What is a Baker's cyst, a fluid-filled swelling behind the knee typically resulting from knee joint disorders such as osteoarthritis, rheumatoid arthritis, or meniscal tear, commonly affecting adults between 35 and 70 years old?

What is a Baker's Cyst?

A Baker's cyst is a fluid-filled swelling located in the back of the knee, specifically representing a distension of the gastrocnemio-semimembranosus bursa that communicates with the knee joint space through an opening in the posterior joint capsule. 1, 2

Anatomical Location and Structure

• The cyst is positioned in the posteromedial region of the knee, specifically between the medial head of the gastrocnemius muscle and the semimembranosus tendon 1, 3, 4
• This bursa uniquely communicates with the knee joint via an opening in the joint capsule posterior to the medial femoral condyle, creating a valve-like mechanism that allows fluid to enter but restricts its return 4
• The characteristic comma-shaped appearance can be visualized sonographically in the posterior transverse scan between these anatomical structures 1, 2
• The cyst can extend far into the thigh and calf muscles, well beyond its origin 1, 2

Underlying Causes and Associated Conditions

In adults, Baker's cysts are almost always secondary to intra-articular knee pathology rather than primary conditions 3, 4, 5:

• Osteoarthritis is a common underlying cause 2, 4
• Meniscal tears frequently contribute to cyst formation 3, 4, 5
• Rheumatoid arthritis and other inflammatory arthropathies are associated conditions 4
• Cartilage lesions and chronic knee effusions create the fluid accumulation 2, 5

In children, the presentation differs significantly—popliteal cysts are usually incidental findings on physical examination without associated intra-articular pathology and generally have no clinical relevance 3, 4

Clinical Presentation

The American College of Cardiology notes that Baker's cysts present with swelling and tenderness behind the knee that may worsen with exercise but, critically, can also be present at rest—distinguishing them from vascular claudication which resolves quickly with rest 1, 2:

• Palpable, fluctuant mass in the popliteal fossa, especially visible with knee extension 2
• Feeling of tightness or fullness behind the knee 2
• Pain that may worsen with knee movement or exercise 2
• Unlike intermittent claudication, symptoms do not quickly resolve with rest and position changes typically provide no significant relief 1, 2
• Possible limitation in knee range of motion 2

Critical Complications

Cyst rupture is the most important complication, as it clinically mimics deep vein thrombosis (DVT) with sudden calf pain, swelling, and inflammation—making differentiation essential 1, 2:

• Rupture causes dissemination of fluid into the calf musculature 2, 6
• The American College of Radiology identifies ruptured Baker's cysts as a "pathologic mimic for DVT" that limits the reliability of clinical diagnosis alone 2
• Other rare complications include leg ischemia, nerve entrapment, and compartment syndrome 7
• Hematoma formation within the cyst can occur, though rarely 7

Diagnostic Approach

Ultrasound is the preferred initial diagnostic tool to confirm the presence of a Baker's cyst, with comparable accuracy to MRI for this specific purpose 2, 6:

• Ultrasound can accurately diagnose the cyst, detect rupture, and determine vascularity 2
• The comma-shaped extension between the medial gastrocnemius head and semimembranosus tendon must be visualized to confirm diagnosis 1, 6
• Plain radiographs of the knee should be performed first to evaluate for underlying joint pathology including osteoarthritis, erosions, and other causes 2
• MRI without IV contrast is recommended when additional evaluation is needed after ultrasound or when concomitant internal knee pathology is suspected 1, 2
• MRI accurately depicts the extent of effusion, presence of synovitis, and presence or rupture of the cyst 1, 2

Important Clinical Pitfall

The American College of Cardiology warns that clinicians should not assume all popliteal masses are benign Baker's cysts—imaging must exclude popliteal artery aneurysm, especially in patients with a history of other arterial aneurysms 2. Additional red flags suggesting malignancy include: symptoms disproportionate to cyst size, absence of joint damage explaining the cyst, unusual topography, bone erosion, size greater than 5 cm, and tissue invasion 3.