Shoulder muscle synergies before and after reverse total shoulder arthroplasty

Reverse total shoulder arthroplasty (rTSA) has become a standard treatment for symptomatic massive rotator cuff tears (MRCT). While clinical outcomes are generally satisfactory, significant variability in functional recovery raises questions about the extent of inter-individual neuromuscular adaptations and motor restoration. This study aimed to assess the neuromuscular impact of rTSA by analysing superficial shoulder muscle activity and synergies. Asymptomatic shoulders (n = 20) and symptomatic shoulders undergoing rTSA (n = 20) were assessed before and after surgery. Participants performed four functional tasks while surface electromyography was used to quantify muscle activity. Synergies of the anterior (DELTA), middle (DELTM), and posterior deltoid (DELTP), middle (TRAPM) and upper trapezius (TRAPS), and serratus anterior (SERRA) were computed. Symptomatic MRCT was associated with a significant increase in activity of DELTA (p = 0.014), DELTM, TRAPS, and SERRA (p < 0.001). Postoperatively, DELTM (p = 0.002), TRAPS (p = 0.039), and SERRA (p = 0.025) remained significantly more active. Although the number of muscle synergies (n = 2) remained stable across groups, significant differences in muscle weightings were found preoperatively for TRAPS and DELTP (p < 0.001). Also, cosine similarity increased postoperatively by 15.96% for synergy 1 and 37.42% for synergy 2, indicating a greater resemblance to the asymptomatic synergy pattern (p < 0.001). Changes in muscle activities and synergy patterns, marked by the emergence of compensatory roles for scapular stabilisers and the deltoid, highlight the disruption of motor control associated with MRCT. Following rTSA, motor coordination was substantially restored, supporting improved functional autonomy in daily tasks.