Question
A 58 year old woman has a swelling behind her knee. Two weeks ago her calf suddenly became painful and swollen.
- Describe the MRI and explain the anatomy.
- What is the differential diagnosis of the calf symptoms?
- How would you manage the cyst?
Answer
MRI and anatomy
- Large lobulated fluid filled cyst in the posteromedial knee
- Lies between the medial head of gastrocnemius and semimembranosus
- Communicates with the joint through a valve like opening
- Secondary to intra-articular pathology, meniscal tears and osteoarthritis
Calf symptoms
- Ruptured popliteal cyst (pseudothrombophlebitis)
- Deep vein thrombosis, exclude with duplex ultrasound
- Popliteal artery aneurysm, check pulsatility
- Compartment syndrome
- Soft tissue sarcoma if the mass is atypical
Management
- Treat the underlying joint pathology
- Aspiration and steroid injection under ultrasound
- Arthroscopic treatment of the meniscus and enlargement of the valve
- Open excision has high recurrence if the joint problem is not treated
- In children, cysts are primary and resolve spontaneously
Related pages
Author Contributions
Orthofracs team
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.