An Analysis of Variability and Terminology Inconsistencies in the Amount and Type of Constraint in Polyethylene Bearing Liners Across Reverse Shoulder Arthroplasty Implant Systems.

Abstract

This study aims to evaluate the amount and type of constraint between different Reverse Total Shoulder Arthroplasty (rTSA) systems in order to address a current gap in uniform nomenclature and help guide surgical decision-making in rTSA. Data was collected on commercially available polyethylene liners for rTSA implant manufacturers in the United States. Constraint ratios were calculated using liner depth and glenosphere radius, and for select designs, lip heights were used. All available sizes and diameters were analyzed, and data was cross-referenced with published literature and product manuals. The mean constraint ratio for standard liners was 48.74% (SD = 5.60%), with values ranging from 40% (Medacta Shoulder System) to 62% (Stryker Reunion S). Several devices fell outside one standard deviation from the mean, indicating notable inter-company variability. A one-way ANOVA confirmed significant differences among standard liner designs (p < 0.001). For retentive liners, the mean constraint ratio was 61.90% (SD = 5.24%), ranging from 47% to 71%. Again, multiple devices fell outside the expected range, and ANOVA results demonstrated significant variation across manufacturers (p < 0.001). Significant variability exists between implant systems when defining "standard" and "retentive" liners in rTSA with calculated constraint ratios. This inconsistency can complicate surgical decisions, especially during revisions or when switching systems, potentially leading to unintended biomechanical outcomes. We advocate for enhanced transparency and standardization by moving away from utilizing the terms "standard" and "retentive" and instead utilizing the implant's neck shaft angle and capture ratio.