Mark A bursa is a fluid-filled sac that usually overlays a bone or a joint and acts as a shock absorber.
Anatomical bursae occur normally around the body where tendons cross bones or joints.
The complex knee joint has 15 bursae, for example. Adventitious bursae are not part of the normal body structure but develop when the soft tissue overlying a bone suffers repeated friction or trauma. An example of this type is when one arises over the pelvic bone in the buttocks muscles because someone has been sitting on a hard chair for several hours a day. If a bursa becomes inflamed then it is called bursitis and certain occupations predispose people to this. It's best not to stay in the same position for long periods of time if bursitis is to be avoided.