Baker's Cyst (Popliteal Cyst)

Baker cyst anatomy - extension of the semimembranosus bursa through the posterior knee capsule

Figure: Baker cyst as an extension of the semimembranosus bursa (Tintinalli's Emergency Medicine)

Definition & Anatomy

A Baker's cyst, first described by Adams in 1840 and later named after Baker (1877), is a distended bursa or herniation of the synovial membrane through the posterior aspect of the knee joint capsule. Multiple bursae exist in the popliteal space, but symptoms develop most often in:

The bursa beneath the medial head of the gastrocnemius, or
The semimembranosus bursa (located between the semimembranosus tendon and the medial tibial condyle, and between the semimembranosus tendon and the medial gastrocnemius head)

The cyst typically forms when intra-articular pathology causes excess synovial fluid, which escapes through a normal communication between the bursa and the knee joint - acting like a one-way valve.

Campbell's Operative Orthopaedics 15th Ed 2026, p. 595

Pathophysiology

Excess synovial fluid (from any cause) accumulates and pushes through the posterior capsule or through the normal bursal-joint communication
A valvular mechanism is often present - capsular folds allow fluid to pass from the joint into the cyst but not freely back
In adults, the cyst almost always communicates with the knee joint; in children, this communication is less common
Giant synovial cysts extending into the calf are classically associated with rheumatoid arthritis

Causes / Associated Conditions

Category	Examples
Degenerative	Osteoarthritis, degenerative posterior horn medial meniscus tear
Inflammatory	Rheumatoid arthritis, other inflammatory arthropathies
Mechanical	Meniscal tears (especially posterior horn), chondromalacia patella
Traumatic	Post-traumatic effusions
Any knee effusion	Gout, pseudogout, infection

Clinical Features

Posteromedial popliteal mass - typically soft, fluctuant, and transilluminable
Pressure, pain, and limitation of knee ROM
Fullness or tightness behind the knee, worse with activity

Complications

Rupture - fluid dissects into the calf, causing sudden calf pain and swelling - can mimic DVT or compartment syndrome ("pseudothrombophlebitis")
Acute compartment syndrome (rare, from rupture)
Popliteal vein thrombosis - can occur concurrently with a dissecting cyst
Spontaneous venous bleeding in patients on anticoagulants (blood dissects into calf)
Note: DVT and Baker's cyst can coexist, so DVT must always be excluded
Rosen's Emergency Medicine, p. 725

Differential Diagnosis

DVT / thrombophlebitis
Popliteal artery aneurysm
Lipoma
Xanthoma
Fibrosarcoma / other soft tissue tumors
Vascular tumor
Pyogenic abscess
Ganglion cyst

Diagnosis

Modality	Role
Transillumination	Simple bedside test - cyst lights up (confirms cystic nature)
Ultrasound	First-line - confirms cyst, rules out DVT, guides aspiration; shows anechoic/hypoechoic popliteal mass
MRI	Gold standard - confirms cyst AND identifies intra-articular pathology (meniscal tears, OA, etc.); preferred modality
Arthrography	Demonstrates communication with the joint; largely replaced by MRI
Venous duplex	Excludes concurrent DVT; may identify cyst remnants

Baker's cyst is a diagnosis of exclusion in the ED - DVT must be ruled out when there is clinical suspicion.

Treatment

Conservative (first-line)

Relative rest, compression, NSAIDs
Treat the underlying intra-articular pathology (the cyst is a secondary problem)
Observation alone is appropriate in children - most resolve spontaneously

Aspiration / Injection

US-guided aspiration with a large-bore needle, followed by corticosteroid instillation (e.g., methylprednisolone acetate), is effective and safe for symptomatic cysts
Not recommended in the ED; done on an outpatient basis

Surgical Options (when conservative fails)

Approach	Details
Arthroscopic treatment of intra-articular pathology	Most common - treats the root cause (meniscal debridement, partial meniscectomy, chondral treatment)
Arthroscopic cystectomy	Resection of cyst wall + enlarging the articular-cyst communication via posterior portals; best outcomes; recurrence <2%
Open cystectomy	Posteromedial (Henderson) incision or posterior incision for large midline cysts; recurrence rate ~40% when done alone
Synovectomy	Added for RA-associated giant synovial cysts to prevent recurrence

Key surgical principles:

Arthroscopic cystectomy significantly outperforms open surgery
The valvular capsular fold should be resected arthroscopically with a motorized shaver
Loose bodies may travel between the joint and cyst - can be retrieved via accessory posteromedial portal
Campbell's Operative Orthopaedics 15th Ed 2026, p. 595-596
Rheumatology 2-Volume Set (Elsevier 2022)

Key Clinical Pearls

In adults - always look for intra-articular pathology (most common: degenerative posterior horn medial meniscus tear)
In children - cysts rarely communicate with the joint, intra-articular pathology is rare, and most resolve with watchful waiting
A ruptured cyst mimics DVT - but both can coexist, so always do a duplex ultrasound
Patients on anticoagulants are at risk for hemorrhage into the cyst with calf dissection
RA patients can develop giant synovial cysts extending deep into the calf - require synovectomy to prevent recurrence

This is a shared conversation. Sign in to Orris to start your own chat.