Reverse Shoulder Arthroplasty Biomechanics

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.

Keywords: biomechanics; reverse total shoulder arthroplasty; shoulder.

Figure 1

Inferior and medial translation of the CoR and humeral position with rTSA, relative to the native shoulder.

Figure 2

Increase in deltoid moment arm length with rTSA, relative to the native shoulder.

Figure 3

Modified scapulohumeral rhythm with rTSA, where additional scapular rotation occurs with rTSA relative to the scapulohumeral rhythm of the native shoulder (dotted line) during arm elevation.

Figure 4

Increase in deltoid abductor moment arm length by medializing the CoR: inset CoR glenosphere (left), standard offset glenosphere (middle), and expanded glenosphere (right).

Figure 5

Wrapping of the middle deltoid around the lateral proximal humerus generates a stabilizing compressive force; where a greater amount of deltoid wrapping results in a larger compression vector that imparts greater joint stability: native shoulder (left), medial glenoid/medial humerus design (middle), and medial glenoid/lateral humerus design (right).

Figure 6

rTSA glenoid prosthesis design classification, representative images of four glenosphere designs having equivalent articular curvatures, demonstrating that the relationship between glenoid thickness and articular radius is directly related to the lateralization of the CoR relative to the glenoid fossa.

Figure 7

rTSA humeral prosthesis design classification, examples of a medial humeral component with an inlay humeral liner (left) and a lateral humeral component with an onlay humeral liner (right).

Figure 8

rTSA prosthesis design classification.

Figure 9

Comparison of lateral and distal humeral positioning associated with two different MGLH reverse shoulder prosthesis designs.

Figure 10

Use of an augmented humeral tray with rTSA in the case of proximal humeral bone loss to improve rTSA biomechanics by restoring the greater tuberosity shape and increasing deltoid wrapping to improve stability while increasing the deltoid moment arm to improve deltoid efficiency.