[Popliteal aneurysm simulating a Baker's cyst in a patient with rheumatoid arthritis: a case presentation].

Baker's cyst is the most common mass located in the back side of the knee, the popliteal fossa, in patients with degenerative and inflamatory disease of the knee. Popliteal mass may also be due to proliferation of adipose tissue, popliteal artery aneurysm, thrombotic vein, or tumor. These lesions are rarer and may easily be misinterpreted as cysts. We show a man with rheumatoid arthritis who is presenting a palpable mass in the popliteal fossa. Ultrasonography examination demostrated a popliteal artery aneurysms.