There is a relationship between the acromial index, the critical shoulder angle, and rotator cuff tears.

Understanding anatomical parameters is essential not only for the treatment but also for the prevention of new rotator cuff tears. There are still controversies and conflicting results regarding the association or causal inference between the acromion index, the critical shoulder angle, and this disease. The study aimed to (1) investigate the relationship between the acromial index, the critical shoulder angle, and rotator cuff tears, as well as (2) analyze the intra- and inter-rater reliability of the measurements performed. This was a cross-sectional study conducted at a single diagnostic center using digital radiographs of shoulders with rotator cuff tears, calcific tendinitis, and controls. Three researchers independently and blindly measured the acromial index and the critical shoulder angle twice, with a 7-day interval between measurements. Intra- and inter-rater agreement was calculated using the intraclass correlation coefficient. Tukey's test was used to compare pairs of means to identify differences between the evaluated groups. The significance level for rejecting the null hypothesis was set at 5%. The sample included 56 shoulders with rotator cuff tears, 49 shoulders with calcific tendinitis, and 58 control shoulders. Intra- and inter-rater agreement was excellent in the vast majority of cases. The rotator cuff tear group had the highest mean values for both measurements compared to the other two groups (p < 0.05). There is a relationship between the acromial index, the critical shoulder angle, and rotator cuff tears, with excellent intra- and inter-rater reliability. Level of Evidence III.