Diagnosis is by plain radiographs (AP, lateral, and mortise views):

• Note whether fibular fractures are distal, proximal, or at the level of the tibia-fibula syndesmosis. 
• The lateral view is best for assessing the posterior tibial articular surface. 
• On the mortise view (taken with the leg in 15° internal rotation), measure the distance between the talus and medial malleolus (the “medial clear space”); >4mm suggests instability.

     An additional view to assess stability is a mortise stress view.  This is taken with the examiner holding the leg internally rotated with one hand while the foot is externally rotated with the other hand.  Medial clear space > 5 mm on stress view suggests ankle instability, either from medial malleolar fracture or deltoid ligament rupture. This view can often be deferred until orthopedics follow-up.