Causes of Grade 3 Acromioclavicular (AC) Joint Separation
Grade 3 AC joint separation is primarily caused by direct trauma to the shoulder, typically from a fall directly onto the point of the shoulder with the arm in an adducted position. 1, 2
Mechanism of Injury
- Direct trauma to the shoulder is the most common cause, particularly when force is applied to the point of the shoulder with the arm at the side 2, 3
- Falls from a height onto the shoulder are a frequent mechanism of injury 4, 2
- Sports-related injuries, especially in contact sports like football, hockey, or rugby, commonly lead to AC joint separations 3, 5
- Motor vehicle accidents can result in AC joint separations due to high-energy impact 2, 6
- Cycling accidents where the rider falls onto the point of the shoulder 3, 5
Classification and Diagnosis
- Grade 3 AC separation is defined by complete disruption of both the acromioclavicular and coracoclavicular ligaments, resulting in superior displacement of the distal clavicle 2, 3
- The Rockwood classification system is commonly used to grade AC separations, though it has been shown to have poor to fair inter-rater reliability (kappa=0.08-0.35) 2
- Proper diagnosis requires radiographic confirmation with specific views:
- Standard anteroposterior (AP) views in internal and external rotation 1
- Axillary or scapula-Y view, which is vital for proper evaluation of AC joint alignment 1
- Zanca view (15° cephalic tilt AP view) to better visualize the AC joint 2
- Weight-bearing (stress) views may be helpful, though many surgeons don't routinely perform them 7
Risk Factors
- Young athletic population is at higher risk due to participation in contact sports 3, 5
- Previous history of shoulder or clavicle injuries may predispose to AC joint separation 4
- Age is a factor, with younger patients more commonly affected due to higher activity levels and participation in high-risk activities 6
- Inadequate protective equipment during sports participation 3, 5
Clinical Presentation
- Patients typically present with pain at the AC joint following trauma 2, 6
- Visible deformity with superior displacement of the distal clavicle ("step-off") 2, 3
- Limited range of motion due to pain, particularly with horizontal adduction and overhead movements 6, 5
- Tenderness to palpation directly over the AC joint 2, 5
- Pain exacerbated by cross-body adduction test 6, 5
Imaging Considerations
- Radiography is the preferred initial imaging modality for suspected AC joint separation 1
- Upright positioning is essential during radiography as malalignment can be underrepresented on supine imaging 1
- CT may be considered for complex cases but is not typically necessary for initial diagnosis of grade 3 AC separations 1
- MRI is generally not indicated for isolated AC joint injuries unless concomitant soft tissue injuries are suspected 1
Understanding the mechanism of injury and proper diagnostic approach is crucial for appropriate management of grade 3 AC separations, which remains controversial between operative and non-operative treatment options 2, 3, 6.