Semi-inlay and onlay humeral stems in reverse shoulder arthroplasty achieve comparable outcomes through different patterns of glenoid and humeral lateralization and distalization: A retrospective study from the FP-UCBM Shoulder Study Group.

Abstract

Background: Reverse shoulder arthroplasty (RSA) has evolved through different humeral stem designs to optimize biomechanics and minimize complications. Although inlay and onlay configurations differ in geometry and bone preservation, their clinical outcomes often appear similar. This study aimed to compare clinical, functional, and radiographic outcomes between onlay and semi-inlay humeral stems, hypothesizing that comparable results may be achieved through distinct glenoid and humeral lateralization and distalization patterns.

Methods: A retrospective analysis was conducted on 199 patients who underwent primary RSA between January 2021 and January 2024. Radiographic parameters lateralization shoulder angle (LSA), distalization shoulder angle (DSA), glenoid and humeral lateralization angles (GLA and HLA), and glenoid and humeral distalization angles (GDA and HDA) were measured on standardized anteroposterior radiographs. Clinical evaluation included Constant-Murley Score, Simple Shoulder Test, Visual Analog Scale, and range of motion. Statistical comparisons were performed using non-parametric or parametric tests according to data distribution (α = 0.05).

Results: At a mean follow-up of 28 ± 8 months, 164 patients met the inclusion criteria (41 semi-inlay; 123 onlay); the groups showed no significant difference in clinical outcomes. Radiographically, semi-inlay stems demonstrated higher glenoid lateralization (GLA 53.31 ± 4.43 vs. 50.26 ± 6.92, p = 0.001) and lower humeral lateralization (HLA 34.52 ± 6.49 vs. 35.62 ± 9.11, p > 0.05), while onlay stems showed greater glenoid distalization (GDA 76.84 ± 12.03 vs. 70.01 ± 8.05, p = 0.001) and lower humeral distalization (HDA 22.04 ± 8.40 vs. 29.95 ± 6.53, p < 0.001). No significant differences were found for global LSA or DSA (p > 0.05). Scapular notching incidence was comparable in the groups. Interobserver reliability for angular measurements was excellent (κ = 0.82).

Conclusion: Onlay and semi-inlay RSA designs achieved equivalent clinical outcomes through different biomechanical adaptations. Semi-inlay stems appear to compensate for their intrinsic lower humeral lateralization with increased glenoid lateralization, whereas onlay stems may balance their humeral geometry through greater glenoid distalization. These complementary strategies highlight that, rather than the selection of a specific stem design, it is the achievement of the desired lateralization and distalization obtained through different approaches that is key to successful RSA outcomes.