Review Question - QID 217419

The lateral tibiofibular ratio (measurement 4/5) is the most reliable way to confirm reduction of the syndesmosis on plain radiographs.

Syndesmotic ankle injuries are not uncommon, occurring with 13% of all ankle fractures. Most commonly, an external rotation injury is the cause of the ligamentous injury. With disruption of the syndesmosis, it no longer functions to resist axial or rotational forces. Patients often have pain diffusely, but may localize it over the syndesmosis or have a positive squeeze test. Syndesmotic instability can occur and be judged in both the coronal and sagittal planes. Radiographs can be used to evaluate for syndesmotic instability, with decreased tibiofibular overlap and increased tibiofibular clear space both having been described as predictors in the past. However, recent studies have demonstrated that the most reliable radiographic judge of syndesmotic reduction is actually the tibiofibular overlap seen on a lateral radiograph.

Grenier et al. performed a radiographic study to explore the usefulness of an anteroposterior tibiofibular (APTF) ratio in order to assess syndesmotic reduction. They noted the average APTF ratio was 0.94, with the anterior fibular cortex normally crossing in the middle of the tibial shadow. They concluded that significant abnormalities outside of this normal range could indicate syndesmotic injury or malreduction.

Krahennbuhl et al. performed a systematic review of the different syndesmotic imaging methods. They concluded that typical AP radiographic measurements can not reliably predict syndesmotic injury. They suggest using MRI in confirming syndesmotic injury, as its sensitivity and specificity near 100% in diagnosis.

Figures A and B are AP and lateral ankle radiographs demonstrating measures of syndesmotic disruption (1) tibiofibular overlap, (2) tibiofibular clear space, (3) medial clear space, (4), lateral segment from anterior tibia to anterior fibula, (5) lateral segment from anterior fibula to posterior tibia.

Incorrect Answers:
Answer 1: This measurement represents the tibiofibular overlap which should be >6mm on an AP and >1mm on a mortise view.
Answer 2: This measurement represents the tibiofibular clear space which should be <6mm on both AP and mortise views.
Answer 3: This measurement represents the segment between the anterior tibial cortex and anterior fibular cortex on the lateral radiograph.
Answer 4: The tibiofibular overlap and clear space would not form a useful ratio.