PURPOSE: The primary aim was to analyse the lateral malleolus morphology with a focus on the shape of the distal fibula for IM fixation of the fibula in infra- and transsyndesmotic fracture patterns. The secondary aim was to propose a treatment algorithm according to the lateral malleolar anatomy. METHODS: 77 healthy, skeletally mature volunteers underwent CT scanning of the ankle. The fibula medullary canal and its cortical thickness were quantitatively analysed at 4 different levels measured from the fibular tip (1.5 cm, 3.0 cm, 4.5 cm, and 6.0 cm). A geometric classification was proposed, and a decision algorithm was developed. Statistical significance was set at a p-value < 0.05. RESULTS: The smallest diameter of the medullary canal of the fibula was at 6.0 cm from the tip of the fibula, in 98.2% of the ankles. The distal fibula can be classified into triangular and rectangular type, according to the cortical thickness index (p < 0.0001). In 16.7% ankles, the internal diameter of the fibula at 6.0 cm was equal or narrower than 3.5 mm (p < 0.05). CONCLUSION: The shape of the distal fibula as evaluated by CT-guided analysis allows for IM osteosynthesis of the lateral malleolus in 83% of individuals. In our study, the smallest diameter of the medullary canal of the fibula was located 6.0 cm from the tip of the lateral malleolus. Coronal CT evaluation is advantageous in indicating which type of IM implant should be used safely for infra- and transsyndesmotic fracture patterns, potentially reducing intraoperative risks.