Implant design influences muscle activation patterns during functional motions after reverse shoulder arthroplasty

The purpose of this study was to evaluate how surgical technique and prosthesis design affect muscle activation after reverse shoulder arthroplasty (RSA) during hand-to-head (H2H) and hand-to-back (H2B) movements. Surface electromyography data from 8 muscles were collected while 28 RSA patients performed H2H and H2B. Muscle activation onset (AO) and total muscle activation (TMA) were calculated. Multiple regression was used to identify surgical technique and prosthesis design factors that predicted muscle AO and TMA. Correlation was used to identify associations between muscle activation and patient reported outcomes (CMS and DASH). During H2H, neck shaft angle predicted anterior deltoid, pectoralis, and latissismus muscle AO and lateral humeral offset (LHO) predicted latissimus AO. Glenosphere eccentricity, glenosphere tilt, glenosphere size, and LHO predicted TMA for anterior deltoid, middle deltoid, teres minor, and pectoralis muscles, respectively. During H2B, glenosphere lateralization predicted trapezius, anterior deltoid, and teres minor muscle AO, and glenosphere eccentricity and humeral retroversion change predicted trapezius TMA. During H2H, later middle deltoid AO was associated with better CMS, and later pectoralis major AO was associated with worse DASH scores. Additionally, higher teres minor TMA was associated with worse internal rotation score. During H2B, later latissimus AO was associated with better DASH scores. Improved understanding of how implant design and surgical technique affect muscle activation and clinical outcomes after RSA may help surgeons and therapists to optimize shoulder function after RSA.