Strain Field Mapping Indicates Direct Transmission of Supraspinatus Force Through the Rotator Cuff

Operative repair of the rotator cuff (RC) of the shoulder can return a patient to normal function but is not without complications. An understanding of the tissue strains in the RC of the shoulder under normal and pathological conditions can inform surgeons about the conditions needed for restoration of function and the state of the tissue under repair. The current work applied digital image correlation (DIC) to quantify rotator cuff strains with the goals of (1) determining whether the tension created by the supraspinatus (SS) muscle is transmitted without diversion by the structure of the cuff from the supraspinatus to the cuff insertion; and (2) whether releases of the SS tendons at their insertions alter the strain field in the region away from the cuff insertion. DIC methods recorded the bursal-side cuff strains created using a shoulder simulator, which could apply physiologic loads to the cuff muscles. The SS and infraspinatus (IS) insertions of the humerus were sequentially released while muscle loads were applied. The first principal strains and their directions showed changes after releases of both the anterior and posterior SS insertions and after the IS release. The results demonstrated (1) that the RC transmits SS muscle forces without diversion and (2) that RC releases do affect the strain field. Release of both heads of the SS led to statistically significant changes in strain magnitude and direction.