Effect of anterior supraspinatus tendon partial-thickness tears on infraspinatus tendon strain through a range of joint rotation angles

Background: Rotator cuff tears are common shoulder problems whose propagation is difficult to predict because of the structural and mechanical inhomogeneity of the supraspinatus tendon. We have previously shown that the supraspinatus and infraspinatus tendons interact mechanically when the supraspinatus tendon is intact or exhibits a full-thickness tear, so that an increase in supraspinatus tendon strain is paralleled by an increase in infraspinatus tendon strain. Such interaction is critical and suggests that an increase in infraspinatus tendon strain that accompanies an increase in supraspinatus tendon strain may shield the supraspinatus tendon from further injury, but increase the risk of injury to the infraspinatus tendon. In this study, the mechanical interactions between the supraspinatus and infraspinatus tendons were evaluated for the commonly occurring supraspinatus tendon partial-thickness tears through a range of rotation angles. Methods: For each joint rotation and supraspinatus tendon tear size evaluated, the supraspinatus tendon was loaded, and images corresponding to 5 N, 30 N, 60 N, and 90 N of supraspinatus tendon load were isolated for the speckle painted supraspinatus and infraspinatus tendons. A region of interest outlining the insertion site was isolated and displacements between the 5 N loaded image and each of the others were measured, from which normalized average principal strains were quantified in both tendons. Results: The observed effect on infraspinatus tendon strain paralleled that observed on strain in the supraspinatus tendon. Introducing a supraspinatus tendon partial-thickness tear and increasing load caused an increase in normalized average maximum and a decrease in normalized average minimum principal strain in the infraspinatus tendon. Increasing rotation angle from internal to external rotation caused a general decrease in normalized average maximum and increase in normalized average minimum principal strain in both tendons. Conclusion: The supraspinatus and infraspinatus tendons mechanically interact for the intact and partially torn supraspinatus tendons for neutral and rotated glenohumeral joint.