A Baker’s Cyst is a swelling at the back of the knee caused by fluid from the joint leaking backwards and collecting in a ‘pocket’ (cyst). Baker’s cysts are sometimes also called ‘popliteal cysts’ (because they are found in the hollow at the back of the knee called the popliteal fossa.
Baker’s cysts are entirely benign and safe. They only tend to cause issues in two specific circumstances:-
- First, if the cyst is very large then it can cause discomfort around the back of the knee, with a feeling of fullness, tightness or pressure, particularly with kneeling/squatting.
- Second, if a Baker’s Cyst is large than it can sometimes burst. If this happens then the cyst releases fluid (synovial joint fluid) into the back of the leg, into and around the calf muscles. This can be painful, and the swelling and firmness that it causes in the calf can be confused with a potential DVT (Deep Vein Thrombosis – blood clot). If the calf is swollen then it is important to exclude a potential DVT, and this can be tested for with a blood test (to test the D-Dimer level) and an ultrasound scan of the veins. If a ruptured Baker’s Cyst is confirmed then no specific treatment is needed, as the pain and swelling pretty much always simply get better on their own with time.
What’s the cause?
The underlying cause of a Baker’s Cyst is that joint fluid has leaked out of the back of the capsule of the knee, into the popliteal fossa. This implies that either there is widespread degeneration in the joint (Baker’s Cysts are very common in arthritic knees), or else there might potentially be a meniscal cartilage tear. Particularly with horizontal tears posterior horn meniscal tears, the tear can sometimes act as a flap valve, so that when pressure is put on the knee joint fluid is pushed through the tear and out into the back of the knee. When pressure is eased off the joint, the tear tends to close up, preventing the fluid at the back of the knee from going back into the joint.
Particularly, if there is specific pain with a Baker’s Cyst then this implies even more strongly that there is a probable underlying meniscal tear.
If one were to surgically excise a Baker’s Cyst, then this has to be done by opening up the back of the knee. The back of the knee is where all the major nerves and blood vessels are. Therefore, one cannot do this through a small hole (as they say – big mistakes can be made through small holes!) … and therefore when opening up the back of the knee one has to use a fairly big incision, to be able to see what you’re doing. This leaves a big scar at the back of the knee, which is a tender and awkward place to have a scar.
Given that Baker’s Cysts are not actually in any way dangerous, and given that they rarely cause any actual major symptoms themselves … surgical excision of a Baker’s Cyst is rarely ever a good idea. In addition, even if a Baker’s Cyst is excision through an open incision at the back of the knee, there is a fairly high rate of recurrence afterwards.
Baker’s Cysts can be aspirated (the fluid sucked out with a needle). Given all the important and dangerous structures round the back of the knee that sit right next to where these cysts are found, this should only ever be done under ultrasound guidance, for safety. If fluid from a Baker’s Cysts is drawn off, then most people will also inject a small volume of steroid into the cyst cavity – as this apparently slightly reduces the risk of recurrence (although personally, I’m not really convinced of this!). The recurrence rate after cyst aspiration is, however, very high (because one has not actually done anything to address the cause/origin of the cyst).
It is possible to decompress a Baker’s Cyst arthroscopically (through keyhole surgery), approaching the knee from the front …. If the hole that communicates from the inside of the knee joint to the cyst at the back of the knee can be found and identified, then a probe can be passed in through the hole to allow the fluid from the cyst to empty into the knee cavity, from where it can be washed out. The hole can then either be opened up (to eliminate the flap valve effect) or closed by stitching it up. This hole is very often in the back part (posterior horn) of one of the meniscal cartilages --- and by fixing the meniscal problem (meniscal repair or meniscal trimming) one can often solve the problem of the Baker’s Cyst.