What is it?
The acromioclavicular joint is the area between the collarbone (clavicle) and part of the shoulder blade (acromion). It's stabilised by several ligaments surrounding the joint with injury being referred to as ‘shoulder separation’. The degree of damage can vary from a mild sprain to severe disruption or tears causing deformity.
Why does it occur?
This injury most commonly arises by direct trauma through impact sustained during contact sports or falling on an outstretched arm.
Repetitive overhead use of the arm however, can precipitate arthritis of the acromioclavicular joint and lead to injury. This is common in manual labourers where overloading the shoulder is a common contributory factor. Bony spurs (osteophytes) are generated around the arthritic joint causing damage to the ligaments.
What are the symptoms?
Pain, either localised tenderness over the AC joint or a generalised aching of the shoulder. A step deformity, bruising, swelling and audible clicking or crepitus during movement are all signs of joint instability. You may experience a restricted range of movement particularly in abduction of the arm.
How is it diagnosed?
History of the injury and physical examination of the shoulder joint is required. Further imaging with radiographs may be taken whilst holding a weight in the affected arm. Any increase in AC joint space can be indicative of ligament disruption and arthritis may be seen. AC joint stability can be assessed using ultrasound, whereas magnetic resonance imaging is useful to delineating the extent of soft tissue injury and also allows shoulder joint assessment.
How is it treated?
This depends on the grade of ACJ injury, which can range from a mild sprain to complete disruption (graded 1 to 6). Most injuries are classified as grade 1 or 2 which settle with conservative management involving a sling, rest, ice and analgesia. A phased return to normal activity is advised with recovery taking anywhere from 6 to 12 weeks. Grade 3 injuries can be managed either way, with surgery being considered after a trial of conservative management where symptoms persist.
The most severely disrupted grades 4 to 6 are rare due to the high energy injury needed to cause this extent of damage to the joint. All injuries of these grades benefit from surgical fixation using pins or repair of the ligaments surrounding the AC joint.