Strain distribution in the bursal rotator cuff based on whole-muscle and muscle subregion-specific loading: A cadaveric study.

Rotator cuff (RC) tears are common injuries leading to significant dysfunction of the shoulder. Rotator cuff tears alter tension and strain in muscles and tendons. Anatomical studies demonstrated that rotator cuff muscles are comprised of anatomical subregions. However, the strain distribution within the rotator cuff tendons generated from the tension from each anatomical subregion is unknown. We hypothesized that subregions would present distinct 3-dimensional (3D) strain distributions within the rotator cuff tendons, and that the anatomical insertion configuration of the supraspinatus (SSP) and infraspinatus (ISP) tendons might dictate strain, thus tension, transmission. 3D-strains in the bursal side of the SSP and ISP tendons of eight fresh-frozen cadaveric intact shoulders were obtained by applying tension on the whole SSP and ISP muscles, and on their subregions using an MTS system. Strains in the anterior region of the SSP tendon were higher than in the posterior region with whole-SSP anterior-region (p < 0.05) and whole-SSP muscle loading. Higher strains were observed in the inferior half of the ISP tendon with whole-ISP muscle (p < 0.05), middle-subregion (p < 0.01), and superior-subregion (p < 0.05) loading. Tension generating from the posterior-region of the SSP was primarily transmitted to the middle facet via an overlap between the SSP and ISP tendons insertions, while the anterior-region mainly distributed its tension into the superior facet. Tension generating from the middle and superior-regions of the ISP was distributed into the inferior portion of the ISP tendon. These results emphasize the importance of the distinct anatomical subregions of the SSP and ISP muscles in distributing the tension to the tendons.