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Biomechanical Summary of Reverse Shoulder Arthroplasty

March 15, 2015

Contributors: Lynn A. Crosby, MD; Pierre-Henri Flurin, MD; Thomas W Wright, MD; Joseph D Zuckerman, MD; Howard D. Routman, DO; Christopher Roche, MS, MBA; Matthew L. Hansen, MD; Matthew L. Hansen, MD

INTRODUCTION: Reverse shoulder arthroplasty (rTSA) inverts the concavities to create a fixed fulcrum to restore stability and facilitate arm elevation. Inverting the concavities causes an inferior-medial shift in the center of rotation (COR) and position of the humerus that changes muscle lengths and moment arms, altering the relationship of each muscle relative to its normal physiologic function.

DISCUSSION: Different rTSA prosthesis designs impact how the position of the COR and humerus change relative to the anatomic shoulder. Designs that lateralize the COR also lateralize the humerus to better tension the rotator cuff and minimize scapular impingement. However, lateralizing the COR increases the torque on the glenoid fixation interface and decreases the length of the deltoid moment arms. Decreasing the deltoid moment arms reduces its efficiency and requires a greater force to elevate the arm. Prosthesis designs that lateralize the humerus without lateralizing the COR may better restore rotator cuff tension while maintaining deltoid moment arm lengths and minimizing torque on the glenoid fixation interface. Glenoid implant positioning alters rTSA biomechanics. Inferior placement avoids scapular notching but further shifts the COR and humerus which elongates the deltoid, reduces deltoid wrapping, and may increase the risk of acromial stress fractures. Additionally, inferior glenoid placement increases the adduction capability of the subscapularis and infraspinatus, counteracting the work of the deltoid. Due to this increased adduction capability, repair of the subscapularis with rTSA is controversial. Subscapularis repair is recommended with prosthesis designs that medialize the humerus to decrease the instability risk; however, repair may not be necessary with lateral humerus designs which better tension the rotator cuff. Not repairing the subscapularis also reduces the force required by the deltoid as it no longer counteracts the deltoid.

CONCLUSION: Using rTSA in eroded glenoids medializes the joint line, shortens the rotator cuff, and reduces deltoid wrapping. Excessive joint medialization can eliminate deltoid wrapping and cause the deltoid to generate a distraction force. When using rTSA in worn glenoids, the joint line should be lateralized with bone graft, augmented glenoid baseplates, or thicker glenospheres.

Results for "Total Shoulder Arthroplasty"

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