Semimembranosus-Gastrocnemius Bursa

The semimembranosus-gastrocnemius bursa is located at the medial aspect of the popliteal space, between the semimembranosus tendon and the medial head of the gastrocnemius (Guerra et al. 

Lindgern and Willen 1977). From the anatomic standpoint, the semimembranosus-gastrocnemius bursa consists of three main portions the base, the neck and the body (Fig. 14.11b,d). The base is small, anteriorly located and communicates with the femo-rotibial joint the neck is a narrow tract between the semimembranosus tendon and the medial head of the gastrocnemius the body is the largest posterior portion of the bursa and lies dorsal to the gastrocnemius. 

The posterior knee is the predominant site for cystic masses (Tschirch et al. 2003). There is some confusion about the correct terminology of popliteal cysts in the radiological and clinical literature. A popliteal cyst can be defined as a cystic lesion located on the posterior aspect of the knee joint, within the popliteal space. In this location, the most common popliteal cysts are Baker cysts. These cysts result from the distension of the semimembranosus-gastrocnemius bursa and are not synonymous with popliteal cysts (Baker 1877). Posterior extra-articular ganglia are rare and may be located anywhere in the popliteal fossa but not at the level of the semimembranosus-gastrocnemius bursa. Differentiation between these two entities is clinically relevant because they have different characteristics, pathogenesis, imaging features and therapeutic implications. 

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