Trimalleolar ankle fracture with syndesmotic injury

Discussion:

The x-ray in this case severely underplays the severity of the ankle injury, which on MRI consists of:

  1. Trimalleolar fracture with undisplaced fractures of the posterior and medial malleoli as well as avulsion fractures of the lateral malleolus involving the anterior talofibular ligament.
  2. Syndesmotic injury with complete rupture of the anterior inferior syndesmotic ligament and hyperintensity of the interosseous and posterior inferior syndesmotic ligaments in keeping with grade 2 injuries.
  3. Anteromedial talar dome osteochondral injury with subchondral fracture.

Occult ankle fractures are uncommon and it is difficult to maintain a high level of suspicion with most ankle x-rays negative for fracture (between 2.5-20% are positive for fracture depending on clinical setting, being highest in emergency department referrals). Medial/lateral ankle soft tissue swelling does not help pick up occult fractures but an ankle joint effusion of >13-15 mm (measuring anterior and posterior components perpendicular to the joint line) on a lateral radiograph without a fracture seen should prompt further referral to CT. 

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