What would cause a cyst to form under my knee cap?is there any help for nerve pain one year after total knee replacement?

Arthritis: A cyst under the patella (kneecap) will often develop as a result of articular cartilage degeneration which can occur with arthritis or other conditions that lead to damage of the cartilage. These can be found in any joint. The cyst is a fluid filled cavity that will often connect with the joint at the area of cartilage damage. If the cyst has occurred after tka it is likely something else.