WHAT IS A POPLITEAL CYST?
A popliteal cyst is a benign fluid-filled lump that appears at the back of the knee in some children. It connects with a cyst that we all have at the back of our knees. This cyst oils the hamstring tendons as they move over the back of the knee so they don't fray through. The cyst has a fancy name - the semi-membranosis bursa - but most people find "popliteal cyst" less of a mouthful!
WHY DOES IT APPEAR?
We don't know. Possibly the normal cyst enlarges because the child can over-straighten the knee which increases the load on the tendons. The extra fluid may be needed to improve lubrication of the tendons and prevent them fraying. This does not explain why only one knee is usually affected. Nor does it explain why not all children who can over-straighten their knees get a popliteal cyst. There is still a lot we don't know in medicine.
WHAT SHOULD BE DONE ABOUT IT?
Nothing! The cyst is totally benign. In most children it causes no symptoms. Occasionally after unusual exercise the cyst will get bigger and be uncomfortable. However, a day's rest will settle both the pain and enlargement. The cyst will eventually disappear on its own, though it can take 2 years.
WHAT ABOUT SURGERY?
Surgery to remove the cyst is possible. Unfortunately the cyst comes back in half the children, probably for the same reason it came initially. It seems unfair for your child to suffer the pain and risks of surgery only to end up with the same lump and also a scar. Especially if the lump is totally benign and will disappear in time on its own. Very occasionally the cyst becomes so big and uncomfortable that an operation is necessary. When this happens we just have to accept the risk of the cyst coming back.