Bursitis occurs when the bursa becomes inflamed, usually due to a specific or repetitive trauma. The swelling and inflammation reduce the space between the humerus and acromion, producing a pinching action, and also cause thickening of the bursa, which further reduces the space. Some people have an acromion that also reduces the space. The inflammation, and thickening produce a cyclic process  which often leads to progressive pain and disability.

There are a number of conditions which can produce pain in the shoulder. Among them are: