Acromioclavicular Joint (AC Joint) Overview and Injuries

Posted on 24 Oct 2013 13:44

The abbreviation AC or AC joint stands for the acromioclavicular joint. The acromioclavicular is one of the three articulations of the shoulder girdle. See the shoulder complex for a general overview of the shoulder girdle and its joints.

The acromioclavicular joint links the clavicle to the scapula. The end of the lateral clavicle bone, which is at the top of your shoulder, articulates with the acromion process of the scapula. It is a separate joint from the shoulder joint, or glenohumeral, which is where the humerus bone articulates with the glenoid fossa of the scapula. The following is a general overview of the joint and its injuries. For a more comprehensive analysis of this and other joints of the body, see Joint Structure and Function: A Comprehensive Analysis by Pamela K Levangie, et al.

The AC joint is a gliding or plane joint, which is a type of diarthrodial joint. Its function is to allow the scapula to rotate in three dimensions during arm movements. It also allows forces to be transmitted from the upper extremity to the clavicle. The actual degree of motion is limited, unlike most diarthrodial joints. The joint has an articular disc with a poorly understood function. This disc starts out as hyaline cartilage but transforms to fibrocartilage, starting with the acromial side by age 17 and the clavicular side by age 23. Since movements of the scapula and clavicle occur in synchrony, little relative motion actually occurs at the joint itself and most of the scapulothoracic movement occurs at the sternoclavicular joint. Therefore, complete fusion of the AC joint seems to cause little loss of shoulder function.

labeled acromioclavicular joint (ac joint)

Labeled acromioclavicular joint

The distal end of the clavicle is connected to the acromion by the acromioclavicular ligament. There is also another ligament, called the coracoclavicular ligament. This ligament does not actually belong to the acromioclavicular joint anatomy itself, but it provides superior and inferior stability (vertical), by firmly attaching the clavicle and scapula. This ligament divides into medial and lateral bands. The medial portion of the ligament is called the conoid ligament and the lateral portion is called the trapezoid ligament. The two bands are separated by a large bursa. Much of the protection against large translations of the joint is attributed to this coracoclavicular ligament.

dips or shoulder dips exercise

Dips, a common strength training exercise, can be stressful
to the AC joint for some people, especially those with previous
trauma or aggravations to the joint.

The AC joint is quite susceptible to trauma or degenerative change. It is, in fact, one of the most frequently injured joints in the body. This usually happens before the age of 30 and the most common mechanism is a fall on the shoulder or a blow to the shoulder with the arm extended, which can result in traumatic dislocations. In fact, the term "shoulder separation" often is referring to a subluxation or dislocation of the AC joint, and these account for up to 12% of separations in the shoulder girdle. These kinds of injuries are common in contact sports players.

Sprains of the acromioclavicular joint are categorized in various ways. The following is a general description of the most common classified injury types:

  • Type I sprain: the acromioclavicular ligament is partially injured (sprained) but no instability or displacement of the joint occurs. The coracoclavicular ligament is intact. Immediate first aid is icing and immobilization. A sling may be desired. As soon as possible, range of motion and strengthening exercises should begin.
  • Type II sprain: the acromoclavicular ligament is ruptured and coracoclavicular ligament is sprained, but instact, so that no significant superior displacement of the clavicle occurs. Same treatment as a type I sprain.
  • Type III sprain: the acromiclavicular and the coracoclavicular ligaments are both ruptured and the clavicle is displaced superiorly to some extent, from 25 to 100%. Surgery is sometimes performed for type III acute displacements of the AC joint but there is little evidence of superior results between surgery and nonoperative conservative treatment.
  • Type IV sprain: both ligaments are completely ruptured and displacement of the clavicle posteriorly, often with the clavicle moving posteriorly into the trapezius, which is called a "buttonhole." Since chronic pain can result, surgery is often required.
  • Type V sprain: complete rupture of both ligaments and displacement of the clavicle superiorly (much more so that with type II injuries) until the clavicle is in a subcutaneous position (just under the skin). Since the clavicle could erode through the skin in this position, surgery is usually recommended.
  • Type VI sprain: both ligaments are completely ruptured and the clavicle is displaced inferiorly to the acromion or coracoid of the scapula. This type of injury is extremely rare and requires a large input of energy, which will probably result in additional injuries to the shoulder girdle. Sugery is probably needed.1,2

Treatment of AC joint injuries remains controversial and whether or not to use operative treatment is hotly debated. As well, many different surgical techniques have been described.1,3

Also, small changes over time, as from overuse, can narrow the joint space causing dysfunction and pain with shoulder use, which can be provoked by certain exercises. Although it may seem logical to expect AC problems in overhead throwing athletes, disorders of the AC in this population is relatively uncommon. The throwing motion does not seem to place much stress on the joint. For more information, see The Shoulder and the Overhead Athlete by Krishnan, et al.1,3

In older adults, the most condition affecting the joint is degenerative arthritis, which may or may not present symptoms. Injuries to the AC joint are often confused with other shoulder injuries. Exercises that may aggravate the problematic joint are push-ups, dips, and the bench press.1,2

1. Levangie, Pamela K., Cynthia C. Norkin, and Pamela K. Levangie. Joint Structure and Function: A Comprehensive Analysis. Philadelphia: F.A. Davis, 2011.
2. Schepsis, Anthony A., and Brian D. Busconi. Sports Medicine. Philadelphia, PA: Lippincott Williams & Wilkins, 2006.
3. Krishnan, Sumant G., Richard J. Hawkins, and Russell F. Warren. The Shoulder and the Overhead Athlete. Philadelphia: Lippincott Williams & Wilkins, 2004.

Images used under license, not for reuse. Acromioclavicular joint © Alila Medical Media - Dips exercise © Alen Ajan -

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This page created 24 Oct 2013 13:44
Last updated 22 Jul 2016 19:13

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