Deltoid Bursitis (Subacromial bursitis) is an inflammatory condition that is a cause of shoulder pain. It is usually caused by an amount of repetitive overhead activities, including minor trauma. A bursa is a fluid-filled sac located in several parts of the human body, most specifically, it can be found between tissues (bone, skin, tendons, and muscle).
These pillowy membranes defend bones from knocking against each other, as well as from rubbing directly against other soft tissues as it increases the flexibility and durability of the joint. The subdeltoid bursa is located in between the shoulder socket joint and the deltoid muscle, which then forms the rounded cap onto the top of the shoulder.
Therefore, the subdeltoid bursa reduces its friction within the shoulder and allows this complex joint to navigate more simply. When the bursa becomes inflamed, damaged, or irritated, it is then claimed to be the start of the development of deltoid bursitis.