Reverse shoulder arthroplasty glenoid lateralization influences scapular spine strains

Background: Scapular spine insufficiency fractures following reverse shoulder arthroplasty are poorly understood. There exists limited literature regarding the role of reverse shoulder arthroplasty lateralization on scapular spine strains and fractures. The purpose of this cadaveric biomechanical simulator study was to evaluate the role of glenoid lateralization on scapular spine strain. Methods: Eight cadaveric shoulders were tested using an in-vitro simulator. A custom modular reverse shoulder arthroplasty was implanted that allowed for in-situ glenoid lateralization adjustment. Scapular spine strain was measured by strain gauges placed in clinically relevant Levy zones along the scapular spine. All specimens were tested in loaded forward elevation and abduction. Results: Glenoid lateralization from 0 to 5mm caused negligible changes in scapular spine strains. Lateralization from 5 to 10 mm, however, caused significant increases in strain at 0 degrees forward elevation in all strain gauges (p < 0.026). Strains measured in Levy zone 2 were significantly higher than all other locations (p < 0.039). Additionally, forward elevation resulted in significantly higher strain values than abduction (p = 0.001). Conclusions: Glenoid lateralization is an important parameter in reverse shoulder arthroplasty; however, our results demonstrate higher degrees of lateralization may place higher strains on the scapular spine. An understanding of reverse shoulder arthroplasty lateralization and scapular spine strains is important to optimize parameters and to mitigate negative effects.