Case 12 :
Patient presents with a swelling at the back of the knee. USG is performed.
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Hemorrhagic popliteal cyst

Popliteal cysts, commonly known as Baker's cysts, are bursal collections of synovial fluid that usually occur in the semimembranous-gastrocnemius bursa. They may develop in other locations as well, including the bursa beneath the popliteal tendon, the bursa between the lateral head of gastrocnemius and biceps femoris, and the tibiofibular joint cavity. 30 to 50% of adults have communications between the knee joint and the semimembranosus-gastrocnemius bursa. Such communications are seldom seen in younger patients, suggesting that the channels are acquired through trauma or degeneration of the posterior joint capsule. The exact cause of the cyst formation is not known, but there are several well-established associations. Baker's cysts have been found in patients with internal knee joint derangement (meniscal or cruciate ligament tears, or loose bodies), osteoarthritis, rheumatoid arthritis, chondromalacia, granulomatous synovitis, osteochondritis dissecans, PVNS, and septic arthritis. Idiopathic cysts have been found in children. Cysts may directly compress the popliteal vein causing obstruction and thrombosis. Rupturing of the cyst allows synovial fluid to dissect between the soleus and gastrocnemius muscles. This gives rise to local pain and inflammation mimicking venous thrombosis. Treatment can involve surgical excision.
US: Cystic lesion in popliteal fossa with extensions into adjacent soft tissues. Does not clarify any underlying intraarticular pathology.
Arthrography: More sensitive than ultrasound (with communicating cysts) but may miss cysts which do not readily communicate with the joint. Studies show well defined lobular structures filled with fluid. They may have irregular surfaces due to hypertrophy of synovial lining.
CT: More sensitive than arthrography, but may not clearly demonstrate the communication with joint space.
MRI: Excellent demonstration of soft tissue anatomy and cyst connections with the joint space. TI hypo-to isointense and T2 hyperintense cystic structures. Associated hemorrhage (as with this case) or rupture will alter typical MRI findings. MRI can reveal underlying meniscal tears, other changes of rheumatoid arthritis, cysts in atypical locations, and cysts containing osteochondral fragments.

Dr. Ravi Kadasne



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