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.