The semimembranosus bursa lies between the semimembranosus muscle and the medial head of gastrocnemius. This bursa my become enlarged and present as a painless lump in the medial part of the popliteal fossa.
On examination the lump is fluctuant. However, it cannot be pushed into the joint. The knee joint is normal.
The lump may require excision if symptomatic.
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