The acromioclavicular (AC) joint is the junction between the shoulder blade and the collar bone. The AC joint is supported by a capsule and other ligaments, which ensure that the joint is stable and can assist in the movement of raising the arm above the head. When these supporting elements are injured, the AC joint can be sprained, also known as a shoulder separation

Depending upon the severity of the sprain, there are different signs of an AC sprain. For a grade 1 sprain, there is no visible displacement and there is localized pain whereas in a grade 3 sprain, there is an extensive involvement of the AC joint and surrounding areas, and visible displacement with bruising is observed.

Contact sports such as football or hockey where there is constant shoulder contact, or falling on your shoulder, or on an outstretched hand often cause AC sprains