The olecranon bursa is the top part of the ulna bone in the lower arm, that forms the bony area at the back of the elbow. This is the area that supports the arm when elbows are leaned on a table. A bursa is a fluid filled sac that provides cushioning and lubrication at a joint. There are several bursae in the body, including one over the elbow joint which decreases the friction between the bones and overlying skin.