Polyethylene glenospheres in reverse total shoulder arthroplasty have greater long-term implant survivorship than metallic glenospheres: A New Zealand Joint Registry Study.

BACKGROUND: Reverse total shoulder arthroplasty (rTSA) is increasingly common and clinically effective. Conventional-bearing rTSA (CB-rTSA) employs a metallic glenosphere articulating with a polyethylene humeral liner. Inverted-bearing rTSA (IB-rTSA), using a polyethylene glenosphere with a metallic or ceramic humeral liner, was developed to eliminate polyethylene wear debris at the scapular neck and thereby reduce polyethylene-induced osteolysis and the severity of scapular notching. Comparative data on survivorship and patient-reported outcomes between these configurations remain limited. This study compared implant survival, revision risk and functional outcomes between CB-rTSA and IB-rTSA designs.

METHODS: This retrospective comparative cohort study identified all primary rTSA procedures performed in New Zealand between January 2007 and December 2024 from the New Zealand Joint Registry. Revision-free implant survival and postoperative function, measured by the Oxford Shoulder Score (OSS), were analysed. Patients were grouped by bearing configuration: polyethylene glenosphere (IB-rTSA) and metal glenosphere (CB-rTSA). Subgroup analyses compared SMR Reverse Polyethylene and SMR Reverse Metal (LimaCorporate, Italy) with other CB-rTSA designs, predominantly Delta Xtend (DePuy Synthes, USA). Revision rates per 100 component-years were calculated, and multivariable Cox proportional hazards models compared revision risk, adjusting for age, sex and diagnosis.

RESULTS: A total of 8,601 rTSA procedures were included: 3,437 (40.0%) using a polyethylene glenosphere and 5,164 (60.0%) a metallic glenosphere. Indications for surgery were comparable between groups, whereas the sex distribution differed significantly. Unadjusted revision rates per 100 component-years were higher for the metal group (0.71) than the polyethylene group (0.46) (P = .001). The adjusted hazard ratio for polyethylene compared with metal was 0.57 (95% CI 0.43 - 0.75), representing a 43% lower revision rate. In secondary, unadjusted cause-specific analyses, polyethylene glenospheres were associated with fewer revisions for deep infection (0.11 vs 0.20 per 100 component-years; P = .031) and dislocation (0.09 vs 0.16; P = .045). Mean OSS at 6 months, 5 years and 10 years were similar between groups, with no statistically or clinically significant differences.

CONCLUSION: In this large national registry study, IB-rTSA using a polyethylene glenosphere was associated with significantly lower revision rates than conventional metal glenosphere designs, with no statistically or clinically significant differences in patient-reported functional outcomes. In secondary, unadjusted cause-specific analyses, polyethylene glenospheres were also associated with lower rates of revision for deep infection and dislocation. Although it could not be examined using registry data, this lower revision burden may relate to the absence of polyethylene-induced osteolysis and consequently less pronounced scapular notching.