Reverse shoulder arthroplasty (RSA) is a well-established treatment for rotator cuff arthropathy. However, the effectiveness of RSA has not been compared to non-surgical treatment in a randomised controlled trial. Shoulder exercises may be an effective treatment for reducing pain and improving function in glenohumeral osteoarthritis. The primary aim of this trial is to examine if RSA followed by standard postsurgical rehabilitation is superior to a 12-week exercise programme in patients with rotator cuff arthropathy eligible for unilateral RSA. The investigators hypothesise that surgical intervention followed by standard rehabilitation, results in clinically relevant (14-point, on a scale from 0-100) improvement compared to the exercise intervention.