Biomechanical analysis of progressive rotator cuff tendon tears on superior stability of the shoulder

Background: The biomechanical relationship between irreparable rotator cuff tear size and glenohumeral joint stability in the setting of superiorly directed forces has not been characterized. The purpose of this study was to quantify kinematic alterations of the glenohumeral joint in response to superiorly directed forces in a progressive posterosuperior rotator cuff tear model. Methods: Nine fresh-frozen cadaveric shoulders (mean age; 58 years) were tested with a custom shoulder testing system. Three conditions were tested: intact, stage II (supraspinatus) tear, stage III (supraspinatus + anterior half of infraspinatus) tear. At each condition, range of motion and humeral head positions were measured with a "balanced'' loading condition, and with a superiorly directed force ("unbalanced loading condition''). At each of the 0 degrees, 20 degrees, and 40 degrees of glenohumeral abduction positions, all measurements were made at 0 degrees, 30 degrees, 60 degrees, and 90 degrees of external rotation (ER). Two-way repeated measures analysis of variance with Tukey post hoc tests were performed for statistical analyses. Results: With the balanced load, no significant change in superior humeral head position was observed in stage II tears. Stage III tears significantly changed the humeral head position superiorly at 30 degrees and 60 degrees ER at each abduction angle compared with the intact condition (P <= .028). With superiorly directed load, stage II and stage III tears both showed statistically significant increases in superior translation at all degrees of ER for all degrees of abduction (P <= .035), except stage II tears at 0 degrees ER and 40 degrees abduction (P = .185) compared with the intact condition. Stage II tears showed posterior translations with 30 degrees and 60 degrees ER, both at 20 degrees and 40 degrees of abduction. Stage III tears also showed posterior translations with 90 degrees ER for all abduction angles (P <= .039). Conclusion: With superiorly directed loads, complete supraspinatus tendon tears created superior translations at all abduction angles, and posterior instability in the middle ranges of rotation for 20 degrees and 40 degrees of abduction. Larger tears involving the anterior half of the infraspinatus tendon caused significant superior and posterior translations within the middle ranges of ER for all abduction angles. In addition to superior instability, posterior translation should be considered when selecting or developing surgical techniques for large posterosuperior rotator cuff tears. (C) 2021 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved.