The ankle is one of the most frequently injured areas of the skeleton and the site of the most common intra-articular fracture of a weight-bearing joint. Ankle injuries are responsible for over 5 million emergency department visits each year.
Although many of these injuries are ligament sprains, the radiologist plays a key role in the thorough evaluation of complex injuries and the detection of subtle fractures (see the images below).
Diagram showing the typical locations for ankle fractures occurring from the 4 major injury mechanisms. Note that the supination external rotation (SE) fracture is shown as a dashed line, because it is best seen in the lateral projection. PA= pronation abduction; PE= pronation external rotation; SA= supination adduction.
Anteroposterior radiograph from a 37-year-old man with a supination adduction stage 2 ankle injury as a result of a motor vehicle collision. This image shows a small avulsion fracture at the tip of the lateral malleolus (stage 1) and an oblique fracture across the base of the medial malleolus (stage 2).
The shapes of the ankle bones and the supporting ligamentous structures are important anatomic features of the ankle area. The distal tibia has a large, flat articular surface (the plafond), a prominent medial malleolus, and a less prominent posterior malleolus. The talar dome is wedge-shaped, wider anteriorly than posteriorly.
The distal fibula or lateral malleolus is bound to the distal tibia by the anterior and posterior inferior tibiofibular ligaments, an inferior transverse ligament, and a syndesmosis ligament. The fibula is also bound to the talus by the anterior and posterior talofibular ligaments and to the calcaneus by the calcaneofibular ligament. The medial malleolus is bound to the talus, calcaneus, and navicular by the superficial and deep portions of the deltoid ligament.
Brandser et al emphasized the necessity of obtaining 3 conventional radiographs in anteroposterior (AP), internal oblique (mortise), and lateral projections.
Other imaging studies, such as arthrography, ultrasonography, computed tomography (CT) scanning, magnetic resonance imaging (MRI), and nuclear medicine, are rarely used. Radiographic stress views may be done, although they can be difficult to obtain. Park et al reported stress views with dorsiflexion and external rotation of the ankle best show tears of the deltoid ligament by resultant widening of the medial clear space when measured at 5 mm or more.
Limitations of techniques
Despite the use of the standard 3-view conventional radiographic survey, some ankle fractures cannot be seen at the time of initial evaluation. The presence of a large ankle-joint effusion on the initial lateral radiograph suggests an occult fracture.
One third of patients with an effusion measuring 13 mm or more had occult fractures in a series reported by Clark et al.
Many of these occult fractures involve the talar dome. The radiographic appearance often suggests the presence of associated ligamentous injuries, but in a series of 59 patients, Gardner et al showed that MRI is much more specific for ligamentous injuries.
Additionally, although radiographic widening of the syndesmotic space of greater than 5 mm is reported to be abnormal, in an MRI series of 70 patients, Nielson et al found no association between the MRI findings of syndesmotic injury and the radiographic measurements.
In a more recent prospective series of 51 patients with ankle fractures, Hermans et al confirmed that radiographic measurements of the syndesmotic space, amount of tibiofibular overlap, and width of the medial clear space did not correlate with ligamentous injuries that were shown on concurrent MRI studies.
Differential diagnosis and other problems to be considered
When evaluating for ankle fractures, also consider conditions such as ankle impingement syndrome, ankle sprain, metatarsalgia, metatarsal fractures (eg, fifth metatarsal fractures), talar dome osteochondral injuries, and injuries to other surrounding ligaments and/or tendons.
As some ankle fractures are initially occult, patients with significant injury should be treated symptomatically and asked to return for additional radiographs in 7-10 days if symptoms persist. The physician should pay special attention to certain target areas, such as the medial and lateral edges of the talar dome, the anterior process of the calcaneus, and the base of the fifth metatarsal, in order to check for subtle fractures.