Radiographic Evaluation of the Acromioclavicular Joint
For AC joint evaluation, obtain standard anteroposterior (AP) radiographs with at least three orthogonal views: AP views in internal and external rotation, and an axillary or scapula-Y view to adequately assess both vertical and horizontal stability. 1
Standard Imaging Protocol
The orthogonal view concept for AC joint evaluation requires multiple complementary projections because standard AP radiographs alone are inadequate due to overlapping structures 1:
- The Zanca view (AP with 10-15° cephalic tilt) is the most accurate single view for examining the AC joint itself 2
- An axillary or scapula-Y view is essential as the second orthogonal projection to differentiate injury types, particularly distinguishing type III from type IV injuries where posterior displacement occurs 2
- Bilateral comparison views are critical, as the vertical coracoclavicular interval difference between sides should be less than 50% in normal patients 3
Key Measurements and Reliability
The vertical coracoclavicular interval demonstrates excellent inter-observer reliability (ICC 0.918) and is the most reproducible parameter for assessing AC joint injuries 4, 3:
- Vertical instability assessment using bilateral projections shows high reproducibility 4
- Horizontal instability assessment shows much more variable reliability between observers 4
- Digital measurements are more precise and reliable than visual classification alone 4
Specialized Views
The Rockwood view (upright 30° angled caudad radiograph) can supplement standard views to better visualize the sternoclavicular region when needed 1.
Newer axial and tangential projections have been described that can demonstrate the AC joint clearly and facilitate percutaneous procedures, though these are not yet standard 5.
Common Pitfalls
- Weighted views remain controversial with no clear evidence supporting their routine use over standard bilateral comparison views 4
- Standard AP radiographs without orthogonal views miss posterior dislocations (type IV injuries) 2
- In skeletally immature patients, the vertical coracoclavicular interval should be <11 mm (males) or <10 mm (females) in the 90th percentile 3
Advanced Imaging Considerations
MRI changes the Rockwood classification in approximately 48% of cases compared to radiography alone, identifying additional ligamentous injuries in 25% of patients 6. However, radiography with proper orthogonal views remains the initial imaging modality for acute AC joint injuries 4, 2.