Finite element analysis of acromial fracture after reverse total shoulder arthroplasty

Background The prevalence of acromial fracture after reverse total shoulder arthroplasty (RSA) is reportedly 2.6–8.4%, and clinical outcomes differ among fracture sites. However, few studies have investigated the factors affecting fracture sites. This study aimed to reproduce acromial fractures after RSA by replicating the postoperative displacement of the humerus using finite element models (FEM), and to investigate the effect of humeral displacement on the fracture site. Methods Six patients (mean age, 76.3±7.9 years; five women, one man) with acromial fractures after RSA treated at our institution were included. Among them, three had Levy Type 1 fractures, whereas the other three had Levy Type 2 fractures. Preoperative computed tomography (CT) data were used to create a three-dimensional FEM. All elements within the proximal 2 cm of the clavicle and the scapular body were completely constrained. The entire humerus was forcibly displaced following its displacement after RSA, as measured by preoperative and postoperative CT. The fracture sites, total displacement, external constraint of the humerus at the initial fracture, and bone density from the acromion to the scapular spine were investigated. Values of P < 0.05 were considered statistically significant in tests of statistical inference but were interpreted as reference values due to the small number of cases. Results There were no major differences in humeral displacement between Type 1 and Type 2 fractures. Although the fracture sites in the FEM were slightly more medial than the actual fracture sites, they were similar. The total displacement and external constraint of the humerus at the initial fracture were similar in Type 1 and Type 2 fractures. The distribution of bone density from the acromion to the scapular spine differed between fracture types. Conclusion We were able to reproduce acromial fractures after RSA by replicating humeral displacement using FEM. The extent and direction of humeral displacement may not significantly affect the acromial fracture site, whereas the bone density distribution from the acromion to the scapular spine may affect it.