A Baker's cyst (popliteal cyst), a synovial cyst behind the knee, is quite a common finding in both children and adults.
It may cause pain and a feeling of pressure behind the knee but most cysts are small, asymptomatic incidental findings. In children, Baker's cysts are often asymptomatic.
Asymptomatic cysts need not be treated.
In adults, there is usually an underlying joint disease (such as meniscal injury, osteoarthritis or rheumatoid arthritis), and the cyst usually communicates with the knee joint. In such cases, the symptoms are associated with the underlying disease.
A symptomatic cyst in an adult may be treated by aspiration of the knee joint. If there is an underlying inflammatory joint disease, an intra-articular glucocorticoid injection can be given after draining the joint.
Symptoms
The cyst develops when the knee joint cavity communicates with the bursa situated between the gastrocnemius muscle and the semimembranosus tendon, allowing synovial fluid to enter the bursa.
The cyst will be the larger the more fluid there is in the joint.
In some people, there is a one-way valve mechanism in the communicating channel, preventing synovial fluid from flowing back from the bursa into the joint cavity.
If no such communicating channel can be found, there is just gastrocnemius-semimembranosus bursa.
Inside the cyst, there may be separate cystic structures of various sizes. If the cyst is loculated, aspiration of synovial fluid is often more difficult.
The fluid in the cyst is often highly viscous (in contrast to rheumatic inflammation, where the fluid is often watery, see Investigation of Synovial Fluid).
The cyst is often situated medially behind the knee but, in some cases, it may be situated laterally.
In adults, Baker's cysts are often associated with underlying diseases of the knee joint, such as rheumatoid arthritis or osteoarthritis; as many as 40% of patients with osteoarthritis of the knee are diagnosed with a Baker's cyst. Another possible underlying disease is a torn meniscus.
In children, the cysts are often asymptomatic incidental findings.
Symptoms and findings
Most cysts are small, symptomless swellings behind the knee.
The most common symptoms are popliteal swelling (in 2/3), tenderness (in 1/3) and pain associated with full extension of the knee. A large cyst may cause local tightness and restricted movement of the knee.
Popliteal swelling may increase or in some cases totally disappear when bending the knee. If the cyst empties when bending the knee, there is no valve in the communicating channel.
Physical activity may worsen the symptoms. At first, there is pain only on exertion but, as the condition becomes worse, there will also be pain at rest.
The symptoms often restrict the movement of the knee joint and lead to weakening of the thigh muscle.
If there is an underlying disease, such as rheumatoid arthritis or osteoarthritis, the symptoms are associated with that disease.
A Baker's cyst may rupture on exertion of the knee, causing a sudden severe pain behind the knee and in the calf, and leading to extravasation of the fluid along fascial compartments down to the ankle.
Diagnosis
Clinical diagnosis is usually sufficient.
In clinical examination, the bend of the knee should be inspected and palpated with the patient standing with the knee in full extension and in 90° flexion.
A roundish, mobile mass will be found that feels fluid-filled.
When the knee is flexed about 45°, the mass may be reduced or disappear (Foucher's sign).
If the finding remains unchanged when moving the knee joint, it is probably not a Baker's cyst.
MRI is also good for identifying cysts. Imaging with a contrast medium or plain X-raying is usually not needed to detect cysts; however, a plain X-ray is useful for detecting bony changes.
Cysts may also be incidental findings obtained when imaging the knee joint for some other reason. In children, particularly, cysts are often incidental findings obtained in association with another examination.
The pain from a ruptured cyst increases when bending the knee; symptoms of deep vein thrombosis do not usually increase when moving the knee joint but on extreme ankle flexion.
Ultrasonography is good for diagnosing both conditions.
A firm mass behind the knee may be a malignant tumour, such as a sarcoma.
Adipose tissue may sometimes collect behind the knee. In this case the mass feels soft and flexible, and its size does not fluctuate when moving the knee.
An aneurysm of the popliteal artery pulsates and is therefore usually easy to differentiate from a Baker's cyst.
A meniscal cyst, seroma or haematoma may appear as a solid mass behind the knee, resembling a Baker's cyst.
Treatment
Most Baker's cysts require no treatment.
An incidentally found symptomless cyst requires no treatment.
In children, about half of the cysts disappear spontaneously.
A symptomatic cyst in an adult may be treated by aspiration (drainage) of the knee joint and an intra-articular glucocorticoid injection (e.g. 40 mg triamcinolone). After the injection, the knee should be flexed/extended a few times to ensure that the glucocorticoid solution flows through the channel all the way to the Baker's cyst.
The injection may reduce accumulation of synovial fluid at least temporarily, possibly keeping the cyst asymptomatic.
Draining the joint and injecting a glucocorticoid into both the joint and the cyst may be more effective. In such cases the injection site in the cyst should be confirmed by ultrasound.
In patients with rheumatoid arthritis, for instance, if there is knee synovitis not responding to essential drugs and local glucocorticoid injections, the knee should be debrided. The Baker's cyst can be removed at the same time.
The patient should be referred for surgical assessment if:
the Baker's cyst causes severe mechanical restriction of knee movement, or
intensified treatment of the underlying inflammatory disease has not affected fluid accumulation in the cyst, or
the knee joint must be repeatedly punctured, or
biopsy is necessary to confirm the diagnosis, considering other diseases.
The joint can be loaded as permitted by pain.
If extension of the knee is restricted considerably, by more than 20°, and the thigh muscle has become clearly thinner (> 2 cm), strenuous physical exertion should be avoided.
Cold packs, other traditional analgesic methods and exercises guided by a physiotherapist can be used.
If the knee is stable, a knee orthosis is not likely to be useful.
Rupture of Baker's cyst
Rupture is a serious complication of a Baker's cyst that often develops in association with or soon after exerting the knee.
It causes acute, sometimes severe, pain in the popliteal area and the calf.
Fluid from the cyst and the joint will run down within the fascial compartments all the way to the ankle.
The finding resembles that of a deep vein thrombosis or muscle rupture. It can easily be confirmed by ultrasonography.
Fluid running into the fascial compartments may press on veins, causing venous obstruction Deep Vein Thrombosis. In the diagnosis, the possibility of two concomitant diseases should be considered.
The leg should be kept elevated, a compression stocking or a tight bandage covering the whole leg applied, together with cold application and NSAIDs given as symptomatic treatment.
If the knee is swollen, it should be drained to alleviate pressure on the cyst, and a glucocorticoid should be injected into the joint (however, no sooner than 1 month after the last injection and no more than 4 times/year).
Aspiration of the knee will usually alleviate the knee symptoms and prevent further fluid from draining into the calf although the fluid that has run into the intermuscular compartments of the calf cannot be removed by aspiration.
If there is severe knee joint inflammation not responding to treatment and synovial fluid continues to drain from the ruptured cyst into the calf, the patient should be referred for surgical treatment.
References
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Canoso JJ, Goldsmith MR, Gerzof SG et al. Foucher's sign of the Baker's cyst. Ann Rheum Dis 1987;46(3):228-32. [PubMed]
Rauschning W. Anatomy and function of the communication between knee joint and popliteal bursae. Ann Rheum Dis 1980;39(4):354-8. [PubMed]
Jayson MI, Dixon AS. Valvular mechanisms in juxta-articular cysts. Ann Rheum Dis 1970;29(4):415-20. [PubMed]
Rauschning W, Lindgren PG. Popliteal cysts (Baker's cysts) in adults. I. Clinical and roentgenological results of operative excision. Acta Orthop Scand 1979;50(5):583-91. [PubMed]