Baker cyst
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"Baker cyst" OR "popliteal cyst"
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Baker Cyst (Popliteal Cyst)
Definition and Anatomy
A Baker cyst is a fluid-filled swelling in the popliteal fossa formed by distension of the gastrocnemius-semimembranosus bursa. Specifically, it arises from the fusion of the semimembranosus bursa with the subtendinous bursa deep to the medial head of the gastrocnemius, lying in the medial popliteal fossa at the level of the medial posterior femoral condyle.
The cyst was described by Adams in 1840 but named after Baker, who described it in 1877. It can form by one of two mechanisms:
- Herniation of the synovial membrane through the posterior joint capsule
- Escape of fluid via normal communication between the gastrocnemius-semimembranosus bursa and the knee joint (a ball-valve mechanism through a capsular fold)
Anatomical illustration showing the Baker cyst communicating with the knee joint via the posterior capsule (Tintinalli's Emergency Medicine)
Pathophysiology
Increased intra-articular pressure from any cause drives synovial fluid through the valvular capsular communication into the bursa, where it accumulates. Common underlying intra-articular pathologies include:
- Osteoarthritis (most common in adults)
- Degenerative tear of the posterior horn of the medial meniscus (very common association)
- Rheumatoid arthritis (may produce giant synovial cysts extending down the calf)
- Patellofemoral chondromalacia
- Any cause of knee effusion
In children, the cyst rarely communicates with the joint, and intra-articular pathology is uncommon - most resolve spontaneously.
Clinical Features
- Posterior knee mass - palpable in the posteromedial corner of the knee
- Pressure, pain, and limitation of range of motion (particularly extension)
- Many go unnoticed until they rupture
Rupture leads to escape of fluid into the calf, producing:
- Unilateral lower extremity edema
- Calf pain and swelling mimicking DVT ("pseudothrombophlebitis syndrome")
- Rarely, acute compartment syndrome
Patients on anticoagulants can bleed into popliteal cysts, leading to dissection into the calf. Concurrent popliteal vein thrombosis can occur.
Differential Diagnosis
| Condition | Distinguishing Feature |
|---|---|
| DVT | Doppler ultrasound; may coexist |
| Lipoma | Solid on ultrasound, no communication with joint |
| Vascular tumor / aneurysm | Pulsatile, Doppler flow |
| Fibrosarcoma | Solid, irregular margins |
| Popliteal artery aneurysm | Pulsatile mass |
| Pyogenic abscess | Fever, tenderness, septic signs |
Diagnostics
Ultrasound is the first-line modality in the ED - readily available, confirms the cystic nature, rules out DVT, and can be used for aspiration guidance. On ultrasound, the cyst appears as an anechoic or hypoechoic structure in the popliteal fossa:
Transverse ultrasound of a Baker cyst (3.21 x 1.62 cm) (Tintinalli's Emergency Medicine)
MRI is the preferred modality for complete evaluation because it also delineates intra-articular pathology (e.g., meniscal tears, cartilage lesions) - important for surgical planning. On T2-weighted MRI, the cyst appears as a bright area of high signal intensity communicating with the joint space.
- Baker cyst is a diagnosis of exclusion in the ED setting
- Venous duplex to exclude DVT if clinically suspected
- The cyst can sometimes be transilluminated clinically
Treatment
Conservative (first-line)
- Relative rest, compression, NSAIDs
- Treat the underlying intra-articular pathology
- US-guided aspiration can be performed outpatient if diagnosis is certain (not recommended in the ED)
- Intra-articular corticosteroid injection (e.g., triamcinolone 40 mg) reduces cyst size by decreasing synovial fluid production
- In children: benign neglect - the majority resolve spontaneously
Surgical (when conservative fails)
The most important principle: treat the underlying cause, not just the cyst.
- Arthroscopic treatment of intra-articular pathology - most common approach in adults (partial meniscectomy, debridement of cartilage fragments)
- Arthroscopic cystectomy - resection of the cyst wall, enlarging the cyst-joint communication, and removal of the capsular fold (valvular mechanism) via posteromedial portals. This produces the best results; recurrence <2% when combined with posterior capsule repair.
- Open excision (Henderson posteromedial incision) - for cysts that do not communicate with the joint, or when arthroscopic access is insufficient. A 40% recurrence rate is reported with open cystectomy alone.
In rheumatoid arthritis patients: synovectomy should accompany cyst removal to prevent recurrence.
Recent evidence (2024): A systematic review and meta-analysis (PMID 38700675) comparing cyst wall preservation vs. cyst wall resection during arthroscopic decompression found that neither approach was clearly superior - both methods are acceptable during arthroscopic popliteal cyst surgery (Arch Orthop Trauma Surg, 2024).
Key Clinical Pearls
- Always exclude DVT when a Baker cyst ruptures - they can coexist
- In adults, always look for and address the underlying intra-articular pathology; the cyst will recur without this
- In children, conservative management is almost always sufficient
- Giant synovial cysts extending into the calf = think rheumatoid arthritis
- Loose bodies can travel between the knee joint and the cyst intermittently
Sources: Rosen's Emergency Medicine (9th ed.), Campbell's Operative Orthopaedics (15th ed., 2026), Tintinalli's Emergency Medicine, THIEME Atlas of Anatomy - General Anatomy and Musculoskeletal System
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