The acromioclavicular (AC) joint is the joint at the top of the shoulder between the scapula (shoulder blade) and clavicle (collar bone) and forms the socket component of the shoulder’s ball and socket joint. AC joint separation is a disruption of the ligaments that connect the scapular to the clavicle causing the two bones to separate. There are three grades of AC separation:
- Grade 1 AC joint separation: slight joint displacement with stretching or tearing of the AC ligament with the nearby coracoclavicular ligament remaining intact.
- Grade 2 AC joint separation: partial dislocation of the joint with complete tearing of the AC ligament with the nearby coracoclavicular ligament remaining intact.
- Grade 3 AC joint separation: a complete separation of the AC joint with tearing of both the AC ligament and the coracoclavicular ligament and tearing of the surrounding joint capsule.
AC joint separation is typically caused by direct trauma, such as a fall or collision, where the tip of the shoulder is contacted. Another common mode of injury is a fall onto an outstretched hand. AC joint separation symptoms vary depending on the severity of injury but typically presents as pain on the top of the shoulder made worse by lifting the arm, swelling or bruising over the top of the shoulder, loss of range of motion in the shoulder and a possible “step” deformity.