I have very bad arthritis in my left knee, and lately my calf and the area behind my knee have been hurting as well. I also had some swelling and redness. I used lots of ice and the swelling went down, but the pain is still there. Could this be something serious, like a blood clot?

— Stella, New York

The symptoms you are describing fit very well a complication of knee arthritis known as Baker's cyst or popliteal cyst.

There are six bursae in the back of the knee. A bursa is a sack with a small amount of fluid in it that acts as a cushion. Most bursae are between bones and skin or between muscles. Approximately two-thirds of people have a narrow connection between the back of the knee and a bursa with a very long name — the gastrocnemio-semitendinosus bursa. The narrow connection can act as a one-way valve from the knee to the bursa.

Normally, there is a small amount of fluid (synovial fluid) in our joints that facilitates the smooth motion of the joint. When there is arthritis in the knee, or trauma, or rupture of structures within the knee, very often there is fluid accumulation, rarely as much as five ounces. When we bend the knee, fluid goes into the bursa, but when we stretch it, the fluid remains in the bursa because of the one-way valve mechanism. Therefore, every time we take a step, fluid goes into the bursa, and because the fluid is accumulating gradually, the bursa gets distended, becomes larger, and is designated as a cyst. At this point it can often be felt as fullness in the back of the knee and may be painful.

Two things can happen as the cyst enlarges: It may slide lower in the calf between the two parts of the main calf muscle (gastrocnemius) — this is called dissection — or it may rupture, spilling synovial fluid in the surrounding tissues. Rupture typically happens after a deep knee bend but may occur without it. The fluid causes inflammation of the tissues, resulting in pain, swelling of the calf, heat, and redness. This set of symptoms can imitate those of a blood clot in a vein (thrombophlebitis).

Any presence of fever, excessive redness, and severe pain should raise the possibility of infection and should be treated as an emergency. Joint fluid should be removed for laboratory tests (bacterial stains, culture, and sensitivity to antibiotics) to help identify the responsible bacteria. Antibiotic treatment should start immediately and can be modified later, if needed.

In addition to an expert examination by a rheumatologist or an orthopedic surgeon, the definitive diagnostic test is an ultrasound of the back of the knee (popliteal fossa) that will demonstrate whether the swelling is a popliteal cyst or a thrombophlebitis.

Treatment of a popliteal cyst before it ruptures includes removal of joint fluid and injection of a steroid (cortisone-like medication formulated for prolonged action) into the knee or the cyst, as long as there is no infection. If the cyst is of great size and causes persistent discomfort, it may be surgically removed by an orthopedic surgeon. At the same time, whatever caused the popliteal cyst in the first place should be treated, either with medication or surgery or both, as indicated.

The treatment of thrombophlebitis is totally different and includes anticoagulation (blood thinners), rest, treatment of varicosities (enlarged veins), and prescription elastic stockings.

Learn more in the Everyday Health Knee Pain Center.