What is the Acromioclavicular Joint?

The AC joint is short for the acromioclavicular joint. Separation of the two bones forming this joint is caused by damage to the ligaments connecting them. It is sometimes also referred to as a shoulder separation injury.

The acromioclavicular joint is formed by the outer end of the clavicle (collar bone) and the acromion process of the scapular (shoulder blade). The acromion is a bony process which protrudes forwards from the upper part of the scapular. This joint forms the highest part of the shoulder.

The two bones are attached by the acromioclavicular (AC) ligament. There are several other ligaments which can be of importance in AC joint injuries, including the coracoclavicular (CC) ligament (divided into conoid and trapezoid sections) which joins the clavicle to the coracoid process, another forward protruding part of the scapula, slightly below and to the inside of the acromion.

A third ligament is the coracoacromial ligament which attaches the acromion process to the coracoid process, although it is rarely involved in this type of injury.The most common way of injuring the AC joint is by landing on the shoulder, elbow, or onto an outstretched hand.

Symptoms include:

  • Pain at the end of the collar bone
  • Pain may feel widespread throughout the shoulder until the initial pain resolves, following this it is more likely to be a very specific site of pain over the joint itself
  • Swelling often occurs
  • Depending on the extent of the injury a step-deformity may be visible. This is an obvious lump where the joint has been disrupted and is visible on more severe injuries
  • Pain on moving the shoulder, especially when trying to raise the arms above shoulder height

AC joint injuries are graded from 1-6 using the Rockwood scale which classifies injuries in relation to the extent of ligament damage and the space between the acromion and clavicle, as shown in the pictures opposite.

Grade 1 is a simple sprain to the AC joint, grade 2 involves rupture of the AC ligament and grade 3 rupture of both AC and CC ligaments which often results in a superior displacement. From this point onwards the scale and grade of injury depends on the degree of displacement of the clavicle.

Grade 4 involves posterior displacement and grade 5 superior displacement, to a greater degree than grade 3, with an increase in coracoclavicular space by 3-5 times the norm. A step deformity may be apparent with grade 3, 4 & 5 injuries. Grade 6 (not shown) involves full rupture of both AC and CC ligaments with the clavicle being displaced inferiorly.

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