Total shoulder arthroplasty is an effective procedure to improve patients function and to relieve shoulder pain in cases of glenohumeral arthritis. The main challenge of this procedure is implanting the glenoid component. Indeed, surgical exposure is difficult and proper visualization of the glenoid is demanding. The current free-hand method is not precise and especially unreliable in glenoids with severe deformity. Inappropriate reaming and glenoid positioning can cause more bone loss during surgery. In anatomic prostheses, glenoid component fixation is demanding and is the first component to present loosening at long term follow-up. When it comes to reverse arthroplasty, the glenoid component, once again, requires the most attention. Malpositioning of the glenosphere can cause notching, loosening, and instability. In order to prevent these complications, precision in glenoid implantation is key. To this end, intraoperative computer navigation would be a helpful tool, but the increased expenses in computer equipment represent massive costs and possibly increases in surgical time. A more affordable option is a personalized guide, for which the only extra expense is the customized guide. These guides are based on precise 3D CT scan templates. Previous studies have evaluated the precision of the guide in vivo and in vitro, but none have measured it in a randomized study. As it represents additional costs, its efficacy needs to be proven before widespread use. This method may also provide secondary benefits, such as decreasing operating time, lowering short- and long-term complications, and improving efficacy. The objective of this study is to evaluate the performance of a patient specific glenoid guide.