Acromioclavicular joint

The acromioclavicular joint, or AC joint, is a joint at the top of the shoulder. It is the junction between the acromion (part of the scapula that forms the highest point of the shoulder) and the clavicle.

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Function

The AC joint allows the ability to raise the arm above the head. This joint functions as a pivot point (although technically it is a gliding synovial joint), acting like a strut to help with movement of the scapula resulting in a greater degree of arm rotation.

Ligaments

The joint is stabilized by three ligaments:

• The acromioclavicular ligament, which attaches the clavicle to the acromion of the scapula.
• The coracoacromial ligament, which runs from the coracoid process to the acromion.
• The coracoclavicular ligament, which consists of two ligaments, the conoid and the trapezoid ligaments.

Injuries

Main article: Separated shoulder

A common injury to the AC joint is dislocation, often called AC separation or shoulder separation. This is not the same as a "shoulder dislocation," which refers to dislocation of the Glenohumeral joint.

AC dislocation is common in sports such as football, soccer, horseback riding, mountain biking, and biking. AC dislocations are rated in terms of severity on a scale of 1 to 6, 6 being the most severe. The most common mechanism of injury is a fall on the tip of the shoulder or FOOSH (Falls on an outstretched hand).

AC dislocations are also graded from I to III. Grading is based upon the degree of separation of the acromion from the clavicle with weight applied to the arm. Grade I is a tear of the AC ligament. It has the normal separation of <4mm. Grade II is a complete dislocation of AC ligament with partial disruption of coracoclavicular ligament. The AC gap is >5mm. Grade III is complete disruption of AC and CC ligaments. On plain film the inferior aspect of the clavicle will be above the superior aspect of the acromion. The joint will be very tender and swollen on examination.

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