Sex-associated differences in deltoid and rotator cuff muscle elongation after reverse shoulder arthroplasty

Advancements in pre-operative planning and implant design have improved the ability to tailor reverse total shoulder arthroplasty (rTSA) to patient anatomy. Post-operative changes in deltoid and rotator cuff muscle length are important for function and stability and may vary based on implant configuration and patient-specific factors; however, the influence of sex on these muscle length changes remains poorly defined. The purpose of this study was to evaluate sex-based differences in implant selection and muscle length changes following rTSA and to determine whether these differences persist after accounting for anatomic and implant-related variables.

A retrospective analysis of 507 patients (233 males, 274 females) who underwent computed tomography-based pre-operative planning for rTSA using the INHANCE Reverse Shoulder System (DePuy Synthes) was performed. Standardized 3-dimensional scapular coordinates were used to measure glenoid and scapular morphology. Percent elongation of the deltoid, infraspinatus, teres minor, and subscapularis was calculated between native and planned states. Implant characteristics, including glenosphere size, lateralization, and eccentricity, were recorded. Sex-based differences in anatomy, implant selection, and muscle elongation were evaluated with t-tests, analysis of variance, and multivariable regression.

Males demonstrated significantly larger scapular and glenoid dimensions and were more likely to receive larger and more lateralized glenosphere configurations (P < .001). Deltoid elongation was associated with implant size and anatomic features and did not differ by sex after adjustment (P = .140). In contrast, posterior rotator cuff muscles demonstrated significant sex-based differences, with males exhibiting greater shortening of the infraspinatus (-8.1% vs. -5.1%, P < .001) and teres minor (-14.7% vs. -10.1%, P < .001). Implant size was not consistently associated with muscle length changes, and no significant interaction between sex and implant size was observed.

Muscle length changes following rTSA vary based on both implant configuration and patient-specific factors, with distinct sex-based differences in posterior rotator cuff response. While deltoid elongation is primarily influenced by implant selection, posterior cuff muscle changes appear to be independently associated with sex. These findings highlight the importance of considering patient-specific anatomy when planning rTSA and may help guide implant selection and soft tissue tensioning strategies.