Relative contributions of the supraspinatus cord and strap tendons to shoulder abduction and translation

Abstract

The supraspinatus (SS) is formed by a larger anterior bipennate muscle with a cord like tendon and a posterior unipennate muscle with a strap like tendon. There is a tendinous connection between the two SS subunits. Yet, the relative mechanical contribution of the SS cord and SS strap muscular tendinous units to load transmission and subsequent shoulder abduction force is unknown. We hypothesized that a simulated SS cord versus a SS strap tear will generate less shoulder abduction force; and further, an intact SS cord will offset the expected abduction loss from a SS strap tear, but the inverse will not be true. Twenty fresh-frozen cadaveric specimens were tested in a shoulder simulator with physiological load vectors applied to the upper and lower subscapularis, SS cord, SS strap, infraspinatus, and teres minor. The roles of the SS cord and SS strap muscles were delineated by varying their loads, while keeping constant loads on other muscles. The randomized testing trials included a Native condition and four test Cases which simulated tears by dropping the load and force transfer via the SS cord-to-SS strap connection by adding the load. Testing was completed at both 0° and 30° of abduction. During each test shoulder abduction force, rotator cuff strains and humeral translation were measured. Simulated isolated SS cord and SS strap tears lead to a significantly lower shoulder abduction force (p<0.001). A simulated cord tear at 0° and 30° reduced the abduction force by 53% and 38%, respectively. A simulated strap tear at 0° and 30° dropped the abduction force by 27% and 23%, respectively. The decline in abduction force was larger for the SS cord tear versus SS strap tear (p≤0.001). A SS cord tear with full load transfer to the strap was able to recover to native values at both 0° and 30° (p≥0.288). Likewise, a SS strap tear with full load transfer to the SS cord showed a similar recovery to native values at both 0° and 30° (p≥0.155). During full load transfer, the tendon strain followed the loading pattern. A SS cord tear or SS strap tear did not cause a change in humeral translation (p≥0.303). A mechanical findings support the efficacy of nonoperative treatment of small (<10mm) SS tears,11 because an intact SS strap tendon can effectively offset the abduction loss of a torn SS cord tear, and vice versa.