AC joint Dislocation

Definition

Acromioclavicular dislocation (ACD) is a non-physiological translation of the clavicula resulting in minor or major damage of the ligamentous apparatus of the acromioclavicular joint capsule.

Aetiology

Dislocations of the acromioclavicular joint stem from injuries with direct impact to the superior or lateral aspect of the shoulder with the arm adducted or falling on the outstretched arm [1]. Often contact sports such as rugby, American football and ice hockey are associated with these types of injuries [1]. The degree of force correlates with the damage caused in the process, as low force may only lead to a sprain and as force increases, ligaments may rupture [1, 2].

According to the Rockwood Classification, the pathologies can be divided into six different types. For a precise radiological description of each Rockwood type please visit Radiopedia.

Epidemiology

AC-dislocations are very common and affect around 3-4/100’000 inhabitants [1]. 25% to 52% occur while participating in some sort of sport activity [1], as these type of injuries account for 4-12% of all shoulder girdle injuries [3].

History

The medical history should contain:

• Traumatic event?

  • • The most common cause is a fall onto the acromion with the arm in adducted position

• When was the traumatic event?

• How many traumatic events?

• Degree of force

• Previous shoulder pathologies?

Disease specific diagnosis

Diagnosis can be made clinically as a blatant elevation of the distal end of the clavicula compared to the healthy shoulder is pathognomic for a dislocation of the AC-joint [3]. Additional signs such as tenderness on palpation, bruises and patients' history may substantiate suspicion [3].

Specialized testing includes (videos at the end of the page):

• Paxino’s sign

  • • 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.

• 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. 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 noty full alleviated by supination, one should lean towards pathologies of the AC-joint.

Specialized tests for the acromioclavicular joint should not be used as a singular diagnostic tool as sensitivity and specificity are comparably low. However, a combination of both the Paxino’s sign and the O’Brien’s test has shown to have the highest likelihood ratio (2.71) and the highest specificity, when performed in series [4]. If pre-test probability is already indubitable due to the patient’s history and rough examination, specialized physical examination is of no importance [4].

To classify injuries into Rockwood types and rule out any collateral damage to the clavicula, two x-ray images, one in ap and the other in weight-bearing Alexander/Zanca view should be performed [3]. Instructions on how to interpret x-ray images can be found on Radiopedia. In addition, ultrasound and MR imaging might be used to further assess damage to ligaments or the rotator cuff [5].

Differential Diagnosis

• Fracture of clavicula

• AC joint osteoarthritis

• AC joint synovitis with all its possible causes

There is a lack of data surrounding long term outcome of acromioclavicular dislocations. This is most certainly due to the different treatment options of each Rockwood type injury. However, prognosis seems to be satisfactory, although arthrosis has been reported after both conservative and surgical treatment.