Ankle radiographs are frequently performed in emergency departments, usually after trauma. The radiographic series is comprised of three views: an anteroposterior, mortise, and a lateral. They may be performed to assess degenerative or inflammatory arthritis as well as to look for sequelae of local infection. 

Systematic review

Choosing a search strategy and utilising it consistently is a helpful method to overcome common errors seen in diagnostic radiology. The order in which you interpret the radiograph is a personal preference. A recommended systematic checklist for reviewing musculoskeletal exams is: soft tissue areas, cortical margins, trabecular patterns, bony alignment, joint congruency, and review areas. Review the entire radiograph, regardless of perceived difficulty. Upon identifying an abnormality, do not cease the review, instead put it aside and complete the checklist.

The ring structure of the ankle is made up of three bones (tibia, fibula and talus) and three ligaments (medial and lateral collateral ligaments and interosseous ligament)

• if there is one break in the ring, look for a second

Soft tissue

Assess all soft tissue structures for any associated or incidental soft tissue signs such as swelling, stranding, or emphysema.

Ligaments

Like all joints, structural integrity is achieved by the ligaments that hold it together. The ankle has three main sets of ligaments:

• medial: deltoid ligament

• lateral: posterior talofibular, anterior talofibular and calcaneofibular ligaments

• syndesmotic ligament

The deltoid ligament is much stronger than the ligaments that support the lateral aspect of the ankle. This results in a relative difference between the degree of pronation and supination that can be achieved.

Bony anatomy

The ankle is a synovial joint composed of the distal tibia and fibula as they articulate with the talus. The distal tibia and fibula articulate with each other at the distal tibiofibular joint which is more commonly referred to as the tibiofibular syndesmosis (or simply the syndesmosis).

As with all radiographs, check around each bone on the radiograph looking at the cortex. Specifically, check the tarsals and the base of 5^th metatarsal.

Beware of accessory ossicles - do not misdiagnose as fractures:

• os trigonum (posterior to talus)

• os subtibiale (distal to the tip of medial malleolus)

• os subfibulare (distal to the tip of lateral malleolus)

Tibia/fibula check

• trace around the distal tibia and fibula on both views

  • fractures may be accompanied by ligamentous injury and may be unstable

• on the lateral view carefully look at the fibula

  • an oblique fibula fracture may be difficult to see

Lateral

• trace the bony cortex of the lateral and medial malleoli, posterior tibia, calcaneum, and base of 5^th metatarsal

• assess Böhler angle: two tangent lines drawn across the anterior and posterior borders of the calcaneus form an angle measuring 25-40°

  • if <20°, think calcaneal fracture

  • confirm with axial view

Alignment

On the radiograph, the horizontal portion of the distal tibia parallel to the dome of the talus is the tibial plafond. Taken with the medial and lateral malleoli, it forms a rectangular socket, the ankle mortise (a.k.a. mortice ¹).

Being a synovial joint, the ankle joint (between the ankle mortise and talar dome) is surrounded by a joint capsule. Like the knee joint capsule, the ankle capsule has an additional cranial extension at the syndesmosis.

Stability

The ankle is most at risk of injury when it is pronated or supinated. Pronation is relatively limited because of the shape of the medial malleolus and the deltoid ligament. This explains why only 20% of injuries occur in pronation compared to 80% when the foot is supinated. 

Trace the mortise

• trace around the mortise and talar dome

• check the joint space is uniform

  • if one side is widened, look for a fracture

• ensure the talar dome surface is smooth

  • if there is a defect or cortical irregularity, think osteochondral fracture

• assess the interosseous ligament

  • 1 cm proximal to the tibial plafond, the distance between the tibia and fibula should be <6 mm

  • if it is >6 mm, think tear or rupture of ankle ligaments and look for a fracture

Common pathology

Lateral malleolar fracture

• isolated lateral malleolar fractures are common

• the Weber classification is used to determine treatment

  • Weber A: below the ankle joint with intact syndesmosis

  • Weber B: at the level of the ankle joint 

  • Weber C: above the ankle joint with medial malleolus fracture

• more: lateral malleolar fracture

Maisonneuve fracture

• distal tibiofibular syndesmosis disruption

• associated medial malleolar fracture, medial collateral ligament rupture, AND proximal fibular fracture

• the result of traumatic external rotation

• complex, unstable injury

• more: Maisonneuve fracture

Pilon fracture

• distal tibial fracture

• compression injury usually after a fall from a height

• check for associated talar or calcaneal fracture

• more: pilon fracture

Calcaneal fracture

• 60% of all tarsal fractures; majority intra-articular

• fall from height (lover’s fracture)

• Böhler angle <20° indicates fracture

• more: calcaneal fracture

Talar neck fracture

• most common fracture of the talus

• forced dorsiflexion with axial load

• high risk of avascular necrosis and degenerative arthritis

• more: talar neck fracture

Special paediatric fractures

• Tillaux fracture: Salter-Harris type III fracture of tibia

• Triplane fracture: Salter-Harris type IV fracture of tibia; most common fracture in adolescence

Don’t miss…

Osteochondral fracture

• focal areas of damage to cartilage and adjacent subchondral bone

• occur secondary to compressive or rotational microtrauma

• commonly affects talar dome; subtle therefore frequently missed

• more: osteochondral fracture