrances and exits, no single point of failure. This anatomy suggests that there’s a substantial need for extra circulation to the patella — or it wouldn’t have evolved — but blood vessel anatomy varies greatly. Not everyone’s arteries may be arranged optimally, and some may fail to keep the blood flowing.
That would explain some of the data, and potentially a lot of otherwise mysterious anterior knee pain.
I discuss some other ideas in a book chapter devoted to this topic, updated today … for the first time in eleven years! I also added a discussion about the effect of leg exercise on patellar circulation.