Summary

Lateralization was achieved with thicker glenospheres, whereas lateralization with distalization were associated with humeral components modification. Lateralized implant options should be selected according to patients’ needs.

Abstract

Introduction

Lateralization of the center of rotation (COR) was proposed to mitigate scapular notching, reduced range of motion, and instability after reverse total shoulder arthroplasty (RTSA) via different implant options. The objective of our study was to quantify the biomechanical effectiveness of lateralization in RTSA with respect to glenoid and humeral component configurations.

Methods

Eight cadaveric shoulders were tested in a custom shoulder testing system. Three parameters, including the glenosphere thickness, humeral tray offset, and insert thickness, were accessed by implanting eight configurations on each specimen. Humeral position, maximum internal rotation, and maximum external rotation (ER) before impingement were quantified at 0°- and 30°-abduction. The adduction angle at which the humeral component contacted the inferior scapular neck and the abduction angle where acromial notching occurred were also measured. The simulated active range of motion, including abduction and ER capability, was tested by increasing the loading applied to the remaining posterior cuff and middle deltoid. Stability was evaluated by the forces that induced anterior dislocation at 30° abduction.

Results

SECTION: The thicker glenosphere affected only lateralization, whereas the centric humeral tray and thicker insert significantly affected humeral lateralization and distalization simultaneously. Greater adduction and ER angles were found in more lateralized humerus. A significant positive correlation between humeral lateralization and ER capability was observed; however, lateralization did not significantly improve implant stability in this cadaveric testing system (p > 0.05).

Discussion

Humeral components modification is associated with lateralization and distalization, whereas only lateralization is achieved with thicker glenospheres. Humeral lateralization with distalization by a thicker insert may be beneficial for patients with pseudoparalysis and lateralized COR. The centric humeral tray would be selected for less distalization to avoid over-lengthening, whereas an eccentric humeral tray is the most effective for distalization and medialization in reducing abduction notching to the acromion. Lateralization is an effective strategy for reducing adduction notching while increasing active ER.