Zone-specific acromial cortical thickening after reverse shoulder arthroplasty

PURPOSE: Acromial and scapular spine fractures after reverse shoulder arthroplasty (RSA) can impair post-operative outcomes; however, quantitative assessment of pre-fracture cortical changes remains challenging. This study evaluated longitudinal changes in inferior acromial cortical thickness after RSA using a computed tomography-based (CT-based) acromial coronal view. METHODS: This multicentre retrospective cohort study included 44 patients who underwent RSA. The acromion was divided into three zones: I (lateral acromion), II (acromial base), and III (scapular spine). Inferior cortical thickness was measured at the centre of each zone. Changes were analysed from immediately post-operatively to one year and from one to two years in shoulders with corresponding CT scans. Cortical area was assessed as a secondary outcome, and the fracture cases were described separately. RESULTS: From immediately post-operatively to one year (n = 41), cortical thickness was unchanged in Zone I but increased in Zone II from 2.0 ± 0.5 to 2.4 ± 0.7 mm and in Zone III from 3.8 ± 1.0 to 4.6 ± 1.6 mm. From one to two years (n = 28), Zones II and III showed further increases, whereas Zone I remained unchanged. Thickness changes correlated with cortical area changes (Spearman's ρ = 0.55). Five shoulders (11.4%) developed fractures within two years, and delayed Zone II and III fractures showed marked preceding cortical thickening. CONCLUSION: Inferior acromial cortical thickening after RSA progressed selectively in Zones II and III, suggesting region-specific post-operative acromial remodelling. These findings may help clarify the differences in fracture pathomechanisms according to the location.