Which muscle is the external rotation compensator after superior capsular reconstruction?

Background: Superior capsular reconstruction (SCR) is a surgical option for massive irreparable rotator cuff tears, particularly involving the supraspinatus and infraspinatus. In this procedure, the torn infraspinatus is not repaired or reconstructed. However, an improvement in postoperative external rotation (ER) angle and strength is observed. There is a lack of studies explaining how ER is restored after SCR. The aim of this study is to identify the ER compensator by assessing the muscle volume of the posterior deltoid and teres minor. Methods: Sixty-eight patients with massive rotator cuff tears underwent SCR during 2016-2021. Of these patients, 28 who met the following inclusion criteria were retrospectively reviewed: (1) massive rotator cuff tears, including the supraspinatus and infraspinatus, (2) severe muscle atrophy and fatty change, (3) intact or reparable subscapularis tendon, and (4) Hamada of grade 3 or lower. Posterior deltoid and teres minor volume were measured using open-source medical image processing software preoperatively and 1-year postoperatively. The percentage of the posterior deltoid and teres minor muscle volume change was compared between patients with 1-year postoperative ER manual muscle testing (MMT) of grade 5 and of grade < 5. The relationship between grade of fatty change, percentage of the muscle volume change, and ER angle and strength were evaluated. Results: There was a significant increase in the 1-year postoperative teres minor volume compared with the preoperative volume (24.6 ± 10.3 cm³ vs. 20.9 ± 8.3 cm³, P