Anatomic versus reverse total shoulder replacement with an intact rotator cuff: a comparative musculoskeletal...

Reverse total shoulder replacement (rTSR) is increasingly being conducted, including in selected elderly patients with intact-cuff osteoarthritis, due to concerns about subsequent cuff failure or glenoid component loosening after anatomic total shoulder replacement (aTSR). However, the biomechanical differences between rTSR and aTSR remain insufficiently understood. This computational study aimed to compare the biomechanics of aTSR and rTSR with an intact rotator cuff under identical conditions, representing selected intact-cuff osteoarthritis cases in which either implant design may be considered.

A validated musculoskeletal multibody simulation framework of the upper extremity was used to compare the glenohumeral (GH) biomechanics during arm elevation in the scapular plane. The GH joint was modeled in detail with six degrees of freedom and explicit contact between the articulating implant components. This enabled the evaluation of muscle forces, deltoid-to-cuff force ratio, GH joint contact forces, shear-to-compression joint force balance, contact area, contact pressure, center of pressure, and GH translations.

Compared with aTSR, rTSR reduced the mean middle deltoid muscle force by 16% and decreased overall rotator cuff force requirements, while increasing deltoid-to-cuff dominance throughout most of the motion. Furthermore, rTSR showed 26.5% and 33.4% lower compressive and resultant GH contact forces, but earlier and 98.7% higher peak superior shear force and a higher shear-to-compression joint force ratio. Contact pressure was 3.5 times lower in rTSR, whereas the contact area was nearly three times larger. Center-of-pressure trajectories differed substantially between designs, in both magnitude and direction. GH translations were more constrained in rTSR, with negligible posterior-anterior displacements and smaller overall translations.

Under an intact-cuff condition, aTSR and rTSR produced altered mechanical strategies for arm elevation. rTSR was characterized by more deltoid-dominant loading, lower joint compression, altered shear force patterns, and more constrained joint translations. This study provides a biomechanical insight for interpreting clinically relevant differences between the two mechanically distinct TSR designs. These findings may help surgeons better understand design-specific trade-offs in joint loading, shear-to-compression joint force balance, and muscle function, and may support clinical decision-making, particularly in intact-cuff conditions.