Inter-rotator Cuff Muscle Force Sharing Mitigates Superior Posterior Cuff Tendon Strain in U-Shaped Supraspinatus Tears.

Abstract

U-shaped supraspinatus (SSP) tears predispose the rotator cuff (RC) to strain-related failure, yet most biomechanical studies evaluate the SSP in isolation, overlooking inter-muscle force sharing. We investigated how coordinated RC loading affects strain magnitude and distribution in U-shaped SSP tears. We hypothesized that isolated SSP loading would increase posterior cuff strain, whereas coordinated RC loading would redistribute strain and reduce localized overload. Eight fresh-frozen cadaveric shoulders with standardized U-shaped SSP tears were tested at 45° glenohumeral abduction. Surface strains were measured using three-dimensional DIC during loading of the SSP (whole, anterior/posterior regions) under two conditions: (1) coordinated loading of SSP, infraspinatus (ISP), and subscapularis (SSC), and (2) SSP-only loading with ISP and SSC unloaded. Major strains were quantified at the SSC, coracohumeral ligament, SSP, and superior/inferior ISP tendons. Normalized strain ratios assessed load sharing. Paired t-tests compared conditions. SSP-only loading increased strain within the superior ISP region, whereas coordinated loading redistributed strain inferiorly. During whole SSP loading, combined cuff loading reduced superior ISP strain (2.91% vs. 3.48%, p < 0.01) and increased inferior ISP strain (4.26% vs. 3.02%, p < 0.05). Similar superior-to-inferior redistribution occurred during anterior loading. During posterior loading, coordinated loading reduced strains in both ISP regions, particularly the superior ISP (1.64% vs. 3.27%, p < 0.01). Strain ratios confirmed a shift toward superior posterior cuff dominance with SSP-only loading. Combined loading of the SSP, ISP, and SSC was associated with reduced superior strain concentration in U-shaped tears and a more balanced load transmission compared with SSP-only loading. These findings suggest that inter-rotator cuff load sharing may influence regional strain patterns and provide a biomechanical rationale for future studies evaluating surgical and rehabilitation strategies.