A Grammont-style Reverse Total Shoulder Arthroplasty (RTSA) radically alters the native biomechanics of the glenohumeral joint. By medializing the center of rotation and distalizing the humerus, the procedure primarily enhances the mechanical advantage of which muscle?

Explanation:
The correct option is d. The fundamental biomechanical principle of the Grammont Reverse Total Shoulder Arthroplasty is the profound medialization and distalization of the joint's center of rotation. This geometry dramatically lengthens the lever arm (moment arm) of the deltoid muscle and recruits more of its anterior and posterior fibers for elevation and abduction. This allows the powerful deltoid to effectively bypass and compensate for a massive, irreparable rotator cuff tear, restoring active forward elevation in a shoulder that was previously pseudoparalytic. Reference: Boileau P et al. Grammont reverse prosthesis: design, rationale, and biomechanics. J Shoulder Elbow Surg.