Baker's cysts and other knee bursae


Nurse Director
Jun 8, 2007
The North
United Kingdom United Kingdom
Fact is that the 'cyst' is a normal part of your knee and acts as a reservoir for the fluid in your knee. You know how most car engines now have an overflow tank for their radiator? So if the engine overheats, instead of the water all blowing off as steam, it overflows into the bottle, cools down and then gets sucked back into the cooling system again which stops the radiator ever boiling dry.

Well it's a little bit like that with the popliteal bursa. Only sometimes the channel between it and the knee joint gets obstructed with inflammation and the fluid can't flow back. Being lined with synovial membrane which also produces the fluid the bursa will swell up like a balloon. Sometimes the channel unblocks when the pressure gets high enough, and/or the inflammation goes down so it empties again, and sometimes it doesn't, so it just gets bigger and painful and that's when it is a 'Baker's cyst'. Before that it's just a normal bursa.

But while most of these cysts are free draining, some can become partially closed and drain fairly easily when the pressure gets to a certain point. Normally they only have a few mls of fluid in them and an increase to about 10mls or more is when it can become noticeable and uncomfortable and initiate a back flow.

So, a Baker's cyst is actually part of the normal anatomy of the knee.


So, to remove or not? It can be a mistake as it's in an extremely difficult area to access, difficult to get at the bursa and what surgeons in my younger days would ruefully refer to as "tiger country" on account of the very major arteries, veins and nerves that are there, very often in direct contact with this bursa. Definitely brown trouser time if you are a surgeon! :shocked:

popliteal fossa anatomy 2.jpg

As a final consideration, the incisional wound can be problematic in healing and extremely painful during rehab. Adhesions can be an issue too. All in all, much better left alone as it will most likely drain itself anyway.

The same applies to the smaller bursae at the front of the knee. There is one above the knee cap (patella) called the supra-patellar and one below it called the infra-patellar bursae which just mean above and below the knee cap.

Knee Bursa 2.jpg

The infra-patellar bursa is often referred to as "house maid's knee" because it can be caused by kneeling on hard floors a lot - a hint from the "Upstairs Downstairs" days when posh houses employed house staff and a lot of the cleaning had to be done on hands and knees! Nowadays is often suffered by gardeners, carpet layers, builders and such. If they get large and troublesome, those can easily be surgically removed.

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