AC Joint dislocations - Classification & Natural History

Authors: O Levy

References: SECEC 2005, Rome


The acromioclavicular joint is a diarthrodial joint between the medial clavicular facet of the acromion and the distal clavicle.  The joint contains a fibro cartilaginous disc that has been observed to vary considerably in size and shape.  There are 2 types of disc: complete and partial (meniscoid).  This intraarticular meniscus undergoes rapid degeneration until it is essentially no longer functional beyond the 4th decade.


AC joint stabilisers Capsular (Acromioclavicular) Ligaments

The AC joint has a thin capsule stabilized by ligaments.  The most robust of the acromioclavicular ligaments is the superior, which is reinforced by attachment of the fibres of the deltoid and trapezius muscles.  The horizontal, anteroposterior stability is provided by the acromioclavicular ligaments.


Extracapsular (Coracoclavicular) Ligaments

The coracoclavicular (conoid and trapezoid) ligaments pass from the inferior surface of the clavicle to the base of the coracoid process of the scapula.  These strong ligaments provide vertical stability to the acromioclavicular joint.  The Coracoclavicular ligaments help to couple the GH and Scapulothoracic motion during shoulder elevation.  This is an obligatory rotation of the clavicle through the AC joint.  The Coracoclavicular ligaments are the prime suspesory ligaments of the upper extremity.


Muscles that cross the joint (Deltoid and Trapezius) providing dynamic suspensory support.  Their importance increases in the presence of torn ligaments.



This injury is more common in young males (5/1 to 10/1).  Often the result of a direct contusion to the apex of the shoulder after a fall or a forceful collision with a hard surface.  It is common among rugby and ice hockey players as well as bicycle or motorcycle accidents.  AC joint injury by indirect trauma either a fall on the elbow or a fall onto the outstretched hand may also occur.  Typically in an acromioclavicular joint injury, the force initially sprains the acromioclavicular ligaments (mild sprain); then stresses the coracoclavicular ligaments (moderate sprain) and finally, if the downwards force continues, the injury progresses to tear the coracohumeral ligaments and then the deltoid trapezial fascia as well.  At this point the upper extremity has lost its suspensory support and drops downwards.



Injuries to the acromioclavicular joint (ACJ) are classified on the basis of the findings from the physical examination and anteroposterior and axillary radiographs.  The degree of damage to the acromioclavicular and the coracoclavicular ligaments as well as the deltoid and trapezius attachments are also considered.  The most common classification is the Allman and Tossy classification (Allman, 1967; Tossy et al., 1963) with injuries type 1, 2 or 3 with Rockwood’s modification, which added 4, 5 and 6 to complete the classification (Rockwood et al., 1998). 



I -      Sprain of acromioclavicular ligament only

II-      Acromioclavicular ligaments and joint capsule.

Disrupted Coracoclavicular ligaments intact.  50% vertical subluxation of clavicle.

III-     Acromioclavicular ligaments and capsule disrupted.

Coracoclavicular ligaments disrupted.  Acromioclavicular joint dislocation with clavicle displaced superiorly and complete loss of contact between clavicle and acromion.

IV-     Acromioclavicular ligaments and capsule disrupted. Coracoclavicular ligaments disrupted.  Acromioclavicular joint dislocation and clavicle displaced posteriorly into or through trapezius muscle (posterior displacement confirmed on axillary radiograph)

V-      Acromioclavicular ligaments and capsule disrupted. Coracoclavicular ligaments disrupted.  Acromioclavicular joint dislocation with extreme superior elevation of clavicle (100 to 300% normal).  Complete detachment of deltoid and trapezius from distal clavicle.

VI-     Acromioclavicular ligaments and capsule disrupted.  Coracoclavicular ligaments disrupted.  Acromioclavicular joint dislocation with clavicle displaced inferior to acromion and coracoid process.


Whiplash injury to the shoulder

Whiplash injury to the shoulder is a newly recognized problem in patients who developed shoulder symptoms as a consequence of indirect trauma sustained in a car accident (Wallace et al., 1998; Levy et al., 2002).  These patients usually have an established episode of trauma usually involving the shoulder wearing the seatbelt at the time of the accident.  The injury originates from the whiplash movement of the shoulder where the clavicle and the body are retrained by the seatbelt.  Most of these patients will have some form of soft tissue injury to the neck as well.  The injuries to the shoulder may involve acromioclavicular joint subluxations and dislocations (mainly type 1 and 2).



Type I & II

Although the majority of patients with Type I and II injuries will recover and most of the acute symptoms will subside with 7-10 days of injury, these injuries are not as benign as thought.  Literature review revealed a surprisingly high rate of complications (need for further surgery, pain, radiographic changes) after grade I and II AC joint sprain treated conservatively.  This is in contradiction with the common perception, which suggests that grade I and II AC sprains have a good prognosis and are not associated with joint instability.  Grade I and II injuries may cause persistent pain and late radiographic changes.  Major symptoms, such as severe pain and instability forcing the individual to give up sports and diminish performance, was up to 9% for grade I injury and up to 42% for grade II injury.  Radiographic changes were noted in 70% of patients with grade I AC joint sprains and in 75% of patients with grade II sprains.  An incidence of distal clavicular Osteolysis of 6% after an AC joint separation (grade I-III) has been reported.  Residual laxity was found in 33%.  Some patients may also have significant weakness in horizontal abduction.  A sprain may cause tears of the capsule ligaments and lesions of the meniscus and the articular cartilage.  Therefore, degenerative changes due to repetitive trauma may occur.


After AC joint sprain, the torn capsular ligament can be trapped within the joint and loose pieces of articular cartilage can be released.  Incongruity in the AC joint after grade II injuries may cause late symptoms.  The painful symptoms may be related to instability of the AC joint, which may lead to articular degeneration after repetitive use.


The severity of grade I & II AC joint injuries is underestimated and may lead to more chronic disability than previously recognised, especially in athletes and heavy labourers who stress their shoulder daily.  Some late surgery as AC joint resection arthroplasty may be needed.  However, more than 50% of the patients having a good or excellent shoulder 6 years after injury.



The same complications may apply to the more severe AC joint injuries.  Furthermore, AC joint instability symptoms may persist, with impingement symptoms secondary to the drop down of the shoulder and the abnormal biomechanics.  Patient may complain of severe deformity in the AC joint and traction symptoms with neck pain and neural brachial plexus symptoms.  There is significant decrease (24%) in horizontal abduction strength at fast speeds.  However, overall 87% with type III dislocation showed satisfactory outcome with conservative treatment of ‘Skilful neglect’.



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9.       Shaw MBK, McInerney JJ, Dias JJ, Evans PA.  Acromioclavicular joint sprains: the post-injury recovery interval.  Injury Int. J. Care Injured 34(2003) 438-442

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