Fig. 1.

(a–d) Biomechanical improvements of the reverse total shoulder implants. a The natural center of rotation (COR) and deltoid lever arm (DL) in a native shoulder. (b) Starting with the Grammont implant, more modern reverse total shoulder arthroplasty implants medialize and distalize the center of rotation, which minimizes shear forces (FS), and increases compressive forces (FC), to create an overall favorable force vector (FV) at the bone-glenoid interface, as well as re-tensions the deltoid to provide a mechanical advantage. (c) In a native shoulder, the middle deltoid (red) and part of the anterior deltoid (light red) provide an abduction force. The posterior deltoid in dark red provides extension force. (d) With medialization of the center of rotation in a reverse total shoulder arthroplasty, a larger part of the anterior deltoid and posterior deltoid are recruited and contribute to the active abduction force. (Adapted from Berliner et al, JSES 2015 and Boileau et al, JSES 2005)