Baker cysts are enlarged bursae in the popliteal fossa. They are filled with synovial fluid and usually communicate with the adjacent joint space. Symptoms include pain, swelling behind the knee, and knee stiffness and decreased range of motion. Diagnosis is usually clinical; however, ultrasonography or magnetic resonance imaging may be needed if clinical findings are inconclusive. If symptomatic, treatment includes nonsteroidal anti-inflammatory drugs (NSAIDs) and, sometimes, drainage, corticosteroid injection, or surgical removal of the cyst.
Baker cysts are fluid-filled popliteal bursae that develop from an accumulation of synovial fluid from the knee. Most Baker cysts are small and 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
Osteoarthritis
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 distal leg swelling, erythema, warmth, and/or Homan sign.
Pearls & Pitfalls
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Diagnosis of Baker Cyst
Clinical evaluation
Sometimes, ultrasonography
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 painful; require differentiation from deep vein thromboses or popliteal fat deposition), 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.
If the onset is acute or subacute, with suspected inflammation, aspiration of the joint or bursa should be performed to exclude infection or crystal-associated arthritis (as is appropriate in any acute monoarticular arthritis).
Treatment of Baker Cyst
Nonsteroidal anti-inflammatory drugs (NSAIDs)
Joint aspiration and corticosteroid injection
Rarely 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.
Key Points
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.
