BackgroundA proper reduction and internal fixation of posterior malleolar fractures can be challenging, as intraoperative fluoroscopy often underestimates the extent of the fracture. Our aim was to assess the value of a modified classification system for posterior malleolar fractures, which is based on computed tomography (CT) images, optimizing screw trajectory during fluoroscopic-guided surgery, and to compare it to the Lauge-Hansen classification system to the CT-based classification.MethodsA retrospective review of all ankle fracture operations from January 2014 to December 2016 was performed. Fractures were included if a CT scan was performed within 1 week of the surgery, and the posterior malleolar fragment occupied one third or more of the antero-posterior talar surface or jeopardize the ankle stability. Eighty-five adult ankle fractures with posterior malleolar fragments were included in this study. Fractures were categorized into one of three types, namely “postero-lateral,” “postero-medial,” or “postero-central,” according to the location of the fracture fragment on axial CT image. An optimal trajectory angle for a single-lag screw fixation was measured on the CT cut between a central antero-posterior line and the line intersecting the posterior fragment perpendicular to the major fracture line. Mean trajectory angles were calculated for each fracture type. Fractures were also categorized according to the Lauge-Hansen system.ResultsThe mean trajectory angle was 21° lateral for “postero-lateral” fragments, 7° lateral for “postero-central” fragments, and 28° medial for “postero-medial” fragments (p < 0.01 for comparisons among the groups). The range of trajectory angles within each group was about 10°, as compared to about 20° within each Lauge-Hansen type. There were no differences in trajectory angle among the Lauge-Hansen groups (p > 0.05 for all comparisons).ConclusionsThere are 3 distinct anatomic subgroups of posterior malleolar fragments, each with an ideal screw trajectory that needs to be used in order to achieve an optimal reduction and fixation.