The biomechanics of the supraspinatus-deficient shoulder treated with superior capsular reconstruction vs. reverse total shoulder arthroplasty—experimental study

Aim of the study Our objective was to compare biomechanical effects of superior capsular reconstruction (SCR) and reverse total shoulder arthroplasty (rTSA) on shoulder motion, in the setting of an irreparable supraspinatus (SS) tear. We hypothesized that rTSA would produce greater improvement in abduction force and shift the humerus inferiorly, while SCR would produce greater range of motion (ROM) and prevent superior migration of the humerus during abduction. Methods Six cadaveric shoulders were evaluated using a custom biomechanical apparatus. Each shoulder underwent four experimental conditions: (1) intact/control, (2) irreparable SS tear, (3) SCR using dermal allograft, and (4) rTSA without SCR. Deltoid abduction force, superior humeral head translation, and passive range of motion were measured in static tendon loading condition at 0, 30, and 60° of glenohumeral abduction. Results Both rTSA and SCR restored abduction force to intact levels at all abduction angles. rTSA significantly increased abduction force compared with the SS-deficient shoulder at 0, 30, 60° ( p = 0.04), while SCR produced a significant increase at 0° ( p = 0.05) abduction. rTSA inferiorly shifted the humeral head by 27 mm ( p = 0.002). SCR restored superior humeral head translation to intact SS levels. Compared with SCR, rTSA resulted in 25° less passive abduction ( p = 0.001) without significant differences in forward flexion/extension. Compared with SCR, rTSA achieved 10° less passive internal rotation at 0° abduction ( p = 0.03) and 26° and 17° greater external rotation at 30° and 60° abduction, respectively ( p = 0.03, p = 0.04). Discussion Our investigation found that abduction force was restored to intact cuff levels by both procedures, without significant differences between the two techniques. SCR restored superior humeral head migration and rTSA translated the humerus inferiorly. rTSA resulted in decreased passive abduction ROM and increased external rotation, compared with SCR. Conclusion Both SCR and rTSA restore key biomechanical parameters following an irreparable SS tear, although SCR offers superior passive abduction ROM.