The pes anserine bursa is a sac (lined space) under the attachments of the pes anserine tendons on the inner aspect of the knee.

The bursa exists between the tibia and the tendons.
Pes anserine bursitis
In situations of overuse the bursa may become inflamed and painful. This is common in class breaststroke swimmers.
Synonyms:
pes bursa
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CLINICAL PUBLICATIONS
The anatomical and imaging study of pes anserinus and its clinical application. Zhong S, Bo Wu B, Wang M, Wang X, Yan Q, Fan X, Hu Y, Han Y and Lih Y. Medicine (Baltimore). 2018 Apr; 97(15): e0352.