A Baker Cyst, also known as a popliteal or parameniscal cyst, is a fluid-filled sac that forms in the posterior aspect of the knee, typically located between the semimembranosus and medial head of the gastrocnemius.

In adults, Baker's Cysts tend to form in association with degenerative conditions of the knee. One of the most common associations seen is secondary to degenerative meniscal tears. Inflammatory conditions and arthritides are also known causes of popliteal cyst formation. When the cyst forms in association with a meniscal tear, the meniscus serves as a one-way valve and the extruded synovial fluid localizes and consolidates to form a viscous, gel-like material.

In children, popliteal cysts more commonly arise as a primary condition. Unlike adults, popliteal cysts in children arise from a herniated posterior knee joint synovium/capsule.

Baker's cysts are encountered most frequently in adults with a history of trauma (e.g., cartilage or meniscus tears, injury to the knee), in association with degenerative/co-existing knee joint disease (e.g., osteoarthritis, rheumatoid arthritis, infectious arthritis, pigmented villonodular synovitis, meniscal tears), or asymptomatically as an incidental finding.

Baker's cysts form and can expand as a result of the accumulation and extrusion of synovial fluid that localizes between the semimembranosus and medial head of the gastrocnemius. In some instances, normal anatomic variants entail physiologic capsular openings or defects in the posterior knee joint capsule that is essentially contiguous with the gastrocnemius-semimembranosus bursa. Popliteal cysts are located on the medial side of the popliteal fossa just below the crease found at the posterior knee.

Epidemiology

Baker's cysts tend to occur most commonly in adults aged 35 to 70 and most often are associated with inflammatory joint diseases such as rheumatoid arthritis, osteoarthritis, or as a result of overuse or injury to the knee. As they are asymptomatic, Baker's cysts are most often detected incidentally on physical examination or imaging studies (e.g., MRI in an adult with suspected osteoarthritis or internal abnormalities of the knee). Prevalence typically increases with age, most likely due to the increase in knee-bursal communication with age.

In children, Baker's Cysts tend to occur most frequently in those aged 4 to 7 years.

A Baker's Cyst typically is diagnosed by having the patient stand and extend the knee full; this is when the mass should be most prominent. When the knee is flexed to 45 degrees, the mass often either softens or fully disappears (e.g., Foucher's sign) due to the relief of tension within the cyst. The patient also should be examined in a supine position, where the knee is passively moved from full extension to at least 90 degrees flexion, to ensure adequate examination of the knee joint.

If a Baker's Cyst is located more laterally or no changes are elicited in the cyst when the knee is examined in a full range of motion, it may be difficult to conclude a diagnosis of Baker's Cyst through physical examination alone, especially in a setting of no prior history of knee pathology.

If this is the case, and a diagnosis of Baker's Cyst remains uncertain, imaging should be performed. This includes plain radiography and ultrasonography, followed by MRI, especially if considering surgery.

Treatment is not usually necessary for a Baker's Cyst unless the patient is symptomatic. Incidental findings in the asymptomatic patient are managed with observation and reassurance alone.

Non-operative management:

• Rest/activity modification

• NSAIDs

• PT/rehab regimens are often effective in patients with minimal symptoms, and in the setting of smaller degenerative meniscal tears

• Aspiration and steroid injection

  • The knee joint itself can be treated with a cortisone injection in the setting of moderate to advanced tricompartmental degenerative knee arthritic conditions. This can help manage patients' symptoms and clinically differentiate the contribution of the popliteal cyst itself on the patients daily symptoms

  • Ultrasound-guided aspiration and injection can be performed by an experienced interventional radiologist

    • Recurrence rates are much lower in younger patient populations

    • Recurrence rates are higher in older patient populations and degenerative meniscal tears with associated cysts

Operative management

• Arthroscopic debridement, cyst decompression, meniscal repair vs partial meniscectomy

  • less invasive than the open approach

  • often results in cyst recurrence, especially in older patients with moderate-advanced knee degenerative conditions

• Open cyst excision

  • Posterior approach to the knee is utilized

  • Not indicated in the setting of underlying knee degenerative conditions secondary to the risk of cyst recurrence

It is important to treat the underlying joint disorder, if one exists, in all patients with symptomatic Baker's cysts. This will help reduce the accumulation of synovial fluid and enlargement of the cyst.

Other Issues

It is important to rule out other possible disorders that may mimic many of the key clinical features of a Baker's cyst. These include deep vein thrombosis (DVT), cystic masses (e.g., synovial or ganglion cysts), solid masses, (e.g. sarcomas and lymphoma), and popliteal artery aneurysms. Given the potential complication of pseudothrombophlebitis (e.g., swelling, warmth, tenderness, and a positive Homan's sign) caused by a Baker's cyst when markedly enlarged, dissected, or ruptured), it is especially important to distinguish this finding from a true DVT that may present with similar symptoms.