Biomechanical consequences of isolated, massive and irreparable posterosuperior rotator cuff tears on the glenohumeral joint A dynamic biomechanical investigation of rotator cuff tears

Background Complex interactions between dynamic and static stabilizers of the shoulder girdle are critical for a biomechanically complex system, allowing for sufficient range of motion in multiple planes. This study assessed the biomechanical consequences of non-retracted rotator cuff tears (RCT), isolated and massive RCT, and irreparable, retracted posterosuperior RCT on the glenohumeral joint using a validated, dynamic shoulder testing system. Methods Eight fresh-frozen cadaveric shoulders were tested using a dynamic shoulder simulator. Each shoulder was tested in the following conditions: (1) intact state; (2) isolated non-retracted supraspinatus tendon (SSP) defect; (3) isolated non-retracted subscapularis tendon (SSC) defect; (4) isolated non-retracted infraspinatus tendon (ISP) defect; (5) massive non-retracted RCT involving all three tendons; (6) irreparable, retracted posterosuperior RCT. Results The SSP, SSC, and ISP simulated defects showed a significant increase in total deltoid force, respectively (p = 0.012; p = 0.007; p = 0.001). Compared with the intact state, the massive RCT showed a significant decrease in glenohumeral abduction angle (p < 0.001) and a significant increase in total deltoid force (p < 0.001). The irreparable, retracted posterosuperior RCT showed a significant decrease in glenohumeral abduction angle, significant increase of total deltoid force, subacromial peak contact pressure, and glenohumeral superior translation (p > 0.001, respectively) compared with the intact state. Conclusion In a dynamic biomechanical shoulder model, isolated non-retracted RCT, located lateral to the rotator cable, can be sufficiently compensated by the remaining intact cuff. However, in irreparable, massively retracted posterosuperior RCT located medial to the rotator cable, devasting effects on the glenohumeral joint can be expected and surgery should be recommended for these patients.