This is a randomized controlled trial comparing the DELTA Xtend Reverse Shoulder System Lateralized Glenosphere Line Extension (intervention group) with the standard DELTA Xtend Reverse Shoulder System (control group). All Danish citizens with rotator cuff arthropathy or degeneration of the glenohumeral joint with severe posterior wear indicating a reverse total shoulder arthroplasty referred to the orthopedic department at Herlev and Gentofte Hospital, Copenhagen University Hospital will be considered for participation in the trial. The following exclude from participation in the study: Below 50 years of age; Cognitive or linguistic impairment; insufficient glenoid bone stock; previous fracture in the upper extremities; autoimmune mediated inflammatory arthritis.A total of 122 patients will be included of which 56 will be part of the roentgen radiostereometric analysis. This will allow a maximum of 20% drop out. The primary outcome is magnitude and pattern of migration of the glenoid component and functional outcome by Western Ontario Osteoarthritis of the Shoulder Index (WOOS score). The secondary outcomes are position of arthroplasty, loosening, inferior scapular notching, patient-reported outcomes, functional outcome, readmission, complications, revisions, changes in bone mineral density of the proximal humerus assessed by duel energy x-ray absorptiometry and economy (cost utility analysis). The patients are examined before the operation and 1 week and 3, 6, 12 and 24 months after the operation.