Shoulder Abduction in Response to Supraspinatus and Coracohumeral Ligament Tears

The rotator cuff consists of four primary muscles: the supraspinatus (SS), the infraspinatus (IS), the teres minor, and the subscapularis (SSc). In addition to these muscles, the coracohumeral ligament (CHL) supports the shoulder capsule and helps provide stability. Within the supraspinatus, there are two distinct structures: the anterior “cord” and the posterior “strap” portion. The aim of this study was threefold: to simulate tears in the cord and strap and measure shoulder abduction force, to create full tears in the cord, strap, and CHL (and once again measure shoulder abduction), and thirdly, to examine the anatomical structures more deeply using 3D scanning technology. Physiological loading was applied to 20 cadaveric specimens using a custom shoulder simulator in order to replicate anatomical shoulder abduction in the scapular plane. Each specimen first underwent a series of loading cases, where different loads were applied to mimic four different load transmission cases. Then, each specimen underwent a cutting sequence where the first incision was randomized between the CHL or the cord. After testing in the shoulder simulator was completed and abduction force was measured, the rotator cuff muscles were removed from each shoulder at their insertion. The supraspinatus (cord and strap), infraspinatus, and CHL were then scanned with a FARO Arm 3D Scanner. Muscle and tendon cross sectional areas, thicknesses, and humeral insertion measurements were taken. Both the modeled SS cord and SS strap tears with full force compensation were able to produce similar abduction strength to the native case, at both 0° (p≥0.291) and 30° (p≥0.423) of abduction. Furthermore, a torn CHL did not produce significant abduction strength loss in comparison to the native state (p>0.999), but a torn SS cord did produce significant strength loss compared to the native (p=0.030). The findings show that if the SS cord is damaged, the SS strap will compensate and will be capable of producing the same abduction strength as a native state. Additionally, an intact SS cord will compensate for a torn SS strap. This is clinically relevant, as it suggests that small anterior rotator cuff tears (