Due to the different force exerted during the posterior malleolus fracture (PMF), the difference in sagittal angle (SA) between the fracture fragments may affect ankle stability. But this aspect is less well studied and the aim of this study was to investigate the relationship between SA and the stability of PMF.The imaging data of 120 patients with PMFs from January 2014 to November 2022 were collected retrospectively and reconstructed. We first measured SA, posterior fragment area (PFA) and fragment area ratio (FAR), reanalyzing the correlation of SA with PFA and FAR, respectively. To better describe the morphological characteristics of the fracture fragments, we further measured the fragment width diameter ratio (FWR), the fragment length ratio (FLR), fragment height (FH), contact area (CA), and finally carried these data into the regression model of SA versus FAR to conduct the intermediary role.SA was negatively correlated with PFA(s) (r = -0.583, P < 0.001), with regression equation s = -0.063SA + 3.066; SA was negatively correlated with FAR (r = -0.204, P < 0.05), with regression equation FAR = -0.002SA + 0.198; A significant correlation was found between FWR, FLR, FH, CA and SA (P < 0.05), as well as between FWR, FLR, FH and FAR (P < 0.05); Further intermediary role analysis showed that FWR, FLR, FH had a partial intermediary role between SA and FAR.As SA increased, PFA and FAR decreased, so the larger the SA was due to the effect of vertical shear force, reflecting higher ankle stability, meanwhile, FWR, FLR and FH should also be considered on the fixation method of fracture fragments.© 2023 The Authors. Orthopaedic Surgery published by Tianjin Hospital and John Wiley & Sons Australia, Ltd.