Baker's cysts (popliteal cysts) are tiny sacs filled with joint (synovial) fluid that form in an extension of the joint capsule behind the knee.
A Baker's cyst results from an accumulation of trapped joint fluid, which bulges from the joint capsule behind the knee as a protruding sac. Causes of the joint fluid accumulation include rheumatoid arthritis, osteoarthritis, and overuse of the knees. Baker's cysts produce discomfort at the back of the knee. The cysts may enlarge and extend downward into the calf muscles.
A rapid increase in the amount and pressure of fluid within the cyst can cause it to rupture. The fluid released from the cyst can cause the surrounding tissues to become inflamed, resulting in symptoms that may mimic those of thrombophlebitis (see Venous Disorders: Superficial Venous Thrombosis). Moreover, a bulging or ruptured Baker's cyst can cause thrombophlebitis in the popliteal vein (which is located behind the knee) by pressing on the vein.
The doctor can usually make a diagnosis by asking the person specific questions about symptoms and feeling a swelling behind the knee or in the calf. Ultrasound, magnetic resonance imaging (MRI), or arthrography, can sometimes aid in the diagnosis and document how far the cyst extends.
When arthritis causes chronic knee swelling, the doctor may need to remove the fluid with a needle (a procedure called joint aspiration) and inject a long-acting corticosteroid (such as triamcinolone acetonide) to prevent the formation of a Baker's cyst. Removing the cyst surgically is an alternative if other treatments are not effective.
If the cyst has ruptured, the pain is treated with a nonsteroidal anti-inflammatory drug (NSAID). If the ruptured cyst causes thrombophlebitis in the popliteal vein, this is treated with bed rest, elevation of the leg, warm compresses, and anticoagulants. Occasionally, antibiotics are needed also.
Last full review/revision April 2008 by Joseph J. Biundo, MD