Dislocation of the acromioclavicular joint occurs when there is a complete separation of the joint surfaces forming this articulation.
The AC joint is held in place by the coraco-clavicular ligaments (conoid and trapezoid), the acromioclavicular ligaments and the joint capsule. Failure of these structures will result in a subluxation or dislocation of the joint. Causes of AC dislocations include axial compression through the humerus, translating up to the AC joint and tearing the associated structures. Or an inferior compression of the acromion such as in rugby scrums or ice-hockey shoulder budges which, drive the acromion inferiorly, and compromise the integrity of the joint.
Typicaly, the patient will present with pain, and oedema over the acromioclavicular joint. A deformity may be appreciated over the affected shoulder, which is characterised by a superior displacement of the clavicle. Such a deformity is usually indicative of a severe tear involving both the trapezoid, and the conoid ligaments as well as the acromioclavicular ligaments. Occasionally, fibres of trapezius and deltoid can be torn. Rarely does the clavicle displace inferiorly to ride under the acromion.
Orthopaedic examination for coracoacromial ligament tears.
The classification scheme by Rockwood and Young is largely accepted for acromioclavicular separation injuries, and subdivides this condition into six subtypes according to the degree of displacement of the distal clavicle. Palpation of the acromioclavicular joint range of motion will produce excessive motion. A definitive diagnosis can be reached via radio-imagery.