The impact of rotator cuff muscle volume on functional outcomes following reverse total shoulder arthroplasty: the use of a validated muscle segmentation software

The indications for reverse total shoulder arthroplasty (rTSA) have progressively expanded, and it is now implanted for a wide range of clinical scenarios, including cases with an intact rotator cuff. The objective of this study was to use a validated computed tomography (CT) muscle segmentation software to evaluate the relationship of rotator cuff muscle volume and postoperative clinical outcomes. A retrospective study was conducted using an institutional database including all patients who underwent rTSA between 2016 and 2022, with a minimum of two years of follow-up. Patients were divided into 2 groups based on the indication: centered osteoarthritis (OA) and cuff tear arthropathy (CTA). Clinical outcomes were assessed preoperatively and postoperatively, including active range of motion (ROM), American Shoulder and Elbow Surgeons score, Subjective Shoulder Value, and visual analog scale for pain. Preoperative CT scans were automatically segmented with a validated CT muscle segmentation software to isolate the scapula, humerus, deltoid, and the 4 rotator cuff muscles. Muscle volume and the percentage of intramuscular fat were measured and normalized to scapular volume. Statistical analyses were performed to investigate correlations between muscle volume, functional outcomes, and ROM. A total of 528 patients (460 OA, 68 CTA) with an average follow-up of 29.1 &#xb1; 8.6 months were included. Normalized muscle volume of the supraspinatus, infraspinatus, and subscapularis was significantly greater in OA patients than in CTA patients (P < .001). No significant differences were observed for the deltoid (P = .92) or teres minor (P = .81). The percentage of intramuscular fat was significantly higher in the supraspinatus, infraspinatus, teres minor, and subscapularis in CTA patients compared to OA patients (P < .001). No significant difference was observed for the deltoid (P = .97). No significant correlation was found between normalized muscle volume (cuff or deltoid) and functional scores or active ROM, either preoperatively or postoperatively following rTSA. In the CTA cohort, a moderate negative correlation was observed between deltoid intramuscular fat percentage and preoperative American Shoulder and Elbow Surgeons score (r = -0.43; P = .031) and preoperative visual analog scale (r = 0.43; P = .026). No correlation was found between intramuscular fat percentage (cuff and deltoid) and postoperative functional outcomes or active ROM. rTSA for OA achieves better outcomes than rTSA for CTA. The difference in outcomes showed no correlation with muscle volumes, indicating that rotator cuff muscles may play a less critical role in functional outcomes after rTSA than previously thought.