First AD

Second AD

No labs, no biopsies . . .  you can make this Doorway Diagnosis just by looking.

Pictured is the posterior right knee of a 69-year-old woman showing a rounded protuberance at the popliteal fossa.

Answer: D. Popliteal cyst (aka, Baker, synovial cyst)

Baker cyst is a swelling of the popliteal bursa which communicates with the posterior knee joint. This communication is exceptionally rare in children and prevalence of the cyst increases with age. Baker cysts occur primarily as a result of the aging or chronically traumatized knee as the integrity of the joint capsule decreases. The cysts are most often comorbid with other intra-articular lesions and inflammatory conditions, such as osteoarthritis, meniscus tears, and rheumatoid arthritis. Excess synovial fluid in the knee joint secondary to inflammation or trauma travels the path of least resistance and the popliteal bursa becomes a cystic swelling.

  The vast majority of Baker cysts are diagnosed by clinical appearance. When there is some question, however, ultrasound and magnetic resonance imaging (MRI) are the most consistent and accurate studies for confirmation of popliteal cysts with MRI now the first-line modality.

When small, Baker cysts may cause few or no symptoms; symptoms may wax and wane depending on the amount of fluid formed in the knee joint. Large cysts may rupture downward into the gastrocnemius and soleus muscles and mimic deep vein thrombophlebitis.

Earlier x-ray images of the patient’s knee showed moderate osteoarthritis. She had been experiencing more and more discomfort in the right knee.

The differential for Baker cysts includes ganglionic cyst of gastrocnemius, meniscal cyst, myxoid liposarcoma, popliteal artery aneurysm, popliteal artery pseudoaneurysm, synovial sarcoma.

Additional information: Herman AM, Marzo JM. 

. 2014;37:e678-84. doi: 10.3928/01477447-20140728-52.

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No labs, no biopsies, no imaging -- only your seasoned clinical judgement is required to make a diagnosis in this case. 

Bulge Behind the Knee