3b - Answer: 53-year-old female presents with posteromedial right knee mass

History: This 53-year-old female presents with a posteromedial right knee mass.

MOST LIKELY DIAGNOSIS: Baker's cyst

Using the diagnostic web viewer, we have provided images that assist in telling our clinical story. Areas of significance are indicated below.

CLINICAL CONSIDERATIONS:

You diagnose the knee mass as a Baker’s cyst with partial dehiscence. Are you out of the woods and ready to move on to the next case? Not quite yet. A Baker’s cyst is a posterior fluid-filled containing structure with heterogeneous etiologies. In other words, there are innumerable types of Baker’s cysts. Which type is this and what are the arrows in the image indicating?

This is the gastrocnemius-semimembranosus bursal type of Baker’s cyst. In order to call it a cyst, it should have substantive size, appear under pressure (“expansile”), and appear to be loculated or isolated from the joint. Normally, when a patient is supine, fluid may gather or accumulate in the gastrocnemius-semimembranosus bursa. This bursal distention, which is not under pressure, should not be confused for a cyst. It simply serves as an outlet valve for fluid. When supine, fluid may passively sit in this bursa. In this case, the object is round, expansile concentrically, and therefore under pressure.

Other forms of Baker’s cyst include those arising from the popliteus reflection and the midline capsule. The arrows are pointing to four different recesses of the gastrocnemius-semimembranosus bursa, any of which can contribute to the development of a posterior bursal or Baker’s type cyst. The four labeled are the anteromedial (pink arrow), anterolateral (green arrow), posteromedial (yellow arrow) and posterolateral recesses. The one that most commonly gives rise to a mass is the posterolateral recess (double red arrows).

It is the posterolateral recess that most often distends as a “symptomatic” bursal or Baker’s type cyst. It can decompress into the calf and simulate a phlebitis. This is called pseudothrombophlebitis.

• While uncommon, anteromedial recess (pink arrow) distension is often confused with a meniscus cyst.
• Posteromedial recess (yellow arrow) distension is often confused with a ganglion.
• Anterolateral recess (green arrow) distension can be confused with a cyst associated with midline posterior capsule or “capsulosynovial cyst,” though this is uncommon.

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Content reviewed: November 3, 2021