Baker cysts, or popliteal cysts, are fluid-filled distended synovial-lined lesions arising in the popliteal fossa between the medial head of the gastrocnemius and the semimembranosus tendons via a communication with the knee joint. They are usually located at or below the joint line.
They represent neither a true bursa nor a true cyst, as they occur as a communication between the posterior joint capsule and the gastrocnemius-semimembranosus bursa.
Two peaks are described: at 4-7 years and 35-70 years .
Baker cysts are most often found incidentally when the knee is imaged for other reasons.
Symptomatic presentation may be acute when rupture occurs, in which case the chief differential diagnosis is deep venous thrombosis. A chronic/subacute presentation can manifest as a popliteal fossa mass or with pain.
Two pathological processes are described :
- a valve-like connection between the knee joint and the gastrocnemius-semimembranosus bursa, resulting in fluid being squeezed in one direction
- no connection, with primary gastrocnemius-semimembranosus bursitis
- degenerative arthropathy
- rheumatoid arthritis
- Charcot joint involving the knee
- post-traumatic in athletes
Normally the first line investigation:
- well-defined cyst with a 'neck' at its deepest extent, extending into the joint space between the semimembranosus tendon and the medial head of the gastrocnemius
- identification of a fluid-filled structure at the posteromedial knee is suggestive of a popliteal cyst, but identification of the 'neck' between the tendons is necessary for a definitive diagnosis
- usually anechoic, but may contain internal debris
Exquisitely outlines the cyst as a mass extending from the joint space with high T2 signal content.
Treatment and prognosis
In children, they can be common, with most spontaneously resolving within 10-20 months. Aspiration may be performed, with steroid injection shown to be beneficial in reducing Baker cyst size and improving symptoms . If the symptoms persist and/or the cyst is very large, a surgical excision is an option.
Recognized complications include:
- dissection: the cyst usually dissects inferomedially but can dissect proximally, anteriorly, intermuscularly or intramuscularly
- rupture: leaking of cyst fluid into the popliteal fossa, between fascial planes and surrounding the hamstrings and medial gastrocnemius muscles; moreover, there is edema of the soft tissue and irregularity of the cyst wall
- compression: of the popliteal vessels and tibial nerve
- compartment syndrome: can be either anterior or posterior
On ultrasound consider:
- large parameniscal cyst
- liquified hematoma in the popliteal fossa
History and etymology
It was first described by Adams, and its intra-articular origin was described by William Morrant Baker .