Partial-thickness tears involving the rotator cable lead to abnormal glenohumeral kinematics

Hypothesis: The objective of this study was to determine the biomechanical function of the rotator cable when a partial-thickness (>50%) tear is present. We compared intact specimens with partial tears of the anterior cable followed by partial anterior and posterior tears in regard to glenohumeral kinematics and translation. The hypothesis was that partial-thickness tears will lead to abnormal glenohumeral biomechanics, including glenohumeral translation and path of glenohumeral articulation. Methods: Five fresh frozen cadaveric shoulders with intact labrum, rotator cuff, and humerus were tested using a custom shoulder testing system in the scapular plane. Glenohumeral translation was measured after applying an anterior load of 30 N at different angles of external rotation. The path of glenohumeral articulation was measured by calculating the humeral head center with respect to the glenoid articular surface at 30 degrees, 60 degrees, 90 degrees, and 120 degrees of external rotation. Results: With an anterior force of 30 N, there was a significant increase in anterior and total translation at 30 degrees of external rotation after the anterior cable was cut (P <.05). When the tear was extended to the posterior cable, there was a significant increase in anterior, inferior, and total translation at 30 degrees and 120 degrees of external rotation (P <.05). With respect to the path of glenohumeral articulation, the humeral head apex was shifted superiorly at 90 degrees and 120 degrees of external rotation after the posterior cable was cut (P <.05). Conclusion: Partial-thickness articular-sided rotator cuff tears with a thickness >50% involving the rotator cable increased glenohumeral translation and changed kinematics in our cadaveric biomechanical model. (C) 2017 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved.