What is the best imaging protocol for evaluating the acromioclavicular (AC) joint?

Imaging for the Acromioclavicular (AC) Joint

Initial Imaging: Standard Radiography

Standard radiography with anteroposterior (AP) views is the first-line imaging modality for evaluating AC joint pathology, and should include bilateral comparison views to assess vertical instability. 1

Essential Radiographic Views

• AP view (Zanca view): The primary view for AC joint evaluation, obtained with the patient upright and the x-ray beam angled 10-15 degrees cephalad to profile the joint space 1, 2
• Bilateral comparison views: Critical for assessing vertical displacement and should be obtained routinely, as unilateral views can miss subtle injuries 3, 4
• Axillary lateral view: Essential for detecting posterior displacement (Rockwood type IV injuries), which are frequently missed on AP views alone 1, 4

Key Technical Considerations

• Upright positioning is mandatory - supine radiographs can underrepresent AC joint malalignment 5
• The axillary or scapular Y view is vital because AC and glenohumeral dislocations can be misclassified on AP views alone 5
• At least 3 views with 2 orthogonal projections should be obtained for trauma evaluation 1

Advanced Imaging for Complex Cases

When to Use MRI

MRI should be obtained when radiographs are noncontributory and there is clinical suspicion of ligamentous injury or when horizontal instability needs assessment. 6

• MRI reclassifies the Rockwood type in approximately 48% of cases compared to radiography alone, detecting additional ligamentous lesions in 25% of patients 6
• MRI is superior for characterizing soft-tissue injuries, including coracoclavicular and acromioclavicular ligament tears 2, 6
• MRI without contrast is the preferred modality (rated 9/9 appropriateness) for suspected labral tears or instability when radiographs are noncontributory 1

Role of Dynamic Imaging

• Dynamic axillary lateral views in supine position with the arm in 90° abduction and 60° flexion/extension can detect horizontal instability missed on static views 4
• Patients with horizontal instability show a gleno-acromio-clavicular angle (GACA) difference >30° compared to <7° in stable joints 4
• This functional assessment may identify previously missed unstable injuries requiring surgical intervention 4

Weighted Views: Controversial Utility

• The evidence for weighted panoramic views remains controversial and inconsistent 3
• Current systematic reviews show no clear consensus on their added diagnostic value for acute AC joint injuries 3
• Standard bilateral comparison views without weights provide adequate assessment of vertical instability with high inter-observer reliability 3

CT and Ultrasound: Limited Roles

• CT is not routinely indicated for AC joint evaluation unless there is concern for complex fractures or when MRI is contraindicated 2
• Ultrasound has minimal utility for AC joint assessment (rated 1/9 appropriateness), though it can exclude joint inflammation if fluid is detected 1, 2

Common Pitfalls to Avoid

• Never rely on AP views alone - posterior dislocations (type IV) are frequently missed without axillary views 1, 4
• Avoid supine radiography as it underrepresents malalignment 5
• Digital measurements are more precise and reliable than visual classification alone for determining injury severity 3
• Standard radiographs have variable inter-observer reliability for horizontal instability assessment, necessitating dynamic views or MRI in equivocal cases 3, 4