Rotator cuff and deltoid muscle changes following reverse total shoulder arthroplasty

With expanding indications, reverse total shoulder arthroplasty (rTSA) is now being performed in greater proportion for shoulders with an intact rotator cuff. However, little is known regarding the dynamic in vivo effects of rTSA implants on the rotator cuff and deltoid musculature. The purpose of this study was to quantify changes in the cross-sectional area (CSA) of the rotator cuff and deltoid following rTSA, and to assess the clinical impact of these changes. Twenty-five patients undergoing rTSA augmented by a glenoid bone graft were included for analysis. Patients underwent preoperative and one-year postoperative CT scans. Range-of-motion measurements and functional scores were collected at approximately one year postoperatively. Cross-sectional area measurements of the supraspinatus (SS) and combined infraspinatus/teres minor (IS/Tm) were made at three separate points on parasagittal CT sections by two independent reviewers. Similarly, CSA measurements of the deltoid were made at three separate points in a plane perpendicular to the humeral shaft. Scalar measurements on CT sections in two orthogonal planes were used to assess lateralization and distalization. Compared to preoperative measurements, the CSA of the SS and IS/Tm decreased in 72% (18/25; mean 13.4% decrease; SD, 19.8%; range, 61.8% decrease to 13.7% increase; P = .02) and 64% (16/25; mean 8.7% decrease; SD, 13.5%; range, 35.3% decrease to 14.9% increase; P = .11) of shoulders, respectively. Eighty-four percent of shoulders saw a decrease in their combined posterosuperior cuff complex area (21/25; mean 11.5% decrease; SD, 11.2%; range, 43.4% decrease to 5.4% increase; P = .035). In contrast, deltoid CSA increased in 80% of patients (20/25; mean 7.9% increase; SD, 20.2%; range, 41.3% decrease to 67.8% increase; P = .43). There were no significant differences in CSA changes between shoulders with cuff-intact glenohumeral osteoarthritis (GHOA) and rotator cuff tear arthropathy (RCTA) (P = .36, P = .58, P = .16, P = .36 for SS, IS/Tm, posterosuperior cuff and deltoid, respectively). In shoulders with GHOA, a smaller decrease in CSA of the SS was associated with a smaller increase in deltoid CSA (R = −0.68; P = .03), and greater improvements in forward elevation (R = 0.66; P = .04). In general, the posterosuperior cuff CSA declines and the deltoid CSA increases following rTSA with a minimally lateralized construct. In the setting of GHOA, changes in CSA of the SS were inversely associated with deltoid CSA changes and positively associated with improvements in forward elevation. For patients with an intact rotator cuff undergoing rTSA, preservation of the SS may optimize active forward elevation motion, while also decreasing deltoid muscle burden.