Normalized preoperative muscle volume correlates with shoulder strength at minimum 2 years after lateralized reverse total shoulder arthroplasty.

Background: Postoperative strength is an important outcome measure after reverse total shoulder arthroplasty (rTSA). The objective of our study was to evaluate whether preoperative deltoid and rotator cuff muscle volume correlate with clinical outcomes and shoulder force testing at a minimum follow-up of 2 years after rTSA. Methods: A retrospective study of 39 patients (27 females and 12 males) undergoing rTSA was performed and the muscle volumes of deltoid and rotator cuff musculature were evaluated using a validated preoperative planning computed tomography protocol (Glenosys 10.6.4; Imascap, Brest, France), which automatically calculates muscle volumes and 3-dimensional fatty infiltration in percent. Absolute deltoid and rotator cuff muscle volumes were normalized to the humeral head size. Clinical follow-up was carried out at a minimum of 2 years. Strength testing in 90° of abduction (ABD 90), in external rotation (ER) with the arm at the side, the Constant score, and the subjective shoulder value were recorded. Results: Normalized preoperative deltoid volume, total muscle volume (deltoid, supraspinatus, infraspinatus, subscapularis, and teres minor), and combined deltoid + infraspinatus volume were strongly correlated with postoperative ABD 90 after rTSA (r = 0.69, r = 0.71, r = 0.72, respectively, all Conclusion: Preoperative measures of normalized deltoid and rotator cuff volume on planning computed tomography scans were strongly correlated with postoperative shoulder ABD strength, moderately correlated with ER strength, and weakly correlated with the Constant score. These findings are important to counsel patients preoperatively. They may also be useful in the comparison of different rTSA designs in future clinical studies.