Associations of Preoperative Body Composition Parameters and Deltoid Cross-Sectional Area With Shoulder Function Before and After Reverse Total Shoulder Arthroplasty

Background Reverse total shoulder arthroplasty (rTSA) provides favorable clinical outcomes; however, postoperative functional recovery varies among patients. Although deltoid cross-sectional area (DA) has been linked to postoperative muscle strength, the associations between systemic body composition parameters, postoperative shoulder function, and local deltoid morphology have not been fully investigated in patients undergoing rTSA. This study aimed to investigate associations between preoperative body composition parameters, DA, and shoulder function before and one year after rTSA. Methods This study included 42 patients (42 shoulders) who underwent rTSA and had available preoperative and one-year postoperative assessments. The study population comprised 15 men (35.7%) and 27 women (64.3%), with a mean age of 78 ± 7 years and BMI of 24.7 ± 6.4 kg/m². The underlying diagnoses were massive rotator cuff tear in 12 patients (28.6%), cuff tear arthropathy in 12 (28.6%), and glenohumeral osteoarthritis in 18 (42.9%). Six patients (14.3%) had diabetes mellitus, and 14 (33.3%) had a history of smoking. Preoperative body composition was assessed using a bioelectrical impedance analyzer (InBody® 970, InBody Japan Inc., Tokyo, Japan), including appendicular skeletal muscle mass index (ASMI), phase angle (PhA), and extracellular water/total body water (ECW/TBW). DA at the midpoint of the total deltoid length was measured using preoperative magnetic resonance imaging (MRI; Philips, Amsterdam, the Netherlands). Shoulder function was assessed preoperatively and at one year postoperatively using the Constant-Murley score, abduction strength, visual analog scale (VAS), and range of motion. Associations of preoperative body composition parameters and DA with shoulder function were assessed using Spearman's rank correlation coefficient, with the Benjamini-Hochberg procedure applied to control the false discovery rate (FDR). Results Preoperative PhA was significantly positively correlated with abduction strength both preoperatively (r = 0.41, adjusted p = 0.02) and at one year postoperatively (r = 0.37, adjusted p = 0.04), whereas preoperative ECW/TBW was significantly negatively correlated with preoperative abduction strength (r = -0.37, adjusted p = 0.04). Preoperative PhA was also significantly negatively correlated with VAS at one year postoperatively (r = -0.40, adjusted p = 0.03), whereas preoperative ECW/TBW was significantly positively correlated with VAS at one year postoperatively (r = 0.40, adjusted p = 0.03). Preoperative DA was significantly positively correlated with abduction strength both preoperatively (r = 0.41, adjusted p = 0.02) and at one year postoperatively (r = 0.34, adjusted p = 0.02). Furthermore, preoperative DA was significantly positively correlated with ASMI (r = 0.64, adjusted p < 0.01) and PhA (r = 0.56, adjusted p < 0.01) and significantly negatively correlated with ECW/TBW (r = -0.35, adjusted p = 0.04). Conclusions Preoperative body composition parameters and deltoid morphology were associated with different aspects of shoulder function before and one year after rTSA. PhA and ECW/TBW were associated with selected measures of muscle strength and pain, whereas DA was associated with abduction strength. These findings suggest that systemic body composition and local deltoid morphology may provide complementary information regarding shoulder function in patients undergoing rTSA.