Baker cysts are small 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, other inflammatory joint diseases and overuse of the knees. Baker cysts often do not cause symptoms but may become noticeable when they become swollen and 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 (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.
Diagnosis of Baker Cysts
A doctor's evaluation
Sometimes imaging tests
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.
If necessary, ultrasonography, magnetic resonance imaging (MRI), or arthrography can aid in the diagnosis, distinguish the cyst from a blood clot in the deep veins (DVT), and document how far the cyst extends.
Treatment of Baker Cysts
Nonsteroidal anti-inflammatory drugs (NSAIDs)
Joint or cyst aspiration and corticosteroid injection
Rarely surgical removal of the cyst
NSAIDs (or another pain reliever if NSAIDs cannot be taken) are the initial treatment of choice for pain relief.
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. The doctor may also aspirate and inject the cyst. Removing the cyst surgically is an alternative if other treatments are not effective.
