Causes of Baker Cyst
Baker cysts form when synovial fluid accumulates in the gastrocnemius-semimembranosus bursa in the popliteal fossa, typically due to increased intra-articular pressure from underlying knee joint pathology that forces fluid through a communication between the joint space and the bursa. 1, 2
Primary Mechanism
- The fundamental cause is elevated pressure within the knee joint that drives synovial fluid into the bursa, creating a fluid-filled sac behind the knee 3
- The bursa beneath the medial head of the gastrocnemius muscle distends when it fills with synovial fluid that communicates with the knee joint 4
- A valve-like mechanism may allow fluid to enter the bursa but prevents its return to the joint space, trapping fluid and forming the cyst 5
Underlying Knee Joint Disorders in Adults
In adult patients, Baker cysts are almost always secondary to intra-articular knee pathology 4:
- Osteoarthritis is a common underlying cause, with recurrent joint effusions leading to cyst formation 1
- Meniscal tears allow synovial fluid extravasation or translocation of synovial cells into surrounding tissues 6
- Inflammatory arthritis (such as rheumatoid arthritis) causes chronic synovitis and increased fluid production 7
- Ligamentous injuries and other internal derangements that produce recurrent effusions 6
- Post-surgical causes: Baker cysts can develop after knee arthroplasty (both total and partial), with a prevalence of 0.6% following primary knee replacement 8
Pediatric Considerations
- In children, Baker cysts can be isolated without underlying knee joint pathology, representing a distinct entity from adult cysts 4
- The knee joint may be entirely normal in pediatric cases 4
Clinical Pitfall
The critical error is treating the cyst in isolation without addressing the underlying knee joint disorder 5. Since the cyst communicates with the joint space, failure to treat the source of recurrent effusions will lead to cyst recurrence 7. Treatment must primarily address the articular lesions causing the effusions rather than focusing solely on the cyst itself 5.