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 often 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. (See also Introduction to Muscle, Bursa, and Tendon Disorders.)
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 (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 the diagnosis of a Baker cyst by asking the person specific questions about symptoms and feeling a swelling behind the knee or in the calf.
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 reduce the size of the cyst or 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) or another pain reliever if NSAIDs cannot be taken. 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).