The area at the back of the knee is referred to by the anatomical term “popliteal fossa,” due to the presence of the flat triangular muscle at the back of the knee called the popliteus. "Fossa" is an anatomical term used to describe a concave depression, such as at the back of the knee, or the inside of the elbow. Swelling or throbbing in the popliteal fossa can result from a number of different conditions.
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A Baker’s cyst is the most common cause of swelling in the back of the knee. Baker’s cysts produce a bulge in the popliteal fossa, which can make the area feel stretched and uncomfortable when bending the knee. The cyst is filled with fluid and is usually a complication of some other condition, such as arthritis, torn knee cartilage, or even Lyme disease, according to the Infectious Diseases Society of America. Baker’s cyst symptoms include swelling behind the knee, knee and calf pain, and clicking or locking of the knee joint.
Baker’s cysts can resolve on their own once the underlying condition has been treated, according to Britain’s National Health Service, but in some cases surgical intervention may be necessary in order to drain the fluid.
A network of nerves and blood vessels crosses the back of the knee joint. Dilation of a blood vessel in this area, such as the popliteal artery, can cause a bulge in the popliteal fossa, according to eMedicine. A weakness in the arterial walls that widens or balloons outward is referred to as an aneurysm.
Those with cardiovascular conditions, such as high blood pressure or elevated cholesterol levels, are more prone to aneurysms. Aneurysms near the body surface, as in the case of the popliteal fossa, manifest in characteristic symptoms, such as throbbing, swollen, distended areas and pain.
Aneurysm rupture can be fatal. Suspicion of an aneurysm should immediately be reported to a doctor.
Tumors in the popliteal fossa are rare, but according to eMedicine, when they do occur, they have the potential to expand rapidly due to the lack of strong connective tissue boundaries in the compartment.
These tumors can be treated with chemotherapy, radiation treatments or surgery, but the Washington Cancer Institute states that surgical removal of popliteal sarcomas, meaning soft tissue tumors, is difficult due to the numerous nerves and blood vessels located in the area.