# Identification of Surgical Plan for Syndesmotic Fixation Procedure Based on Finite Element Method
## Materials and Methods
### Fabrication of 3D Models of Ankle and Syndesmotic Screw for Finite Element Analysis  
The cortical bone, cancellous bone, and ligaments are essential parts of the ankle from clinical and biomechanical perspectives. In particular, the tibia, fibula, talus, and calcaneus need to be included in the 3D model. We selected a suitable patient for the study and captured the CT image of the required part using the CT scanner. The patient was a 32-year-old healthy man, whose condition had been checked in the hospital.  
The CT image was obtained using radiography with an interval of 0.625 mm. This image was imported into the Mimics 22.0 program to extract the necessary bone part and create a 3D model. As the surface of the 3D model at this stage is quite rough and has many pores, we filled the pores and smoothened the surface. If the model has an irregular surface or a complex shape, the results of the FE analysis become less reliable, i.e., the error increases. Therefore, the model should be simplified.  
The complete model comprises the following parts: tibia, fibula, talus, and calcaneus. Each part was modeled in its Hounsfield unit of the cortical and cancellous bones.  
Next, we considered the ligaments, which have the largest effect on the mechanical motion of the bone. The different bones are connected using the ligaments, as shown in Figure 1. The ligaments were classified into anterior latofibrular, calcaneofibrular, posterior latofibrular, deep deltoid, and interosseous membrane. The insertion point of each ligament was determined on the basis of the anatomical data. Figure 1 shows the 3D models of the bones and ligaments.  
The syndesmotic screw, which needs to be included in the 3D model of the ankle, was modeled using the SOLIDWORKS 2019 software. The selected diameters of the screw were 3.5 and 4.5 mm, and the lengths selected were 40, 45, 50, 55, and 60 mm. As the length was determined on the basis of the number of penetrated cortical bones, we considered which of the five classes of length could penetrate three or four cortical bones. If the screw penetrates the fibula and fixes into the tibia, the screw is assumed to have penetrated three cortical bones. However, if the screw penetrates both the fibula and the tibia, it is said to have penetrated four cortical bones. In addition, the levels of the screws were 10, 15, 20, and 25 mm from the tibofibrular joint. As the widths of the tibia and fibula vary depending on the screw level, we considered this fact in determining the length. In summary, 16 types of screws were fabricated on the basis of two different diameters, two different numbers of penetrated cortical bones, and four levels.  
The fabricated screws were included in the complete 3D ankle model. The insertion point was clinically verified from a syndesmotic screw fixation technique. With this point set, the screw level was varied to insert each screw. Table 1 lists the specifications of the model.  
Figure 1: 3D models of the ankle bone and ligaments: (**a**) frontal view, (**b**) anterior ligaments, and (**c**) posterior ligaments.