The AC joint is comprised of the acromioclavicular (AC) and coracoclavicular (CC) ligaments. An AC joint separation is caused by damage to these ligaments. In severe cases, it can also result in a dislocation of the clavicle from the acromion. An AC joint separation is commonly referred to as a separated shoulder, which is not related to a dislocated shoulder. Several grades of injuries can occur. Grades I-III are the most common, but AC joint separations can be classified all the way up to a Grade VI injury.
The lowest grade, Grade I, is caused by a partial tear in the AC ligament. Grade II occurs when the AC ligament ruptures and the CC ligament remains intact. This causes a partial dislocation of the AC joint. Grade III is a complete separation of the joint and both the AC and CC ligaments are completely torn. This leads to an inferior displacement (downward) of the shoulder from its original position and the clavicle rising up and forming a bump in the skin. AC joint separations are commonly caused by trauma to the shoulder like a direct blow to the shoulder pads in football, a cyclist falling at high speeds, or other harsh falls onto the “point” of the shoulder by athletes with an extended arm. An AC joint separation causes pain in the front and top of the shoulder and is exacerbated when reaching upward.
Nonoperative treatments of immobilization, ice, and anti-inflammatory drugs are recommended for Grade I-III separations. Higher grade separations are caused by extreme dislocation of the clavicle that can disrupt shoulder muscles and tendons, and it may require surgery to repair or reconstruct the ligaments.