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What is the diagnosis?

Acromioclavicular (AC) joint injury, also called AC separation or shoulder separation. In this case, grade III injury. AC joint injuries are injuries to the acromioclavicular (AC) and coracoclavicular (CC) ligaments and are classified by the Rockwood system which are graded from I-VI (image 3).       

Differential diagnosis: clavicle fracture, glenohumeral dislocation, rotator cuff tear, labral injury, proximal humerus fracture, sternoclavicular joint dislocation, and coracoid process fracture. 

What is the mechanism of injury?

Direct or indirect trauma leading to disruption of AC ligament and possibly CC ligament common in contact sports, cycling, and skiing. 

• Direct trauma: Fall onto shoulder with arm adducted causes acromion to be forced inferior and medial to the clavicle. 
• Indirect trauma: Fall onto outstretched hand or elbow causes humerus force onto acromion putting stress on AC joint [1]. 

What physical exam findings are expected?

Tenderness to palpation at the AC joint, superior elevation distal clavicle or step off at AC joint, swelling and/or bruising over superior lateral shoulder. 

Special tests: Positive cross-body test (pain with adduction of affected arm across body in horizontal plane), Positive O'brien's test, and positive piano key sign (clavicle rebounds upward after downward pressure is released [2]. 

Which imaging modalities can be used?

Radiographs: 

• Views: AP, axillary, Zanca view (special AC view), and stress views (bilateral weighted AP views). 
• Key findings: Widening of AC joint, (normal <8mm), with greater than 2-4mm asymmetry compared to the contralateral side [3]. Widening of CC distance, (normal <13mm), with greater than 5mm asymmetry compared to the contralateral side. Distal clavicle displacement superior to acromion [1]. (see Image 3 for classification system) 

Ultrasound: dynamic assessment for low grade injuries

Image 2. Normal AC joint in short axis (2A and 2B). Type I AC injury (3A). Type II AC injury (3B). Image adapted from Manske RC, Voight M, Page P, Wolfe C. MSK Diagnostic Ultrasound for the Assessment of the Acromioclavicular Joint. IJSPT. 2024;19(1):1516-1520. 

MRI: used for surgical planning in high grade injuries to evaluate ligament integrity. 

What is the management in the ED/when should you consult orthopedics?

Rockwood Grades I-III = Non-operative

• Type I: AC ligament sprain 
• Type II: AC ligament rupture, CC sprain 
• Type III: AC and CC rupture with CC distance < 25mm (Type III injuries treated non-operatively have at least equal functional outcomes with less complications and early return to sport than surgical treatment [4]). 
• Treatment for types I-III: Sling for comfort (until first follow up appointment, often 3-10 days of use, early ROM exercise and return to activity 6-12 weeks [5]. 

Rockwood Grades IV-VI = Outpatient orthopedic referral for surgical fixation 

• Type IV: posterior displacement of clavicle
• Type V: significant elevation of the clavicle, CC distance >100% of contralateral side
• Type VI: inferior displacement of the clavicle under coracoid
• Consider Type III injuries in upper extremity athletes or laborers with significant symptoms 
• Surgical options: ligament reconstruction with allograft or TightRope and sometimes ORIF with hook plate [7]. 

Image 3. Rockwood classifications showing ligamentous injuries. Case courtesy of Andrew Murphy, Radiopaedia.org, rID: 72436