Is a 32 mm glenosphere optimal in reverse shoulder arthroplasty for patients with small glenoid anatomy? : a range of motion simulation study in petite Asian females.

Aims: In reverse shoulder arthroplasty (RSA), the diameter and position of the glenosphere have a significant effect on impingement-free range of motion (ROM). This effect may be more pronounced in patients with small glenoid anatomy. We tested whether a 32 mm glenosphere provides greater ROM than 36 mm and 39 mm glenospheres across three inferior overhangs (IOs). Methods: Asian female patients undergoing RSA (mean age 75.5 years (SD 5.6; range 66 to 89); mean height 145.9 cm (SD 3.3; range 137.6 to 153.0)) were evaluated using 3D CT planning. Three diameters (32 mm, 36 mm, and 39 mm) and three IOs (1.5 mm, 3.0 mm, and 4.5 mm) were combined (nine simulations per patient). We recorded impingement-free ROM arcs from neutral (0°) to bony impingement (bone-to-bone contact) for extension-flexion (EF), abduction-adduction (AA), and external-internal rotation (RO). Postoperative outcomes were described for the 32 mm subset (n = 11). Repeated-measures analysis of variance assessed the main effects and interactions of diameter and overhang. Results: Both glenosphere diameter and inferior overhang had significant independent effects in each plane (all p < 0.001) without consistent interactions. The 32 mm glenosphere conferred broader arcs of motion in EF, AA, and RO, whereas the 39 mm diameter consistently restricted ROM. A higher IO (≥ 3.0 mm) further expanded motion. Conclusion: In this cohort with small glenoid anatomy, a 32 mm glenosphere and IO ≥ 3.0 mm maximized predicted bony impingement-free ROM, suggesting that downsizing with adequate inferior overhang may be advantageous in a small glenoid vault. Clinical validation accounting for soft-tissue constraints and implant stability is required.