The biomechanics of subscapularis repair in reverse shoulder arthroplasty: The effect of lateralization and insertion site

Functional outcomes of subscapularis (SSc) repair following reverse shoulder arthroplasty (RSA) remains controversial. SSc repair in combination with glenosphere lateralization was reported to yield worse clinical outcomes compared with the non-lateralized glenosphere. The aim of this biomechanical study was to investigate how glenosphere lateralization and different re-insertion sites can affect the biomechanics of the SSc after RSA. Nine patient-specific RSA shoulder models were created from patients' computed tomography scans. Moment arms and SSc length were calculated for abduction, forward flexion, and internal rotation in 20 degrees and 90 degrees abduction for three configurations of glenosphere lateralization (standard/+0, +5, and +10 mm) and three SSc repair sites (native, superior, and inferior) and compared with the native shoulder. When compared with the native shoulder, RSA resulted in large adducting SSc moment arms that were antagonistic to the deltoid. Glenosphere lateralization had no effect on SSc moment arms in any motion. However, lateralization increased SSc tension beyond its anatomic length for +5 and +10 mm of lateralization when attached to its native insertion. A superior SSc repair site created the least adductive moment arm as well as the least amount of SSc lengthening. Increased glenosphere lateralization showed a significant increase in the SSc length, which in combination with its adductive moment arm can be antagonistic to deltoid function. However, a superior SSc repair site may help reduce the adductive SSc moment arm and allow for reduced tension on the repair as its length in that location is less than that of the native SSc. (c) 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res