Axial Rotation Moment Arms of the Shoulder Musculature After Reverse Total Shoulder Arthroplasty

Background Reverse total shoulder arthroplasty changes the lines of action of the shoulder muscles, resulting in increases in the moment arms of the major abductors and flexors of the glenohumeral joint; however, at present little is known about the axial rotation capacity of the musculature after this procedure. The purpose of this study was to measure the instantaneous axial rotation moment arms of all of the major muscles spanning the glenohumeral joint during abduction and flexion after reverse total shoulder arthroplasty. Methods Reverse total shoulder arthroplasty was performed on eight entire cadaveric upper extremities. Specimens were mounted onto a testing apparatus, and the internal/external rotation moment arms of eighteen major muscle subregions involving the subscapularis, supraspinatus, infraspinatus, teres minor, teres major, deltoid, pectoralis major, and latissimus dorsi were measured during abduction and flexion. These muscle moment arms were compared with those measured preoperatively in the anatomical shoulders (i.e., before the arthroplasty). Results Reverse total shoulder arthroplasty resulted in loss of external rotation function in the posterior deltoid subregion. Postoperatively, the inferior subscapularis subregion had the largest internal rotation moment arm overall, whereas the teres minor and the inferior infraspinatus subregion had the greatest external rotation moment arms. The teres minor, infraspinatus, and deltoid subregions were external rotators during abduction, whereas only the teres minor, infraspinatus, and to a small extent the posterior deltoid subregion were external rotators during flexion. Conclusions Reverse total shoulder arthroplasty results in an overall decrease in the external rotation moment arm of the deltoid and increases in the moment arms of the major internal rotators, including the latissimus dorsi and pectoralis major. Reverse total shoulder arthroplasty may result in complete loss of external rotation function if the teres minor and infraspinatus muscles are damaged. Clinical Relevance Care should be taken to avoid damaging the infraspinatus and the teres minor during a reverse shoulder arthroplasty, as these are the only major external rotators after that procedure. Greater external rotation may be achieved postoperatively when the humerus is positioned in the coronal plane rather than the sagittal plane.