gms | German Medical Science

Objectives: Despite advancements, there remain significant challenges in managing irreparable supraspinatus (SSP) tears. Among joint preserving procedures such as partial reconstruction or procedures creating a spacer effect such as SCR or the implantation of a bioabsorbable balloon, tendon transfers yield the dual benefits of restoring active motion and being independent of remaining functional tissue. Recently described methods like middle trapezius transfer (MTT) have been introduced to restore function; however, biomechanical data on its efficacy in the setting of an irreparable SSP tear is lacking. The purpose of this study was to evaluate the efficacy of MTT to restore superior humeral head translation, and sub-acromial contact mechanics in the setting of an irreparable SSP tear using a robotic model.

Methods: Ten fresh-frozen cadaveric shoulders were mounted to a 6-DOF robotic arm (KUKA KR-60), in which rotator muscles, deltoid and the middle trapezius were loaded statically. All specimen underwent biomechanical testing in three states: (1) intact, (2) simulated irreparable SSP tear, (3) MTT. 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 was measured by the robot and subacromial contact area and peak pressure were measured using a contact pressure sensor (Tekscan 5503). A 1-factor random-intercepts linear mixed effects model was created for each outcome and each ABD angle.

Results and conclusion: The humeral head significantly translated superiorly in the SSP defect state across all ABD angles (+1.6 to +3.8 mm, p < 0.0001) and peak pressure significantly increased compared to native (+0.48 to +0.55 MPA, p < 0.05), while there was a significant decrease in subacromial contact area (-15 to -40 mm², p < 0.05). The MTT significantly reduced superior translation across all ABD angles (-0.7 to -1.6 mm, p < 0.05) compared to the defect state. The MTT successfully restored peak pressure conditions comparable to the native state across the entire ABD ROM and a notable reduction in peak pressure was observed at 30° and 45° of ABD when compared to the SSP cut state (0.33 to 0.38 MPA, p < 0.05). After MTT, the contact area was significantly higher compared to the SSP cut state at 30° of ABD (+32 mm², p < 0.005).

In this study, isolated irreparable SSP tears were found to significantly increase superior glenohumeral translation and subacromial peak pressure, while decreasing the subacromial contact area. Performing an MTT reduced superior translation, subacromial peak pressure, and increased subacromial contact area. Combining it’s unique advantage of addressing both static and dynamic components, contributing to the restoration of joint compression dynamic shoulder stabilization, this transfer partially restored the shoulder kinematics to native conditions in the presence of an irreparable SSP tear.