Initial fixation of metallic wedge-augmented versus angled BIO reverse shoulder arthroplasty techniques: A finite element study.

Augmented techniques in the reverse total shoulder arthroplasty (rTSA) have emerged to treat large asymmetric glenoid bone defects and restore shoulder function. However, whether metallic wedge-augmented (W-AUG-rTSA) and angled bony increased-offset (angled BIO-rTSA) rTSA techniques provide equivalent implant fixation remains unclear. This study aimed to directly compare the initial fixation of W-AUG-rTSA and angled BIO-rTSA in a 15° retroverted glenoid, while also assessing the impact of graft stiffness and the number of peripheral screws. Finite element models were developed considering compressive and inferior-to-superior shear loads. Micromotions at the bone-implant interface were compared between the techniques, considering variations in the number of peripheral screws (2 vs 4) and graft stiffness in the angled BIO-rTSA (96 MPa and 1.3 GPa to simulate different bone qualities, and 2.5 GPa to simulate a porous metal wedge as used in the W-AUG-rTSA). The W-AUG-rTSA and angled BIO-rTSA achieved, respectively, maximum micromotions of 63.5 µm and 47.4-65.0 µm (depending on graft stiffness). Assuming a bone ingrowth threshold of 50 µm, 9% and 0%-4% of the bone-implant interface exceeded this threshold for the W-AUG-rTSA and angled BIO-rTSA techniques, respectively, when 4 peripheral screws were used. These results suggest that both augmentation techniques can likely achieve good initial fixation under this screw configuration. Although changes in graft stiffness affected the micromotion distribution in the angled BIO-rTSA, their overall impact on initial fixation was limited. Reducing the number of peripheral screws to 2 resulted in a substantial increase in interface nodes exceeding the 50 µm threshold in both techniques.