These are fluid filled swellings arising from the knee and present in the back of the knee. They are associated with osteoarthritis, gout or meniscal injuries. The condition can be painful. The union of three bones forms the knee joint. A bag surrounds the joint. This bag is lined on the inside by a tissue layer called the synovium. This tissue, secretes a fluid. In degenerative conditions, this fluid may be secreted in excess and this fluid tracks to the back of the knee.

The patient complains of pain in the back of the knee. There may be fullness in the back of the knee. This bag of fluid may burst and cause pain in the knee, resembling a deep vein thrombosis

Those patients with osteo arthritis are at risk.

The fluid in the sac can be aspirated with a needle. Another method is arthroscopic surgery where the joint is visualised from the inside and the fluid aspirated.