Ankle radiograph (an approach)

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 the sequela of local infection.

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).

The joint

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

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.

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 and this results in relative difference between the degree of pronation and supination that can be achieved.

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. 

Systematic review

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

Tib/fib 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

Trace the mortice

• trace around the morticemortise 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 osteochrondralosteochondral fracture
• assess the interosseous ligament
  • 1 cm proximal to the tibial plafond, the distance between tibia and fibula should be less than 6 mm
  • if > 6 mm, think tear or rupture of ankle ligaments and look for a fracture

Lateral

• trace the bony cortex of the lateral and medial malleoli, posterior tibia, calcaneum and base of 5th metatarsal
• assess Bbohlerohler’s angle: two tangent lines drawn across the anterior and posterior borders of calcaneus form angle 20-40°
  • if < 20°, think calcaneal fracture
  • confirm with axial view

Bone review

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

Beware of accessory ossicles - do not misdiagnose as fractures:

• os trigonum (posterior to talus)
• os subtibiale (distal to tip of medial malleolus)
• os subfibulare (distal to tip of lateral malleolus)

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 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)
• bohlerBohler’s angle < 20° indicates fracture
• more: calcaneal fracture

Talar neck fracture

• most common fracture of talus
• forced dorsiflexion with axial load
• high risk of avascular necrosis and degenerative arthritis
• more: talar neck fracture

Don’t miss…

OsteochrondralOsteochondral fracture

• focal areas of damage to cartilage and adjacent subchrondralsubchondral bone
• occur secondary to compressive or rotational microtrauma
• commonly affects talar dome; subtle therefore frequently missed
• more: osteochondral fracture