The shoulder is the most mobile joint in the body and the most unstable. That is why nature created a complex support structure to assure that the upper arm bone in stays in its socket (the rotator cuff). Instability occurs when the support structures fail to hold the upper arm bone in its place. This can happen with a traumatic injury or overuse.
Shoulder impingement syndrome affects about 20% of all people at some time in their lives. Shoulder injuries are common among all age groups. Older adults with age-related changes in the shoulder, and athletes are most commonly affected.
The Acromioclavicular Joint (the “AC joint”) is the bony point of the shoulder where the collarbone and the shoulder blade meet. The joint is surrounded by a capsule that protects the joint and the ligaments that support the joint. This capsule also provides joint stability. All arm motions move the AC joint.