Popliteal (Baker) Cyst

BASICS

Definition: Fluid-filled synovial sac in the popliteal fossa, typically a distention of the gastrocnemius-semimembranosus bursa; not a true cyst.
Primary cyst: Distention of bursa, no intra-articular disorder (mainly children).
Secondary cyst: Communication with knee joint, articular fluid fills the cyst (mainly adults).
Most frequent cystic mass around the knee.
Unilateral or bilateral.
Often associated with synovial inflammation.

Associated intra-articular pathology (adults):
Meniscal tears (esp. posterior horn)
ACL insufficiency
Cartilage degeneration
Rheumatoid arthritis (20%), osteoarthritis (50%), gout (14%)
Other causes: Infectious arthritis, polyarthritis, villonodular synovitis, lymphoma, sarcoidosis, connective tissue diseases
Mechanism: Extension or herniation of synovial membrane; valve-like mechanism may allow one-way fluid passage from joint to bursa
Knee effusions often present
Trauma is primary cause in children (no communication with joint)

Most cysts asymptomatic
Painless popliteal fossa mass (most common)
Dull ache with large cysts or restriction of flexion
Painful if ruptured (swelling, bruising, calf/ankle)
Large cysts: possible tibial nerve entrapment (neuropathy), vascular compression (claudication, thrombophlebitis)
Cyst size may fluctuate with activity

Examine in full extension & 90° flexion
Foucher sign: Mass increases with extension, disappears with flexion
Palpate medial popliteal fossa, lateral to gastrocnemius head, medial to neurovascular bundle
Cyst fluctuant/tender, best felt when knee slightly flexed
Transillumination helps distinguish from solid mass
Ruptured cyst: Pain, swelling, bruising over calf/ankle (crescent sign), pseudothrombophlebitis, rare compartment syndrome

Labs: CBC, ESR if septic arthritis suspected
Ultrasound: Confirms presence/size; Doppler differentiates from aneurysm, DVT, tumors
Aspiration: Only after ruling out popliteal aneurysm; send fluid for cell count/culture if infection/inflammation/mechanical etiology suspected
MRI: Evaluates joint derangements, cyst leakage
X-ray: May show posterior soft tissue density
Arthrography/CT arthrography: Communication with joint or rupture, cyst details
Observation: Preferred in children (most resolve)

Physical therapy: Improves ROM, strength, especially with coexisting pathology
Aspiration: Temporary relief; recurrence common
Corticosteroid injection: Intra-articular/intracystic, often combined with PT or aspiration
Sclerotherapy (ethanol, dextrose/sodium morrhuate): Good results in small studies
Best outcomes: Combination PT + corticosteroid (± aspiration)

Consider excision if symptoms persist or etiology not found
Surgery rarely required in children
Recurrence common if underlying pathology not treated
Arthroscopic excision/drainage with treatment of intra-articular pathology (most successful)

Baker cyst: Synovial sac distention in the popliteal fossa, most often secondary to intra-articular knee pathology (meniscal tear, OA, etc.)
Conservative management preferred in children—most resolve spontaneously.
In adults, treat the underlying cause to resolve cysts.
Pain, bruising, swelling over the medial malleolus ("crescent sign") suggests rupture.