ABSTRACTIt is still unknown whether glenoid inclination or lateral acromial roof extension is a more important determinant for development of rotator cuff tears (RCT) or osteoarthritis (OA) of the shoulder. It was the purpose of this study, to evaluate whether there is a potential predominance of one of these factors in pathogenesis of RCT or concentric OA. We analyzed 70 shoulders with advanced degenerative RCT and 54 shoulders with concentric OA undergoing primary shoulder arthroplasty (anatomical or reverse) using antero‐posterior radiography and multiplanar computed tomography. The two groups were compared in relation to glenoid inclination, lateral acromion roof extension, acromial height, and critical shoulder angle (CSA). All measured parameters were highly significantly different between RCT and concentric OA (p < 0.001). Based on Cohen's d effect size, group differences were most distinct in lateral acromial roof extension (1.36x‐ray, 0.92ct) compared with acromial height (1.06x‐ray, 0.73ct) and glenoid inclination (0.60x‐ray, 0.61ct). However, no single factor showed an effect size which was as high as that of the CSA (1.63x‐ray). Interestingly, a ratio of lateral acromion roof extension and acromial height could enhance the effect size (1.60x‐ray, 1.16ct) near to values of the CSA (1.63x‐ray). In summary, lateral acromial roof extension has a greater influence in pathogenesis of degenerative RCT and concentric OA than acromial height or glenoid inclination. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res