The lateral acromial angle in patients with rotator cuff tears: MRI vs radiographic measurements

Background: The lateral acromial angle (LAA) was first described in 1995 by Banas. The angle is measured using two lines, one is drawn just inferior to the sclerotic line of the acromion undersurface and a second line connects the superior and inferior border of the glenoid fossa. Values of the LAA lower than 78 degrees are associated with a higher prevalence of rotator cuff disease. Originally described on magnetic resonance imaging (MRI), this measurement has since been utilized by other authors on true antero-posterior (AP) radiographs. However, no study has yet described a difference between measuring the LAA on MRI or true AP radiographs, nor has evaluated the importance of scapular kinematics in influencing the value of this angle. Methods: The clinical data of 610 patients with shoulder pain that came to our clinic from January 2018 and March 2023 were retrospectively analyzed. After applying the inclusion and exclusion criteria, 50 patients were included in the final study population. Patients included were those with a full-thickness rotator cuff tear (n=23, 64%) and partial rotator cuff tears (n=17, 34%) diagnosed on an MRI of the shoulder. All the patients had a true AP radiograph of the shoulder taken in our institution. We measured the lateral acromial angle of all 50 patients on MRI and true AP radiograph. Results: The Wilcoxon signed-rank test showed a significant difference in LAA measurements between MRI and true AP radiographs, with MRI granting higher values (median 77.57°) compared to radiographs (median 73.05°), and a median difference of 4.84°. The mean age was 54.5 ± 9.8 years (range, 29-74 years), 54% of them were female. Pearson correlation indicated a moderate correlation between the two methods. The Intraclass Correlation Coefficient (ICC) suggested moderate to good agreement, but with notable variability. A Bland-Altman Plot was used to assess the agreement between the two measurement techniques. Conclusion: MRI provides significantly higher LAA measurements compared to true AP radiographs. Scapular kinematics likely influences these differences, making measurements from MRI and radiographs not perfectly interchangeable with a potential impact on clinical decision making.