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Within the knee joint: bursae visible top right, middle right and bottom right
A bursa (plural bursae or bursas) is a small fluid-filled sac lined by synovial membrane with an inner capillary layer of viscous synovial fluid (similar in consistency to that of a raw egg white). It provides a cushion between bones and tendons and/or muscles around a joint. This helps to reduce friction between the bones and allows free movement. Bursae are found around most major joints of the body.
There are four types of bursa: adventitious, subcutaneous, synovial, and sub-muscular. Among these, only adventitious is non-native. When any surface of the body is subjected to repeated stress, an adventitious bursa develops under it. Examples are Students' elbow and bunion.
Infection or irritation of a bursa leads to bursitis (inflammation of a bursa). The general term for disease of bursae is "bursopathy."
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Imaging of the Bursae via Pub Med https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3177464/