A shoulder dislocation (shoulder instability) occurs when the upper end of the arm bone pops out of the shoulder joint. This injury may be caused by a direct blow to the shoulder, a fall on an outstretched hand or arm, or an exaggerated overhead throwing motion.
Frozen shoulder (adhesive capsulitis) is a condition in which the tissues around the shoulder joint stiffen, scar tissue forms, and shoulder movements become difficult and painful. It can develop when you stop using the joint normally because of pain, other injury, or a chronic health condition, such as diabetes. Any shoulder problem can lead to frozen shoulder if you do not work to maintain its full range of motion.