Rotator cuff tears are common and not well understood shoulder problems. Structural and mechanical inhomogeneity of the supraspinatus tendon complicates accurate prediction of risk of tear propagation and may impact appropriate clinical treatment. The work presented in this thesis dissertation investigates the mechanical interaction between the supraspinatus and the infraspinatus tendons as it may be critical to understanding load bearing at the glenohumeral joint. Principal strains in the infraspinatus and supraspinatus tendons of cadaveric human shoulders were calculated to evaluate whether a mechanical interaction between the supraspinatus and infraspinatus tendons exist with: (1) an increase in anterior-side full and partial-thickness supraspinatus tendon tear sizes; (2) change in glenohumeral abduction and rotation angles; (3) increase in supraspinatus tendon load; and 4) two commonly used supraspinatus tendon repairs. Results showed that a mechanical interaction between the supraspinatus and infraspinatus
tendons existed such that in general, conditions that caused increase in strain in the supraspinatus tendon, such as an increase in supraspinatus tendon tear size and load, also caused an increase in strain in the infraspinatus tendon. Results also showed that joint position significantly affects the mechanical interaction between the supraspinatus and infraspinatus tendons, causing an interruption in the mechanical interactions at joint positions where humeral head contact with the tendon is minimized. Both supraspinatus tendon repair techniques equally restored the mechanical interaction between the two tendons, and mimicked the results of the intact supraspinatus tendon regarding the effect of load and joint position. Changes in infraspinatus tendon strain associated with increases in supraspinatus tendon tear size and loading may shield a torn supraspinatus tendon from further injury or have detrimental chronic effects on the infraspinatus tendon. In conclusion, the supraspinatus and infraspinatus tendons mechanically interact. The observed significant increase in maximum principal strain and decrease in minimum principal strain was concurrent in both the infraspinatus and supraspinatus tendons except when interrupted at certain joint positions.
