Treating posteriorly eroded glenoids with augmented baseplate or bony increased-offset reverse shoulder arthroplasty: a finite element comparison

Augmented reverse shoulder arthroplasty (RSA) implants restore glenohumeral joint alignment in cases of asymmetric glenoid wear. However, no consensus has been reached on whether the use of metallic augmented RSA baseplates and bone graft reconstruction are equivalent in terms of implant fixation and risk of implant loosening. Therefore, the purpose of this study was to compare two augmented RSA designs by assessing the amount of interfacial micromotion generated under realistic physiological loading. Finite element analysis (FEA) models of 9 scapulae with Walch-type B2 or B3 glenoid morphology were virtually implanted with both a metallic augmented baseplate (AUG-RSA) and using the angled bony increased offset RSA procedure (BIO-RSA). Simulation of physiological loading was performed on each of the 18 FEA models. The relative tangential and normal micromotion at the implant-to-glenoid interface was compared in each anatomical quadrant. The AUG-RSA and angled BIO-RSA showed similar magnitudes of micromotion in most anatomical quadrants of the glenoid. Within the superior quadrant, AUG-RSA displayed a higher magnitude of mean and maximum tangential micromotion (mean: 16.6 ± 2.4 μm, p<0.000; max: 35.1 ± 5.3 μm, p<0.000). The proportion of the posterior quadrant experiencing greater than 50 microns of micromotion was also statistically greater with AUG-RSA (5.8 ± 2.5 %, p=0.047). Due to its statistically greater micromotion and portions of contact exceeding the accepted 50 micron threshold, the AUG-RSA may be more likely to have inhibited bone on-growth. However, the clinical importance of these differences remains unclear.