**5. Conclusions**

Modular shoulder arthroplasty is a suitable procedure for the conversion of failed HA to RSA in elderly patients, allowing a successful conversion in the majority of cases. In our study, all measured postoperative clinical outcomes improved significantly. Moreover, retaining the prosthetic stem was associated with a short operating time. Finally, the observed complication rate was acceptable, and no prosthesis-related complications occurred. Although not feasible in every case, conversion RSA is a viable treatment option in the elderly, which can provide successful mid-term results with an acceptable complication rate. Nevertheless, studies with larger sample sizes should be carried out to provide greater insight into conversion RSA outcomes in this rare indication.

**Author Contributions:** Conceptualization, F.R., U.I., M.J.K. and G.K.; methodology, F.R., U.I., M.J.K. and G.K.; investigation, F.R., U.I., M.J.K. and G.K.; writing—original draft preparation, G.K.; writing— review and editing, F.R., U.I., M.J.K. and G.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by Mathys Ltd. Bettlach. Funds helped cover a medical advisor contract and travel expenses for Georges Kohut, Ulrich Irlenbusch, Max Josef Kääb, and Falk Reuther. Mathys Ltd. Bettlach also supported the independent statistical analysis. Related to a certain study (study ID 0701).

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Ethics Committee of the Comité intercantonal d'éthique (Switzerland) on 24 September 2008 (number 01/2008).

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

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical restrictions.

**Acknowledgments:** Medical Minds GmbH provided medical writing and editorial support.

**Conflicts of Interest:** Falk Reuther, Ulrich Irlenbusch, Max J. Kääb, and Georges Kohut declare that they have received benefits for professional use from Mathys Ltd. Bettlach directly related to the subject of this manuscript (see Funding). Besides this, Mathys Ltd. Bettlach had no role in the collection or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.
