A Novel Study on The Trend Surface of Posterior Malleolar Fractures

Background: Posterior malleolar fractures (PMFs) account for 7-44% of all ankle fractures, and are crucial for ankle stability, associated with poorer functional outcomes and higher rates of traumatic arthritis. Despite the exact mechanism and treatment strategy of this kind of fractures still remain controversial, most orthopaedic surgeons tend to treat PMFs with open reduction internal fixation method. To achieve rigid fixation for the fragment, screws should be inserted across the fracture plane perpendicularly. However, the study on the spatial geometry of the fracture surface of PMFs, especially the trend surface, has not been reported before.Objective: This study aimed to construct the trend surface of PMFs and establish an algorithm to better understand the spatial geometry of the fracture surface, which could guide treatment strategies.Methods: A retrospective analysis of CT images of PMFs from January 2021 to January 2024 was conducted. Cases were grouped according to the Bartoníček classification system. The skeletal ankle models were reconstructed, which were then transformed into CAD models. Fracture surfaces were segmented, and point clouds were output to record coordinate values. The K-Dimensional Tree data structure was built for the efficient indexcality of the Depth-First-Search algorithm to find the matched points in the clouds. The average point was computed for matched points, and subsequently, the fracture trend surface and heat map were generated.Results: Twenty-four cases were included. The posterior malleolar fracture trend surface was composed of two surfaces, each approximated as a plane and intersect an angle about 130.5°. The posterolateral surface extended slightly curvingly from the posterior two-fifths of the fibular notch to the posteromedial one-third of the distal tibia, formed an angle of 10.1° with the tibia axis and 10.1° with the coronal plan of the lower leg. The posteromedial surface extended from the posteromedial one-third of the distal tibia towards the intercollicular groove, formed an angle of 5.2° with the tibia axis and 39.4° with the coronal plan of the lower leg. The heat map revealed that the high-frequency fracture regions presented at 16.0mm away from the posterior malleolar distal edge and 11.2mm medial to the posterior margin of the fibular notch.Conclusion: The study provides a novel perspective on PMFs by describing the spatial geometric morphology of the fracture surface. This can be useful for the treatment strategies. 1. The fracture lines on the bone surface help us generate a stereoscopic model of the fracture surface in our brain, but this model may not be the same as the true fracture surface. 2. In A-P fluoroscopic view, 16.0mm away from the posterior malleolar distal edge and 11.2mm medial to the posterior margin of the fibular notch were the high frequency regions where the fracture surfaces passed through. 3. To firmly hold the posterolateral PMF, 3 key screws should be parallelly inserted into the distal tibia through the fragment, oriented medially 10.4°, cephalad 10°. The insertion points of the screws, in back view, are individually identified at 8.3mm, 16.9mm, 8.3mm superior to the inferior margin of the posterior malleolus, and 6.0mm,10.5mm, 15.1mm posterior to the posterior margin of the fibular notch. The depth of the screws should be no more than 40mm to avoid irritating the anterior tendons, vessels and nerves.