An analysis of variability and terminology inconsistencies in the amount and type of constraint in polyethylene bearing liners across reverse shoulder arthroplasty implant systems.

BACKGROUND

This study aims to evaluate the amount and type of constraint between different reverse total shoulder arthroplasty (rTSA) systems to address a current gap in uniform nomenclature and help guide surgical decision-making in rTSA.

METHODS

Data were 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 selected designs, lip heights were used. All available sizes and diameters were analyzed, and data were cross-referenced with published literature and product manuals.

RESULTS

The mean constraint ratio for standard liners was 48.74% (standard deviation [SD] = 5.60%), with values ranging from 40% (Medacta Shoulder System) to 62% (Stryker Reunion S). Several devices fell outside one SD from the mean, indicating notable intercompany variability. A one-way analysis of variance confirmed significant differences among standard liner designs (P < .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 analysis of variance results demonstrated significant variation across manufacturers (P < .001).

CONCLUSION

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 using the terms "standard" and "retentive" and instead using the implant's neck-shaft angle and capture ratio.