**5. Conclusions**

The use of lateralized RSA with a low neck-shaft angle humeral stem results in significantly improved external rotation compared to medialized RSA with a 155◦ humeral stem, even in cases of severe fatty infiltration of the infraspinatus and the teres minor. Humeral lateralization and a low neck-shaft angle should be favored when planning an RSA in patients without a functional posterior rotator cuff. On the other hand, the medialized humerus with a 155◦ inlay stem contributed to a greater anterior forward flexion than the other configuration. However, the change in ROM amongs<sup>t</sup> groups did not affect the postoperative clinical outcome.

**Author Contributions:** Conceptualization: A.L. and P.C.; methodology: H.B.; formal analysis: H.B.; investigation: S.O., H.K. and M.N.; resources: P.C.; data curation: S.O. and H.B.; writing: M.N., R.T. and A.L.; original draft preparation: M.N., S.O. and H.K.; writing—review and editing: P.C., A.L. and R.T.; project administration: P.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was founded by a gran<sup>t</sup> from FORE (Foundation for Research and Teaching in Orthopaedics, Sports Medicine, Trauma and Imaging in the Musculoskeletal System): gran<sup>t</sup> number 2021-1.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee) of Recherche Clinique Vivalto Santé (protocol code CERC-VS-2018-06-1, date of approval: 29 June 2018).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** All the available data have been presented in this study. Details regarding where data supporting reported results can be requested at the following e-mail address: hugo.bothorel@latour.ch.

**Conflicts of Interest:** Alexandre Lädermann reports grants and personal fees from Arthrex, grants and personal fees from Medacta, grants and personal fees from Stryker, outside the submitted work. He is the founder of the FORE, of BeeMed, and Med4cast. Philippe Collin reports that he is a paid consultant for Stryker and Arthrex and receives royalties from Stryker. Rihard Trebše reports personal fees from Zimmer Biomet, personal fees from Medacta, personal fees from De Puy. Other authors declare no conflict of interest.
