Rotator cuff tendon strain distribution changes from the superior to the inferior tendon attachments with increasing shoulder abduction.

Supraspinatus (SSP), infraspinatus (ISP), and subscapularis (SSC) muscles from the rotator cuff (RC) are comprised of anatomical subregions based on muscle fiber alignment and tendon attachments. Previous studies have shown mechanical interactions between the SSP and ISP tendons, with abduction angle influencing these interactions. However, changes in strain distribution originating from the RC muscle subregions, including the SSC, due to differences in abduction angle have not been investigated. Therefore, this study aimed to determine the strain distribution in intact RC cadaveric shoulders with loading from the SSP, ISP, and SSC muscles and their subregions at different abduction angles. Surface strains in the bursal side of the SSP, ISP and SSC tendons of eight fresh frozen cadaveric intact shoulders were obtained by applying tension on these muscles and on their subregions at 25° and 45° glenohumeral abduction using an MTS system. When the SSP muscle or anterior region was loaded, no significant strain differences were observed within the footprint regions at 45° abduction. However, when the posterior region was loaded, strains concentrated on the middle facet as the abduction angle increased. Significantly higher strains were also observed in the inferior portion of the footprint with increasing abduction angle when the ISP or SSC muscles, or their subregions, were loaded. The differences in strain distribution patterns observed between the anterior and posterior regions of the SSP muscle highlight the importance of addressing each region and tendon separately when managing RC tears and repairs.