Acromioclavicular joint injury is one of the common shoulder injuries among physically active people, representing upto 9% of all shoulder injuries.  AC joint separation occurs when clavicle (collar bone) separates from the scapula (shoulder blade), commonly due to a fall directly on the “point” of the shoulder or a direct blow to the shoulder, especially in physically demanding sports.  A good portion of AC joint injuries occur in people in their twenties and it is found that AC joint injuries are five times common in men than in women.  Younger people participating in contact sports are more vulnerable to AC joint injuries.  There is a high chance of AC joint separation in sports like hockey, rugby and skiing.  Footballers and cyclists who fall over the handlebars can also suffer from AC joint separation.  The most common mechanism of injury is by direct contact applied directly over the superolateral border of the shoulder while the humerus is in the adducted position.

Acromioclavicular joint sprains have been classified according to the severity of injury to the acromioclavicular and coracoclavicular ligaments, the acromioclavicular joint capsule, and the supporting muscles of the shoulder (trapezius and deltoid) that attach to the clavicle.  Classification is based on AC injury, beginning with trauma to the AC ligaments, continuing to the coracoclavicular ligaments, and finally disrupting the deltotrapezial fascia.  Degree of the injury is determined in accordance with the severity of injury sustained by the capsular and extracapsular ligaments and supporting musculature.


The severity of AC joint separation depends on which supporting structures are damaged and the extent of that damage.  Separation of the acromioclavicular ligament alone is not a serious injury, but when the coracoclavicular ligaments are ruptured and the whole shoulder unit is involved, there are chances of complicating the separation.

In general parlance AC joint separation is classified in three grades ranging from a mild dislocation to a complete separation:

Grade I:  A mild displacement of the joint. The acromioclavicular ligament may be stretched or partially torn. This is the most common type of injury to the AC joint.

Grade II:  A partial dislocation of the joint in which there may be some displacement that may not be evident during a clinical examination. The acromioclavicular ligament is completely torn, whereas the coracoclavicular ligaments remain intact.

Grade III:  A complete separation of the joint. The acromioclavicular ligament, the coracoclavicular ligaments and the capsule surrounding the joint are torn.  Normally, the displacement is obvious on physical exam. Without any ligament support, the shoulder falls under the weight of the arm and the clavicle is pushed up, causing a bump on the shoulder.

Rockwood Classification of AC joint separation:

Of the three major classifications of AC joint separations, the Rockwood Classification is the most common classification system in use for acromioclavicular joint injuries (Tossy and Alman Classification are the others). This well known six-type system is a modification of the earlier three-class classification system.

It takes into account not only the acromioclavicular joint itself but also the coracoclavicular ligament, the deltoid and trapezius muscles, and the direction of dislocation of the clavicle with respect to the acromion.

Rockwood classification of acromioclavicular separations Types I–III (Table 1)

Structure I II III
Acromioclavicular ligament Sprained Complete tear Complete tear
Acromioclavicular joint Intact Disrupted; widened in the transverse plane Dislocated; clavicle displaced superiorly relative to the acromion
Coracoclavicular ligaments Intact Sprained; slight widening of interval Disrupted; interval widened up to 100%
Deltoid and trapezius muscles Intact Possible partial detachment High probability of detachment from distal clavicle

Rockwood classification of acromioclavicular separations Types IV–VI (Table 2)

Structure IV V VI
Acromioclavicular ligaments Complete disruption Complete disruption Complete disruption
Acromioclavicular joint Dislocated; clavicle displaced posteriorly into or through the trapezius muscle Dislocated; extreme vertical incongruity between lateral clavicle and acromion. Dislocated; clavicle displaced inferior relative to the acromion
Coracoclavicular ligaments Partial or complete disruption with change in interval orientation Complete disruption; interval widened 100% to 300% Intact; interval is decreased or reversed
Deltoid and trapezius muscles High probability of detachment from distal clavicle High probability of detachment from distal clavicle Intact, partial, or complete detachment

 Tossy Classification of AC joint separation:

Grade 1:  Strain and contusions of AC joint.  No deformity visible clinically or on x-ray.

Grade 2:  Localized pain accompanied by swelling and deformity.  X-rays show one-half separation of the AC joint, i.e. clavicle displaced cephalad by one-half the depth of the AC joint.   Coraco-clavicular distance increased as compared to normal side with partial tear of coraco-clavicular ligaments.

Grade 3:  Complete AC joint separation more than one-half the depth of the AC joint; clavicle under skin.  Coraco-clavicular distance significantly increased; complete tear of coraco-clavicular ligaments.

Allman Classification of AC joint separation:

Grade 1:  Sprain of AC joint capsule and AC ligament.  No deformity.  X-rays normal

Grade 2:   Rupture of AC capsule and ligaments.  Mild deformity.  X-rays show upward displacement of clavicle.  Coraco-clavicular ligaments normal

Grade 3:  Complete AC joint dislocation.  Complete rupture of acromioclavicular and coraco-clavicular ligament 

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Q&As on Classification of AC Joint Separation

1) What is the AC joint separation simplified?

An AC joint is a medical term called shoulder separation in layman’s terms. AC stands for acromioclavicular joint. The joint connects the shoulder blade with the clavicle or the collarbone instead of the humerus or the upper arm bone. An AC joint separation mainly includes damage to the ligaments that supports the AC joint, either tears or sprains primarily caused by falling on the shoulder. This often results in pain, deformity of the shoulder, and losing forelimb mobility. AC joint separations vary in severity, from less serious sprains to more serious tears.

2) How do symptoms of AC joint separation help in classifying it?

Some of the main symptoms of AC joint separation are: 

  • Pain
  • Swelling
  • Weakness in the shoulder or upper arm
  • Bruising
  • Visible deformity, mainly a bump on the top of the shoulder

AC joint injuries can be classified accordingly

  • Mild sprains of the AC joint ligament are the minimum form of the injury. There are no visible deformities here, and if you do X-rays, it appears normal.
  • Tears in the AC ligament and other sprains in the coracoclavicular ligament are the second most serious category. Here you can see a bump, and the shoulder blade and the clavicle remain misaligned.
  • The severe form of the injury is where both the CC and AC ligaments get torn. There is a big outer deformity, and the shoulder blade and clavicle remain misaligned.

3) What are various grades of AC joint separation?

In AC joint separation:

  • Grade 1 mainly involves little joint displacement. Here the acromioclavicular ligament might be stretched or torn partially.
  • Grade 2 is partial joint dislocation with slight displacement that might not be obvious during physical examination.
  • Grade 3 is full separation of the joint. Here the acromioclavicular ligament, coracoclavicular ligaments, and the capsule surrounding the joint get torn.
  • While grades 1-3 are very common, grades 4-6 are very uncommon because of any high-energy injury that may occur during motor vehicle accidents. Grade 4-6 AC joint separation is treated surgically because of high disruption of all ligamentous support for the shoulder and the arm.

4) Does all AC joint separation need surgery?

No, maximum AC joint separation functions very well with the traditional treatment of the prevailing symptoms. Most AC joint separation is grade 1, 2, or 3; these do not need surgery and heal with time and care. Generally, the joint swelling remains for two to six weeks, and then it’s possible to return to full activity. Only some grade 3 injuries and high-energy AC separations that often result from motor vehicle accidents need surgery for complete recovery.

5) What mainly causes AC joint separation?

AC joint or shoulder separation mainly results from a sudden and traumatic event caused by any specific incident or action. The two most common explanations of a shoulder separation are either a direct blow to your shoulder, mainly seen in football, hockey or rugby, or a fall to an outspread hand, mainly seen after falling from a horse or bicycle.

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