Presentation
Twisted ankle in eversion injury. Unable to weightbear.
Patient Data
There is cortical irregularity at the tip of the lateral malleolus, which is equivocal for an acute or chronic traumatic injury. No evidence of fracture elsewhere.
Skeletal alignment within normal limits. Small bony spur at the anterior aspect of the distal tibial plafond. No enlarged ankle joint effusion. There is generalized prominence of the soft tissues of the calf and ankle with more marked soft tissue swelling over the lateral malleolus.
Large ankle and posterior subtalar joint effusions. Further small talonavicular joint effusion.
There is an undisplaced fracture through the posterior malleolus of the tibia involving the insertion of the posterior inferior syndesmotic ligament, which itself is heterogeneously hyperintense but intact. There is an avulsion fracture of the tip of the lateral malleolus involving the anterior talofibular ligament insertion; the anterior talofibular ligament is intact but hypointense and thickened suggesting previous injury. There is a full thickness rupture of the calcaneofibular ligament. There is a further undisplaced fracture of the medial malleolus with grade 2 sprain injury of the deltoid ligament. There is bone marrow edema with associated T1 hypointensity of the anteromedial talar dome in keeping with an osteochondral injury with subchondral fracture.
Anterior inferior syndesmotic ligament is completely ruptured. Hyperintensity of the interosseous ligament. Heterogeneity of an intact posterior talofibular ligament in keeping with a low-grade injury.
Dorsal talonavicular ligament is intact. Spring ligament is intact.
Mild hyperintensity of the sinus tarsi. Normal plantar fascia and Achilles tendon.
Anterior/extension tendons are intact. There is an effusion of the common peroneal tendon sheath with mild hyperintensity at the distal myotendinous junction of peroneus longus in keeping with a low-grade strain injury. Peroneal tendons are intact. Tendon sheath effusions of the posterior/flexor tendons, however, the tendons appear intact.
Case Discussion
The x-ray in this case severely underplays the severity of the ankle injury, which on MRI consists of:
- 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.
- 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.
- 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.