A Baker's cyst, or popliteal cyst, is a benign cyctic tumor which forms on the back of the knee joint, in humans only, as far as I can tell. Technically it is not really a cyst - a true cyst is lined with cells which keep forming fluid, while a Baker's cyst is just a contained pocket of fluid. It forms when something tears inside the knee joint, allowing the synovial fluid to leak into places it's not supposed to go. This may result from injury or disease - usually gout or arthritis.

If the cyst is caused by disease, it usually disappears after successful treatment of the disease. If it is caused by injury, it will recur unless the damage to the knee is repaired. Some surgeons fail to recognize this, and simply remove the fluid.

These cysts generally don't hurt, though if they get large they may be uncomfortable - and of course the condition which caused the cyst may hurt like hell.

Treatment is a simple outpatient procedure, involving either removal of the cyst or an injection of a corticosteroid - unless the cyst occurs in a child. Since most childhood cysts disappear on their own within a few years, most doctors choose to observe the cyst carefully over time, rather than fool with nature.

I don't know who Baker is.


thanks to:
http://healthlink.mcw.edu/article/901226567.html
http://www.healthgate.com/choice/uic/cons/mdx-books/sym/surg18.shtml
http://bcn.net/~stoll/archives/bakers.html