Introduction Acromial morphology is an important pathophysiological factor for the development of subacromial impingement syndrome. There are three radiological methods to evaluate acromial morphology: Bigliani, Modified Epstein, and Acromial angle. However, their reliability have not been compared in a single study, nor using standardized radiographs. Consequently, the evaluation of acromial morphology is currently not validated though its widespread use across the world. The objective of this study was to investigate reliability of the three known classifications and the novel Acromial curve classification.

Materials and Methods Three experienced clinicians rated 102 standardized supraspinatus outlet view radiographs with the four classification methods in two separate sessions a month apart. All measurements were blinded. With an expected kappa and ICC > 0.7 (± 0.15), the target sample size was 87 radiographs.

Results The Bigliani classification had interrater and intrarater reliability ranging from fair to good (Kappa 0.32–0.41 and 0.26–0.62). The modified Epstein classification had fair to good interrater and intrarater reliability (Kappa 0.24–0.69 and 0.57–0.63). The Acromial angle classification had moderate to good interrater and intrarater reliability (Kappa 0.53–0.60 and 0.59–0.72). The novel Acromial curve classification showed moderate to good interrater and intrarater reliability (ICC 0.66–0.71 and 0.75–0.78, respectively).

Conclusion The Acromial curve classification was the only classification method with an ICC value > 0.7. The popular Bigliani method had the worst reliability. The Acromial curve classification produces numerical data, as opposed to the other three classification methods. This could potentially be utilized in future research to establishing cut-off values for treatment stratification.