The true axial shoulder projection: Diagnostic aid for acromioclavicular joint dislocation?

Introduction: Acromioclavicular joint dislocations are common. Differentiating between horizontal and vertical instability is challenging, but possible to diagnose on axial radiographs. No clear consensus for axial radiograph parameters currently exists. We aim to establish a reproducible technique to assess whether an axial radiograph represents a true axial view of the shoulder.

Methods: One hundred CT scans of normal uninjured shoulders were examined using multiplanar reformatting to assess the distance between the anterior glenoid (reference line) and anterior-most aspect of the acromion in the axial plane. Measurements were repeated and performed by multiple observers to give of intra- and inter-observer reliability.

Results: The mean distance from the anterior acromion to the reference line was -2.6 mm (i.e. posteriorly placed) (SD = 5.8 mm, range -16.9-13.2 mm). Most (89%) of the measurements were between 9 and -9 mm to the reference line. Intra-observer reliability was high with Cronbach's α measurement as 0.997. Inter-observer reliability gave a Cronbach's α measurement of 0.959.

Conclusion: When the anterior aspect of the acromion lies within 10 mm either side of a line parallel to the scapula blade at the anterior aspect of the glenoid on an axial radiograph, it represents a true axial projection of the glenohumeral joint (GHJ).Level of evidence: IV, Case series.