Abstract
The reverse total shoulder arthroplasty (rTSA) prosthesis has been demonstrated to be a viable treatment option for a variety of end-stage degenerative conditions of the shoulder. The clinical success of this prosthesis is at least partially due to its unique biomechanical advantages. As taught by Paul Grammont, the medialized center of rotation fixed-fulcrum prosthesis increases the deltoid abductor moment arm lengths and improves deltoid efficiency relative to the native shoulder. All modern reverse shoulder prostheses utilize this medialized center of rotation (CoR) design concept; however, some differences in outcomes and complications have been observed between rTSA prostheses. Such differences in outcomes can at least partially be explained by the impact of glenoid and humeral prosthesis design parameters, surgical technique, implant positioning, patient-specific bone morphology, and usage in humeral and glenoid bone loss situations on reverse shoulder biomechanics. Ultimately, a better understanding of the reverse shoulder biomechanical principles will guide future innovations and further improve clinical outcomes.
Highlights
As taught by Paul Grammont, the medialized center of rotation fixed-fulcrum prosthesis increases the deltoid abductor moment arm lengths and improves deltoid efficiency relative to the native shoulder
Such differences in outcomes can at least partially be explained by the impact of glenoid and humeral prosthesis design parameters, surgical technique, implant positioning, patient-specific bone morphology, and usage in humeral and glenoid bone loss situations on reverse shoulder biomechanics
The reverse total shoulder arthroplasty prosthesis is geometrically different than the native glenohumeral joint, and these differences in form substantially modify how the shoulder mechanically functions
Summary
This mechanical stop converts the deltoid force into rotation, and in doing so, functionally restores joint stability for patients with an unstable shoulder. Reversing the articular concavities with rTSA is a functional substitute for the concavity compression stability mechanism that occurs in the native shoulder by the rotator cuff pulling the convex humeral head articular surface into the concave glenoid articular surface. This coordinated action of the rotator cuff muscles converts the superiorly directed pull of the deltoid into arm elevation and rotation [1].
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