Posteroinferior Glenosphere Positioning is Associated with Improved Range of Motion Following Reverse Shoulder Arthroplasty with a 135° Inlay Humeral Component and Lateralized Glenoid.

BACKGROUND: Optimal glenosphere positioning in a lateralized reverse shoulder arthroplasty (RSA) to maximize functional outcomes has yet to be clearly defined. Center of rotation (COR) measurements have largely relied on AP radiographs which allow assessment of lateralization and inferior position, but ignore scapular Y radiographs which may provide an assessment of posterior and inferior position relative to the acromion. The purpose of this study was to evaluate the COR in the sagittal plane and assess the effect of glenosphere positioning with functional outcomes utilizing a 135° inlay stem with a lateralized glenoid.

METHODS: A retrospective review was performed on a prospectively maintained multicenter database on patients who underwent primary RSA from 2015-2021 with a 135° inlay stem. The COR was measured on minimum 2-year postoperative sagittal plain radiographs using a perfect-circle fit method. A perfect circle was made on the glenosphere and the center was marked. From there, four measurements were made: 1) center to the inner cortex of the coracoid, 2) center to the inner cortex of the anterior acromion, 3) center to the inner cortex of the middle acromion, 4) center to the inner cortex of the posterior acromion. Regression analysis was performed to evaluate any association between the position of the COR relative to bony landmarks with functional outcomes.

RESULTS: A total of 136 RSAs met the study criteria. There was no relation with any of the distances with outcome scores (ASES, VAS). In regards to range of motion (ROM), each distance had an effect on at least one parameter. The COR to coracoid distance had the broadest association with ROM with improvements in forward flexion (FF), external rotation (ER0), and internal rotation with arm at 90° (IR90) (p = <0.001, 0.031, <0.001; respectively). The COR to coracoid distance was also the only distance to affect the final FF and IR90. For every 1 mm increase in this distance, there was a 1.8° increase in FF and 1.5° increase in IR90 (ß = 1.78; 95% CI 0.85 - 2.72, p = <0.001, ß = 1.53; 95% CI 0.65 - 2.41, p = <0.001; respectively).

CONCLUSION: Evaluating the COR following RSA in the sagittal plane suggests that posteroinferior glenosphere position may improve ROM when using a 135° inlay humeral component and a lateralized glenoid.