The Effect of Middle Trapezius Transfer using SwiveLocks on Glenohumeral Translation and Subacromial Pressure in Irreparable Supraspinatus Tears – A Static and Dynamic Robotic Biomechanical Investigation

Background To manage irreparable supraspinatus (SSP) tears, recent dynamic methods like the middle trapezius transfer (MTT) have been introduced; however, data on its efficacy in restoring physiological biomechanics are lacking. Purpose The purpose was to assess the effects of MTT on superior humeral head translation and subacromial pressure in the setting of an irreparable SSP tear. It was hypothesized that a SSP tear would increase superior humeral head translation and subacromial pressure and that MTT would reduce superior humeral head translation and subacromial pressure. Methods Ten fresh-frozen cadaveric shoulders were mounted to a 6-degree of freedom robotic arm (KUKA KR-60) and underwent biomechanical testing in three states: (1) intact, (2) simulated irreparable SSP tear, (3) MTT using SwiveLock anchors, loaded with 10N. Each shoulder was brought to 30°, 45°, 60°, 75°, and 90° of abduction (ABD), and 60 N of superior force was applied to the humerus at each position to simulate loaded glenohumeral ABD. Superior humeral displacement (mm) was measured by the robot and subacromial contact area (mm 2 ) and peak contact pressure (MPa) were measured using a pressure sensor. Results In the SSP defect state, the humeral head significantly translated superiorly across all ABD angles (+1.6 to +3.8 mm, p < 0.001), peak pressure significantly increased for ABD <90° compared to native (+0.48 MPa, p=0.009 to +0.55, p=0.030 MPa). and there was a significant decrease in subacromial contact area for ABD >30° (-15 mm 2 , p=0.001 to -40 mm 2 , p < 0.001). MTT significantly reduced superior translation across all ABD angles (-0.7 mm, p=0.048 to -1.6 mm, p < 0.001) but failed to restore native superior translation across all ABD angles (+0.9 mm, p=0.020 to +2.3 mm, p<0.001). MTT restored peak pressure conditions to the native state and a significant reduction in peak pressure was observed at 30° and 45° of ABD when compared to the SSP defect state (+0.33 MPa, p=0.036 to +0.38 MPa, p=0.041). After MTT, the contact area was significantly higher compared to the SSP defect state at 30° of ABD (+32 mm 2 , p=0.003), yet it failed to restore native contact area conditions at 60° (-22 mm 2 , p=0.045) and 90° of ABD (-20 mm 2 , p < 0.001). Conclusion MTT improved shoulder kinematics by reducing superior translation of the humeral head and subacromial peak pressure, and increasing subacromial contact area, partially restoring native conditions in the presence of an irreparable SSP tear.