The distal tibiofibular syndesmosis is a fibrous joint between the distal tibia and fibula, stabilised by the anterior inferior tibiofibular ligament (AITFL), posterior inferior tibiofibular ligament (PITFL), interosseous tibiofibular ligament, and the transverse tibiofibular ligament. The syndesmotic space (tibiofibular clear space) is measured on AP ankle radiograph; normal values are less than 6 mm on AP view and less than 1 mm on mortise view. This space maintains the width of the ankle mortise for stable talar articulation.
The syndesmotic space is the critical measurement in assessing high ankle sprains and syndesmotic injuries on standard radiographs. Widening of the tibiofibular clear space greater than 6 mm on AP or any widening on mortise view indicates syndesmotic disruption requiring stabilisation. The external rotation stress test, squeeze test, and Cotton test assess syndesmotic stability clinically. Syndesmotic screws or suture button devices maintain the fibula in its correct position during healing. Inadequate reduction of the syndesmosis produces chronic ankle pain and accelerated tibiotalar arthrosis.
External rotation injury with or without fracture tears the AITFL and potentially the interosseous membrane, producing syndesmotic instability with widening of the ankle mortise, managed by anatomic reduction and screw or suture button fixation of the fibula in the fibular notch.
Fibular malrotation or inadequate reduction of the syndesmosis after bimalleolar fracture produces a rotated fibula in the fibular notch with a persistently widened or narrowed mortise, causing early tibiotalar arthrosis requiring re-operation for anatomic syndesmotic reduction.
