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

Background: 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 2 SS subunits. Yet, the relative mechanical contribution of the SS cord and SS strap musculotendinous units to load transmission and subsequent shoulder abduction force is unknown. We hypothesized that a simulated SS cord vs. an SS strap tear would generate less shoulder abduction force and, further, an intact SS cord would offset the expected abduction loss from an SS strap tear, but the inverse would not be true.Materials and Methods: 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 4 test cases that 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 degrees and 30 degrees of abduction. During each test, shoulder abduction force, rotator cuff strains, and humeral translation were measured.Results: Simulated isolated SS cord and SS strap tears led to a significantly lower shoulder abduction force (P < .001). A simulated cord tear at 0 degrees and 30 degrees reduced the abduction force by 53% and 38%, respectively. A simulated strap tear at 0 degrees and 30 degrees dropped the abduction force by 27% and 23%, respectively. The decline in the abduction force was larger for the SS cord tear vs. SS strap tear (P <= .001). An SS cord tear with full-load transfer to the strap was able to recover to native values at both 0 degrees and 30 degrees (P >= .288). Likewise, an SS strap tear with full-load transfer to the SS cord showed a similar recovery to native values at both 0 degrees and 30 degrees (P >= .155). During full-load transfer, the tendon strain followed the loading pattern. An SS cord tear or SS strap tear did not cause a change in humeral translation (P >= .303).Discussion: The mechanical findings support the efficacy of nonoperative treatment of small (<10 mm) SS tears,(11 )because an intact SS strap tendon can effectively offset the abduction loss of a torn SS cord tear and vice versa.Level of evidence: Basic Science Study; Biomechanics (c) 2023 The Author(s). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).