Knee
Baker's Cyst (Popliteal Cyst)
베이커 낭종 (오금 낭종)
Synovial fluid collection behind the knee forming a soft cystic swelling
- How common
- Common (5% of adults, 20% of knee OA)
- Typical age
- Ages 35–70
What is it?
Knee joint effusion tracks through a one-way valve into the gastrocnemius-semimembranosus bursa, forming a Baker's cyst.
Commonly affected: Medial popliteal fossa (semimembranosus side)
How it develops
- Intra-articular PathologyKnee OA or meniscal tear increases joint fluid
- One-way ValveA unidirectional valve forms at posterior capsule
- Bursa ExpansionFluid fills the gastrocnemius-semimembranosus bursa
- Popliteal Cyst FormsSoft cyst palpable in popliteal fossa
Symptoms
- Rupture Calf PainAcute calf pain and swelling if ruptures (pseudo-DVT)
- Popliteal MassSoft, fluctuant mass behind the knee
- TightnessTightness with knee flexion
- Underlying Knee PainPain from the underlying OA or meniscal pathology
How it is examined
- Foucher SignCyst firm in extension, soft in flexion
- TransilluminationFluid-filled cyst transilluminates
- Calf Swelling CheckRupture causes calf swelling — must rule out DVT
Imaging
X-ray to assess underlying OA.
- Soft tissue shadow
- Underlying knee OA
MRI/US confirms diagnosis and assesses underlying joint pathology.
- Medial popliteal cyst
- Meniscal tear (underlying)
- Synovitis
- Peri-cystic edema if ruptured
Non-surgical care
- Treat Underlying CauseAddress OA or meniscal tear — primary treatment
- NSAIDs + IceNSAIDs and ice for inflammation
- US-guided Aspiration + SteroidFor symptomatic larger cysts
Conservative options are generally tried first. Medications listed here can have side effects — discuss them with your prescriber.
When surgery is considered
Recurrent, large, or compressing neurovascular structures
Procedures that may be discussed
- Arthroscopic treatment of underlying pathology
- Open cyst excision
Outlook
Most resolve once the underlying joint pathology is addressed.