What is the best glenoid configuration in onlay reverse shoulder arthroplasty?

The purpose of this study was to analyze the effect of different glenoid configurations on arm position and range of motion (ROM) following reverse shoulder arthroplasty (RSA). The hypothesis was that different glenoid configurations would lead to changes in humeral offset, acromio-humeral distance (AHD), ROM, and rotator cuff muscle length.

Using a three-dimensional (3D) computer model, implantation of an RSA was simulated with a 145° onlay humeral stem combined with five different glenoid configurations which varied in diameter and centre of rotation. Glenoid offset, the AHD, ROM, and muscle length were evaluated for each configuration.

Changing glenoid design led to up to a 10 mm change in offset and a 3 mm change in the AHD. There was 7° of improvement in abduction and flexion between the different glenoid designs. Two of the configurations, the 36 mm centered and the BIO-RSA, had an adduction deficit. In extension and external rotation arm with the arm at side, the eccentric 36 mm glenosphere was the best configuration while the centered 36 mm glenosphere was the worst configuration. The 42 mm glenosphere limited external rotation at 90° of abduction.

Varying the glenosphere configurations leads to ROM and muscle length changes following RSA. With a 145° onlay humeral stem, a 36 eccentric glenosphere theoretically optimizes ROM while limiting scapular notching.