Progressive Rotator Cuff Tears Alter Strain Across Footprint and Tear Borders: Subregion-Specific Loading Reveals Size-Dependent Mechanical Risk Zones.

Rotator cuff (RC) tears frequently involve the supraspinatus (SSP) muscle, which consists of anterior and posterior subregions with distinct functional roles. As tear size progresses, it remains unclear how force from these subregions transmits to the RC tendon stump. This study examined strain distribution across SSP tendon stump regions during isolated and subregion-specific muscle loading under increasing tear sizes. Eight fresh-frozen male cadaveric shoulders (mean age: 63 years) were tested. The humerus was fixed at 45° abduction, and the SSP muscle (whole, anterior, or posterior) was loaded (80 N, 60 N, and 20 N, respectively). The infraspinatus and subscapularis were concurrently loaded to preserve physiological balance. Tear models were defined anatomically: Tear-I involved the anterior half of the superior facet; Tear-II encompassed the full superior facet; Tear-III extended into the middle facet. Strain was measured across the intertubercular groove, superior facet, middle facet, and on the anterior and posterior borders of the tears using 3D-digital image correlation. Statistical comparisons were performed using the Friedman test with Bonferroni correction. Whole-muscle and anterior SSP loading showed uniform strain in the intact and Tear-I states but elevated intertubercular groove strain in Tear-II and Tear-III. Posterior SSP loading led to a significantly higher strain in the middle facet across all conditions, particularly in Tear-III. Strain distributions shifted anteriorly in larger tears, disrupting anterior-posterior force couple balance. These findings show that progressive SSP tears alter strain transmission, revealing mechanical risk zones by subregion and tear size, with implications for targeted repair and muscle compensation strategies.