Humeral Stem Designs in Reverse Total Shoulder Arthroplasty: A Focus on the Adjustability of Inclination, Torsion, and Version

Background: While the inclination of the humeral component in reverse total shoulder arthroplasty (rTSA) has been extensively studied, less attention has been given to the parameters of version and torsion. These parameters, which influence outcomes in rTSA, warrant closer examination. This study aims to analyze the designs of humeral stems in rTSA, focusing on their ability to independently adjust inclination (coronal plane), torsion (axial plane), and version (sagittal plane). We hypothesize that while most designs allow for inclination and torsion adjustments, independent version adjustment is rare and may offer unique biomechanical implications. Purpose: To systematically review currently available rTSA humeral stem designs and assess their capacity for independent adjustment of inclination, torsion, and version based on manufacturer specifications and design features. Methods: A comprehensive review of commercially available rTSA humeral stem designs was conducted. Humeral stems from various manufacturers were analyzed for their capacity to allow independent adjustment of inclination, torsion, and version. The analysis relied on technical specifications and data provided by manufacturers, supplemented by a critical review of design features. Results: The review showed that while a few stem designs offer inclination adjustment up to 12.5° via angled liners, there is one implant allowing inclination adjustment within the same stem design, through a modular metaphyseal cup, permitting adjustment from 135° to 155°. Torsion was found to be freely adjustable across all designs, independent of the specific stem configuration. While all systems provide a fixed stem version of 0°, one system offered adjustability of humeral version (0° and 20°) through a modular metaphyseal component. Conclusions: While torsion can be freely adjusted independent of stem design, the ability to modify inclination is limited to certain systems, mostly relying on angled liners for this purpose. Independent adjustability in the sagittal plane (version) remains rare, with only one system offering this feature through a modular metaphysis. The potential clinical implications of adjustable humeral version in rTSA are yet to be investigated.