AC joint Instability

Definition

AC-instability is a result of singular or multiple injuries to the ligamentous apparatus of the acromioclavicular joint leading to a non-physiological translation of the clavicula in relation to the acromion.

Aetiology

Acute (<3 weeks after trauma) or chronic (>3 weeks after trauma) instability commonly stems from prior injuries to the acromioclavicular joint [1]. In most cases, injuries are sustained during sporting activity, where a direct blow to the shoulder or a fall on the outstretched arm leads to a dislocation of the joint [1]. For more information about acute dislocations of the AC-joint, please visit acromioclavicular dislocation. Dislocations or subluxations lead to a sprain or complete disruption of the acromioclavicular- and coracoclavicular ligaments (conoid and trapezoid ligament) [2]. Atraumatic instability only plays a minor role, although there are cases of iatrogenic instability, especially after extensive surgery of acromioclavicular osteoarthritis with resection of more than 1cm of the distal clavicle.

Epidemiology

Acromioclavicular injuries account for 12% of all shoulder injuries, with young, active males being affected the most [1].

History

When taking the patients history, special focus should lay on prior AC-joint injuries and the treatment of such, as chronic instability often is a result of injuries that have not been diagnosed or treated properly [1].

Disease specific diagnosis

Diagnosis of chronic instability is often missed. However, there are several examination techniques, which in combination of the patient’s history were proven to be beneficial in diagnosing instability. For the radiographic Rockwood classification of acute acromioclavicular dislocation or subluxation, please visit acromioclavicular dislocation.

• Cross-body adduction test

  • • • With the patient in either sitting or standing position, the examiner brings the shoulder as well as the elbow into 90° flexion. Subsequently, the elbow is moved horizontally across the patient’s chest, which in a positive result causes pain in the AC-joint [1].

• O'Brien's test

  • • • With the patient in either sitting or standing position, the examiner brings the shoulder in 90° flexion and 10-15° adduction. The patient is instructed to fully internally rotate the shoulder and pronate in the elbow. The examiner subsequently applies downward force on the patient’s forearm as he/she presses against it. This is recreated, although this time the patient fully externally rotates the shoulder and supinates in the elbow. The test is positive if pain can be felt, especially if pain is alleviated in supination [1]. Since O’Brien’s test was originally created to diagnose SLAP-lesions, the positive findings can further be distinguished. If pain ‘sits deep in the shoulder’ it usually indicates a SLAP-lesion. If pain however is situated above or in the AC-joint and the pain is not fully relieved by supination one should lean towards pathologies of the AC-joint.

• Paxinos test

  • • • Patient is sitting upright with the affected arm hanging loosely to his side. The examiner places his/her thumb on the posterolateral side of the acromion, while his/her middle and index finger are placed superior to the mid-segment of the clavicula. With the examiners thumb pressing anterosuperior and the index and middle finger applying inferior directed force, pain is increased in positive finding [1].

• Testing for horizontal instability

  • • • Similarly, to the Paxinos test, both acromion and clavicula are shifted against each other. However, this time the clavicula is moved posteriorly as the acromion is held in position. Recreating the pain is seen as a positive sign [1].

By combining all these techniques, accuracy of clinically diagnosing chronic acromioclavicular instability was shown to increase a lot [1]. There is ambiguity regarding the visualisation of instability on radiographs. Some studies claim that dynamic views in supine position with the arm abducted 90° in the scapular plane are superior in detecting horizontal instability, whereas other sources claim that a modified Alexander view with the arm placed in an adducted horizontal stress position with the scapula anteverted has more informative value [1]. Yet, it is agreed upon that both MRI and CT should only be used in unclear cases [1].

Differential diagnosis

• AC-joint osteoarthritis

• Acute AC-dislocation

Overall, prognosis is favourable. Regardless of which technique was used during surgery, about 19% of patients have shown features of horizontal instability [1]. These patients also had significantly worse outcomes of disability in arm, shoulder, and hand [1].

Chronic instability might lead to osteoarthritis of the acromioclavicular joint [2]. Yet, this course of disease is mainly reported in Rockwood type I and II as other Rockwood types show a greater separation of acromion and clavicula [2].