My uneducated guess
An acromioclavicular joint separation, or AC separation, is a very frequent injury among physically active people. In this injury the clavicle (collar bone) separates from the scapula (shoulder blade). It is commonly caused by a fall directly on the "point" of the shoulder or a direct blow received in a contact sport. Football players and cyclists who fall over the handlebars are often subject to AC separations.
In general, most AC injuries don't require surgery. There are certain situations, however, in which surgery may be necessary. Most patients recover with full function of the shoulder. The period of disability and discomfort ranges from a few days to 12 weeks depending on the severity of the separation
Originally Posted by: PackFanWithTwins