Aims: The objective of this study was to compare simulated range of motion (ROM) for reverse total shoulder arthroplasty (rTSA) with and without adjustment for scapulothoracic orientation in a global reference system. We hypothesized that values for simulated ROM in preoperative planning software with and without adjustment for scapulothoracic orientation would be significantly different., Methods: A statistical shape model of the entire humerus and scapula was fitted into ten shoulder CT scans randomly selected from 162 patients who underwent rTSA. Six shoulder surgeons independently planned a rTSA in each model using prototype development software with the ability to adjust for scapulothoracic orientation, the starting position of the humerus, as well as kinematic planes in a global reference system simulating previously described posture types A, B, and C. ROM with and without posture adjustment was calculated and compared in all movement planes., Results: All movement planes showed significant differences when comparing protocols with and without adjustment for posture. The largest mean difference was seen in external rotation, being 62° (SD 16°) without adjustment compared to 25° (SD 9°) with posture adjustment (p < 0.001), with the highest mean difference being 49° (SD 15°) in type C. Mean extension was 57° (SD 18°) without adjustment versus 24° (SD 11°) with adjustment (p < 0.001) and the highest mean difference of 47° (SD 18°) in type C. Mean abducted internal rotation was 69° (SD 11°) without adjustment versus 31° (SD 6°) with posture adjustment (p < 0.001), showing the highest mean difference of 51° (SD 11°) in type C., Conclusion: The present study demonstrates that accounting for scapulothoracic orientation has a significant impact on simulated ROM for rTSA in all motion planes, specifically rendering vastly lower values for external rotation, extension, and high internal rotation. The substantial differences observed in this study warrant a critical re-evaluation of all previously published studies that examined component choice and placement for optimized ROM in rTSA using conventional preoperative planning software., Competing Interests: S. Poltaretskyi is an employee of Arthrex. P. Raiss is a paid consultant for Arthrex, and also receives speaker payments or honoraria and support for attending meetings and/or travel from Arthrex, and stock or stock options from Zurimed, all of which are unrelated to this study. P. Siegert reports software support from Arthrex for this study. B. C. Werner reports funding from Arthrex for this study, as well as consulting fees from Arthrex and Lifenet, and speaker payments or honoraria from Arthrex which are unrelated to this study. P. Moroder reports software support and article processing charges funding from Arthrex for this study, as well as consulting fees from and a patent submission with Arthrex, which are related to this study. P. Denard reports grant funding from Arthrex for this study, as well as royalties or licenses, consulting fees, and speaker payments or honoraria from Arthrex which are unrelated to this study. P. Denard also reports speaker payments or honoraria from Pacira and stock or stock options from PT Genie, unrelated to this study. B. J. Erickson reports consulting fees from Arthrex, unrelated to this study. J. Griffin reports royalties or licenses, consulting fees, speaker payments or honoraria, support for attending meetings and/or travel, and patents from Arthrex, all of which are unrelated to this study., (© 2024 Moroder et al.)