Digital data of the human body were collected from the Korea Institute of Science and Technology Information (KISTI) and used by agreement. We used CT images of adult cadavers who underwent continuous 1.0 mm slice CT scans (Pronto, Hitachi, Japan) in the supine position. Based on a review of their records, cadavers with joint or tibiofibular bone problems were excluded. A total of 102 cadavers (49 males and 53 females) were enrolled. Their mean age at death was 52.4 years (range, 21–60 years; SD, 9.12) and their mean height was 160.4 cm (range, 146–176 cm; SD, 7.23). CT data in Digital Imaging and Communications in Medicine (DICOM) format were imported into Mimics® software (Materialise Interactive Medical Image Control System; Materialise, Antwerp, Belgium) to reconstruct 3D models, which included the tibia, fibula, and talus. After generating the 3D models, the CT scanning plane was reoriented to produce anatomical axial and coronal images parallel to the tibial plafond in the neutral rotation using the Mimics® software (Fig. 1).
An actual-size 3D cylinder model (Ǿ3.5 mm, length 100 mm) was created using 3D CAD software (SolidWorks 2019®, MA, USA) for simulating syndesmotic screw fixation in the distal tibiofibular joint. After obtaining 3D reconstructions of the bones and implant, virtual implantation of syndesmotic screws (syndesmotic cylinder) was performed via four synchronized windows composed of the axial plane, and coronal, sagiJol, and 3D biplanar images.[10–13] In the true coronal plane, a provisional line 1 mm in diameter at the proximal end of the incisura fibularis was placed for determining the height of the screw. In the axial plane, the provisional line was adjusted for bisecting the fibula and incisura fibularis and defined as the ideal syndesmotic cylinder trajectory. Finally, after the syndesmotic cylinder was definitively placed along the provisional line, it was fine-tuned and verified several times by an experienced surgeon (first author, coauthors, and corresponding author). Next, for considering tibial torsion and its variability[8, 14], the tibial proximal cylinder (Ø3.5 mm/Length 150 mm), which was tangent to the posterior tibial condyles, was traced and the angle between the proximal cylinder and syndesmotic screw was measured for individual comparisons (Fig. 1).
Using the features of free 360° rotations with magnification in any plane, the morphology of the syndesmotic joint was assessed. The 3D models were rotated until the syndesmotic cylinder was parallel to the ground without any tilt, and this projection was defined as the syndesmotic AP projection. In the syndesmotic AP projection, the overlapping degree between the proximal fibula and the lateral border of the tibia was assessed for utilizing as a practical and consistent landmark. In addition, we evaluated various indicators, including the relationship between the adjacent bones, a nonirregular Shenton’s line, and Weber’s indices, and compared them with the conventional projection of the ankle mortise.
All measurements are presented as mean and range or binary variables, including the overlapping point, tibial torsion, and others. All statistical analyses were performed using the SPSS statistical software package for Windows version 25.0 (SPSS Inc., Chicago, IL, USA). Values of p < 0.05 were considered to be statistically significant.