Introduction: Ankle syndesmotic injury is currently assessed by radiographic criteria defined by Pettrone. These indices are based on the assumption that the ankle is in the correct rotation when the radiographs are taken. This study shows that computerized tomographic (CT) scans of an ankle in its mortice orientation demonstrate a greater range of values for the tibio-fibular overlap (TFO), and the tibial clear space (TCS) than that proposed by Pettrone. This study also demonstrates how rotation of the ankle in the transverse axis changes the values for the TFO and TCS.

Materials and Method: 20 uninjured ankle CT scans were studied to evaluate the syndesmosis. The images were orientated so that measurements were taken 1cm above the tibial plafond with the ankle in a mortice orientation. Each image was rotated 5° from 15° external rotation to 20° internal rotation. Bony landmarks were confirmed prior to taking measurements. These were taken by 2 orthopaedic surgeons on 2 separate occasions.

Results: The range for the TFO is 0–11.6mm; the range for the TCS is 2–6.1mm. The range for the values is greater than that proposed by previous studies. The TFO and TCS change with rotation of the leg.

Conclusion: On the AP radiograph syndesmotic disruption is indicated by a TCS > 5mm, a TFO < 10mm and on the mortise view a TFO < 1mm. A normal ankle should therefore have a TCS less than 5mm and TFO greater than 10mm on the AP and greater than 1mm on the mortise view.

There is a greater normal range of syndesmotic width found on CT scans than suggested by previous studies. Values change with rotation of the leg in its transverse plane. Syndesmotic injury cannot be reliably diagnosed using the current radiological criteria.