Acromioclavicular (AC) joint dislocations and subluxations are commonly known as shoulder separations.
The true incidence is much higher than reported because many patients with low grade injury don’t seek medical attention.
Injuries to the acromioclavicular joint usually are the result of downward force on the acromion.
The most common mechanism of injury is a fall directly onto the shoulder.
The clavicle rests against the first rib, and the rib blocking further displacement of the clavicle. If the result is not a clavicle fracture, then acromioclavicular and coracoclavicular ligaments are ruptured.
Mostly these injuries occur in sports but are also common in motor vehicle accidents. This same mechanism of injury can result in sternoclavicular dislocations, fractures of the clavicle or acromion.
These dislocations are classified into six subtypes with severity increasing with the increasing grade.
The acromioclavicular and coracoclavicular ligaments are important stabilizers of this joint and they are disrupted in type 3.
Classification of acromioclavicular injuries.
Type I- Neither acromioclavicular nor coracoclavicular ligaments are disrupted. The radiographs of the AC joint are normal, except for mild soft tissue swelling, as compared with the uninjured shoulder. There is no widening, no separation, and no deformity.
Type II- Acromioclavicular ligament is disrupted, and coracoclavicular ligament is intact.The lateral end of the clavicle may be slightly elevated. The AC joint, when compared with the normal side, may appear to be widened. The widening probably is the result of a slight medial rotation of the scapula and slight posterior displacement of the clavicle by the pull of the trapezius muscle.
The coracoclavicular space of the injured shoulder is the same as that of the normal shoulder
Type III- Both ligaments are disrupted. The distal clavicle appears to be displaced superiorly as the scapula and shoulder complex droop inferomedially. The deltoid and trapezius muscles are detached from the distal clavicle.
Type IV- Ligaments are disrupted, and distal end of clavicle is displaced posteriorly into or through trapezius muscle. The clavicle is posteriorly displaced into or through the trapezius muscle as the force applied to the acromion drives the scapula anteriorly and inferiorly.
This is a very rare injury.
Type V-Ligaments and muscle attachments are disrupted, and clavicle and acromion are widely separated. The distal clavicle has been stripped of all its soft tissue attachments (i.e., AC ligaments, coracoclavicular ligament, and the deltotrapezius muscle attachments) and lies subcutaneously near the base of the neck.
Type VI- Ligaments are disrupted, and distal clavicle is dislocated inferior to coracoid process and posterior to biceps and coracobrachialis tendons.
Radiography
Acromioclavicular joint dislocations could be easily missed in routine radiographs. Therfore special views have been devised for these injuries. In addition to routine anteroposterior and lateral views fl=ollowing readiographic studies may be required.
- Stryker Notch view
- Stress radiography
Treatment
Type I and some cases of Type II can be managed non operatively. Rest of the injuries require operative treatment.
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