To evaluate whether implant design, glenoid positioning, and other factors influenced instability and scapular notching in reverse total shoulder arthroplasty. We retrospectively reviewed records of patients who had undergone reverse total shoulder arthroplasty by the senior author from July 2004 through October 2011 and who had at least 24 mo of follow-up. The 58 patients who met the criteria had 65 arthroplasties: 18 with a Grammont-type prosthesis (Grammont group) and 47 with a lateral-based prosthesis (lateral-design group). We compared the groups by rates of scapular notching and instability and by radiographic markers of glenoid position and tilt. We also compared glenoid sphere sizes and the number of subscapularis tendon repairs between the groups. Rates were compared using the Fisher exact test. Notching severity distribution was compared using the test of association. Significance was set at < 0.05. The Grammont group had a higher incidence of scapular notching (13 of 18; 72%) than the lateral-design group (11 of 47; 23%) ( < 0.001) and a higher incidence of instability (3 of 18; 17%) than the lateral-design group (0 of 47; 0%) ( = 0.019). Glenoid position, glenoid sphere size, and subscapularis tendon repair were not predictive of scapular notching or instability, independent of implant design. With the lateral-based prosthesis, each degree of inferior tilt of the baseplate was associated with a 7.3% reduction in the odds of developing notching (odds ratio 0.937, 95%CI: 0.894-0.983). The lateral-based prosthesis was associated with less instability and notching compared with the Grammont-type prosthesis. Prosthesis design appears to be more important than glenoid positioning.