Baker 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 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 cysts cause discomfort at the back of the knee. The cysts may enlarge to the size of a baseball 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 a blood clot in the calf (thrombophlebitis―see Deep Vein Thrombosis (DVT)). Moreover, a bulging or ruptured Baker cyst can rarely actually 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. Ultrasonography, 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 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, treatment is bed rest, elevation of the leg, warm compresses, and anticoagulants (such as warfarin). Occasionally, antibiotics are needed also.