The AC joint (acromioclavicular joint) is made up of the acromion process (boney projection) of the scapula (shoulder blade) and the clavicle (collar bone.) The AC joint is basically where the collar bone and shoulder blade meet.
The picture above highlights all of the ligaments involved in the static stabilization of the shoulder joint. Ligaments of the shoulder connect three main areas: the clavicle, the coracoid process, and the acromion process (both processes come off the scapula). An isolated AC joint sprain involves the AC ligament which connects the clavicle and the acromion process. Sprains to the ligaments of the shoulder can also involve the other ligaments aforementioned. The more damaged structures *typically* means a longer recovery.
Sprains in general are classified between grades 1, 2, or 3. As the grades increase, the severity of the sprain increases, with a grade 3 being a complete tear of the AC ligament which is synonymous with a AC joint separation.
Many steps can be taken to try and help the athlete play through the pain of the sprain. Taping down of the AC joint, applying a donut pad to the AC joint, a steroid injection into the joint space by the team physician, or bracing are all possibilities.
AC joint sprains can affect any and all positions in football due to the overhead nature of the sport with throwing, above head catching, and simply taking the repeated hits on the shoulder pads.