What Causes a Baker's Cyst in the Knee
Baker's cysts form when synovial fluid accumulates in the gastrocnemius-semimembranosus bursa due to underlying knee joint pathology that causes recurrent joint effusions, with the fluid becoming trapped through a one-way valve-like mechanism at the communication point between the bursa and knee joint. 1, 2
Primary Mechanism of Formation
The bursa communicates with the knee joint space through an opening in the joint capsule posterior to the medial femoral condyle, creating a valve-like mechanism that allows fluid to enter but not easily exit. 2
Repeated knee joint effusions from underlying pathology cause synovial fluid to flow into and distend the gastrocnemius-semimembranosus bursa. 3
Once fluid enters the bursa, the alleged unidirectional valve-like mechanism traps it inside, leading to progressive cyst enlargement. 3, 2
Underlying Causes in Adults
In adults, Baker's cysts rarely occur in isolation and are almost always associated with intra-articular knee pathology: 2, 4
Osteoarthritis is the most common underlying cause, generating chronic joint effusions that feed the cyst. 5, 2
Meniscal tears are frequently associated with Baker's cyst formation. 2, 4
Rheumatoid arthritis and other inflammatory conditions cause synovitis and effusions that contribute to cyst development. 2
Knee arthroplasty can lead to Baker's cyst formation in 0.6% of cases, typically occurring between 6 weeks and 2 years post-surgery. 6
Underlying Causes in Children
In children, popliteal cysts are typically idiopathic and only occasionally associated with intra-articular pathology, representing a fundamentally different etiology than in adults. 2
Pediatric Baker's cysts are more often incidental findings during routine physical examination rather than secondary to joint disease. 2
Clinical Pitfall to Avoid
Do not assume all popliteal masses are benign Baker's cysts—always obtain imaging to exclude popliteal artery aneurysm, especially in patients with a history of other arterial aneurysms. 1
Plain radiographs should be the first imaging study to evaluate for underlying joint pathology causing the cyst, including osteoarthritis, loose bodies, or other structural abnormalities. 1