Where is the problem that causes shoulder bursitis?

The shoulder is interesting in that several bones, muscles, and ligaments contribute to this complex joint.
Impingement syndrome, or shoulder bursitis, occurs when there is inflammation between the top of the humerus (arm bone) and the acromion (tip of the shoulder). Between these bones lies the tendons of the rotator cuff, and the bursa that protects these tendons.
Normally, these tendons slide effortlessly within this space. In some people this space becomes too narrow for normal motion, and the tendons and bursa become inflamed. Inflammation leads to thickening of the tendons and bursa, and contributes to the loss of space in this location. Eventually, this space becomes too narrow to accommodate the tendons and the bursa, and every time these structures move between the bones they are pinched--this is the impingement.