Baker cysts are enlarged bursae that develop from an accumulation of synovial fluid in the popliteal fossa. Most Baker cysts are small and are do not cause symptoms. When they become larger (> 5 cm), they can be noticed by the patient as a swelling behind the knee.
Etiology of Baker Cysts
Most Baker cysts accumulate fluid from the adjacent knee joint space. Increased synovial fluid production is caused by underlying joint disease. Synovial fluid flows from the joint toward the cyst with extension of the knee. Baker cysts can develop without knee joint communication (eg, from the gastrocnemius-semimembranous bursa) in children.
Baker cysts are commonly caused by
Prior knee injury
Rheumatoid arthritis and other inflammatory arthropathies
Overuse of the knee
Signs and Symptoms of Baker Cysts
Baker cysts may be asymptomatic but become noticeable when they become swollen (eg, ≥ 5 cm). Compression of adjacent tissue may cause pain usually with extension of the knee. Patients complain of worsening pain, increased knee stiffness, and decreased range of motion as the cyst becomes larger. Cysts can rupture, simulating deep vein thrombosis, with swelling, redness, warmth, and/or Homan sign.
Pearls & Pitfalls
Diagnosis of Baker Cyst
Sometimes, magnetic resonance imaging
Baker cysts are in the popliteal fossa. The cysts are more prominent and firm when the patient is standing and the knee is fully extended.
If clinical findings are inconclusive (eg, if cysts are small or to differentiate them from deep vein thromboses), ultrasonography can be done. Magnetic resonance imaging is done occasionally, eg, if ultrasonography is inconclusive or to diagnose and characterize internal knee derangements that may require surgery.
Treatment of Baker Cyst
Nonsteroidal anti-inflammatory drugs (NSAIDs)
Joint aspiration and corticosteroid injection
Sometimes, surgical removal of the cyst
Asymptomatic cysts do not require treatment. Nonsteroidal anti-inflammatory drugs (NSAIDs) are the primary treatment for symptomatic Baker cysts.
Joint aspiration can be done to remove fluid and relieve pain and swelling. Arthrocentesis and corticosteroid injection are sometimes used to treat inflammation. Sometimes the cyst is aspirated under ultrasonographic guidance. Removing the cyst surgically is an alternative if other treatments are not effective.
The usual causes of Baker cysts are prior injury, rheumatoid arthritis, osteoarthritis, or overuse of the knee.
If clinical findings are inconclusive, ultrasonography or, less often, magnetic resonance imaging is done.
If symptomatic, treat most cases with NSAIDs, and sometimes arthrocentesis and corticosteroid injection.