The Baker cyst: a bursa, a recess or a true cyst?

A popliteal or Baker cyst is not technically a true cyst, but more of a distended knee joint recess that in most adult people communicates with the gastrocnemius- semimembranosus bursa and has accumulated fluid.

Cause? 
In most cases a Baker cyst is secondary to various intra-articular knee pathologies, including friction between the tendons of the semimembranosus and gastrocnemius, degenerative and inflammatory arthritis, meniscal tears, rheumatological problems such as Gout, and other internal derangements. 

Discomfort?
Although Baker cysts are most frequently asymptomatic, they may cause posterior knee pain, joint stiffness, and reduced range of motion. 

Ultrasound (US)?
The cyst looks like an anechoic comma-shaped fluid collection, located in the posteromedial aspect of the knee, and should not have vascular flow, distinguishing it from a popliteal artery aneurysm. In some cases, the fluid may be complex in appearance and may possibly contain loose bodies. Use SonoSkills pathology checklist for accurate interpretation!