**5. Conclusions**

The strong correlation observed here between the posterior acromion extension, in particular the APA, and glenoid retroversion in normal scapulae suggests that the APA might be used as a predictive anatomical parameter or risk factor for the development and progression of primary glenohumeral osteoarthritis associated with posterior glenoid wear. However, the identified critical APA value at 15 degrees should now be further investigated in larger patient series with osteoarthritic scapulae/glenoids, and if possible elderly controls without any sign of glenohumeral OA. Moreover, long-term clinical studies should evaluate the impact of the APA on clinical function and surgical revision rates after

shoulder arthroplasty. Finally, we should also examine whether the APA is associated with other shoulder disorders or specific treatment outcomes.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/jcm11020351/s1, Figure S1: Correlation matrix among acromion landmarks, acromion angles and glenoid retroversion angle.

**Author Contributions:** Conceptualization, A.T. and A.F.; methodology, A.T., F.B. and P.G.; software, A.T.; validation, F.B. and P.G.; formal analysis, A.T., F.B. and P.G.; investigation, A.T., F.B. and P.G.; resources, A.T.; data curation, F.B. and P.G.; writing—original draft preparation, A.T. and P.G.; writing—review and editing, F.B., F.V. and A.F.; funding acquisition, A.T. and A.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by Wright-Tornier (Wright Medical Group, Inc., Memphis, TN, USA) and the "Lausanne Orthopedic Research Foundation".

**Institutional Review Board Statement:** This retrospective observational study was approved by the institutional ethics committee (CER-VD protocol number 505-15).

**Informed Consent Statement:** Patient informed consent was waived after study approval by the institutional ethics committee (CER-VD protocol number 505-15).

**Data Availability Statement:** Authors agree to make data supporting the results or analyses presented in their paper available upon reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
