Also called a popliteal cyst. Its proposed cause, in some individuals, involves a communication between the semimembranosus/gastrocnemius bursa and the knee joint. Some have postulated a one-way valve effect in which synovial fluid moves from the knee to the bursa. Baker’s cysts can occur secondary to any process that produces synovial fluid (most commonly RA, osteoarthritis, or trauma). A ruptured cyst can occasionally dissect down the calf and be confused with deep venous thrombosis. It is diagnosed with ultrasound or arthrography.
Diseases that can lead to a knee joint effusion with Baker's cyst are the following:
- Diseases of the cartilage (e.g. chondromalacia, arthrosis) and the menisci (e.g. meniscus degeneration)
- Diseases and lesions of the capsule-ligament apparatus (e.g. periarthropathia genu, enthesitis, ruptures)
- Diseases of the synovial tissue (e.g. arthritis in the context of rheumatoid arthritis, peripheral spondyloarthritis or pigmented villo-nodular synovitis)
- Diseases of the bone (e.g. tumours near the joints, fractures, osteomyelitis)
- Crystalline arthropathies (e.g. urate or calcium pyrophosphate arthritis)
- Infections (e.g. septic arthritis)