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Acromioclavicular (AC) joint injuries 

What is the AC joint? 

The AC joint is a plane synovial joint formed between the acromion of the scapula (shoulder blade) and the distal end of the clavicle (collarbone), on top of the shoulder. 

The joint is stabilised by the acromioclavicular (AC) ligaments, coracoclavicular (CC) ligaments and the coracoacromial (CA) ligament (Kiel & Kaiser, 2020). This joint is small but contributes to achieving full shoulder range of motion. 


AC injuries are common in sports such as football, rugby league and rugby union. They may account for up to 40% of all shoulder injuries and nearly 10% of all injuries in collision sports (Kiel & Kaiser, 2020). 

Mechanism of injury 

The AC joint is often damaged by a fall onto the point of the shoulder, or a direct blow to the shoulder (ie a hip and shoulder bump in footy). This compresses the AC joint and can damage the ligaments which stabilise the joint (Brukner & Khan, 2017). 

 Diagnosis is made on a number of features: 

  • Mechanism of injury 
  • Step deformity (visible bump on top of the shoulder – see photo below). The bigger the step deformity generally means a highergrade injury. 
  • Tenderness on palpation over the AC joint 
  • Pain with overhead and horizontal adduction (crossing the arm over the body). This movement compresses the AC joint.


Grades of AC joint injury – grading is based on the Rockwood classification system (see photo below) 

  • Grades 1-2 AC joint injuries involve only minor damage to the ligaments which stabilise the joint. These injuries are managed conservatively (no surgery) with physical rehabilitation. 
  • Grade 3 injuries involve complete separation of the ligaments which stabilise the AC joint. These can be managed either conservatively or surgically. The decision to undertake surgical management is determined by a range of factors including your age, current level of function and goals.  
  • Grade 4-6 injuries are managed surgically due to the larger degree of separation between the clavicle and acromion. These injuries require surgical intervention to stabilise the joint, followed by a comprehensive rehabilitation program (Brukner & Khan, 2017)These high-grade injuries are not common, but without surgery these injuries can cause ongoing shoulder pain and/or instability.  


 It is important to see a healthcare professional to rule out other possible injuries. Some other key things to rule out in an acute shoulder injury include: 

  • Fractures – particularly to the scapula, clavicle & proximal humerus 
  • Rotator cuff tears 
  • Neurovascular damage (nerves and blood vessels) 
  • Glenohumeral joint instability 
  • Glenoid labrum tears 



Rehabilitation timeframes will depend on the severity of the AC joint injury. All injuries will progress through a series of phases which will involve: 

  • Reducing pain and swelling 
  • Increasing muscular control & endurance 
  • Increasing muscle strength 
  • Re-integration into sport-specific skills

Works Cited 

Brukner, P., & Khan, K. (2017). Clinical Sports Medicine. Elsevier.
Kiel, J., & Kaiser, K. (2020). Acromioclavicular Joint Injury. StatPearls. 

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