Background: An acromiohumeral interval (AHI) narrower than 7 mm measured on AP shoulder radiographs has been considered pathology and strongly indicative for rotator cuff tears. No study to date has investigated the effect of radiographic beam position on acromiohumeral interval (AHI) measurement.
Objective: To study the AHI measurement on conventional radiographs compared with 3D computed tomography (3D-CT)
scans and the effectiveness of tilted CT images in physician-performed AHI measurement.
Material and Method: Twenty-eight patients were measured the AHI on conventional radiographs. The same measurement
was performed on 3D-CT image tilted in coronal plane -15 degree, 0 degree (Baseline), +15-degree, +30-degree and +45-
degree views. For baseline 3D-CT scan, the image position was set as 30 degrees medial and 10-degree downward tilts.
Results: The mean AHI on conventional radiographs (8.8+2.4.mm) is significantly more than the AHI on 3D-CT image (7.10+1.5 mm, p = 0.002). The AHI on conventional radiographs and baseline 3D-CT image showed significant moderate to high correlation (r = 0.647, p<0.001). The upward 3D-CT angle affected the AHI significantly (p = 0.002).
Conclusion: The AHI measurement on conventional radiographs is significantly higher than 3D-CT scan, with moderate to high correlation. The different position of the CT image tilts affected the AHI measurements.

Keywords: Shoulder, Acromiohumeral interval, AHI, Computed tomography, Radiographic measurements