Torque provides a more comprehensive assessment of reverse total shoulder arthroplasty lateralization than moment arms or muscle forces.

Abstract

Modern reverse total shoulder arthroplasty (rTSA) designs allow for lateralization of glenoid and humeral components. Prior studies have assessed lateralization biomechanics by evaluating moment arms and muscle forces. However, torque, the product of muscle force and moment arm, is the most direct measure of a muscle's ability to generate joint rotation and a more comprehensive metric to assess rTSA lateralization. Our aim was to evaluate how different lateralization strategies affect the glenohumeral joint torque generated by the rotator cuff and deltoid muscles. To achieve this, we created 16 subject-specific biomechanical shoulder models using computed tomography scans. Four implant configurations were virtually implanted into each subject: medialized glenoid - medialized humerus (MG/MH), medialized glenoid - lateralized humerus (MG/LH), lateralized glenoid - medialized humerus (LG/MH), lateralized glenoid - lateralized humerus (LG/LH). For each design, we evaluated the maximum torque generated by the middle deltoid during scapular plane elevation, alongside the maximum torque generated by the infraspinatus and subscapularis during external and internal rotation at 90° of abduction. The MG/LH configuration generated the greatest middle deltoid torque during early arm elevation, despite having an equivalent moment arm to the MG/MH. Infraspinatus and subscapularis torque were enhanced by progressive lateralization resulting in the MG/MH demonstrating the least torque generating capacity and the LG/LH configuration the greatest, but at the expense of excessive middle deltoid strain, which may increase acromial stress fracture risk. Our findings suggest that joint torque provides a more comprehensive assessment of rTSA lateralization than moment arms or muscle force production alone.