The ankle is a hinge joint formed by three bones: the tibia, the fibula and the talus.
Proximally, the joint comprises the medial malleolus (the distal end of the tibia), the tibial plafond and the lateral malleolus (the distal end of the fibula) which collectively form a rectangular socket called the mortise into which fits the talar dome distally.
This comes from the similarity to the mortise and tenon joint used in carpentry to create stable connections (Figure 1).
Further reinforcement of the ankle joint is provided by a strong fibrous complex between the distal tibia and fibula called the syndesmosis, and the medial (or deltoid) and lateral ligaments which arise from the medial and lateral malleolus respectively.
The ankle joint allows dorsiflexion and plantarflexion and is one of the weight-bearing joints. For more information, see the Geeky Medics guides to the bones of the lower limb and the bones of the foot.
Principles of ankle X-ray interpretation
Begin by checking that you are looking at the correct radiograph of the correct patient.
It is important to confirm the following details:
Patient details (name, date of birth, unique identification number)
Date and time the radiograph was taken
Correct side (right vs. left)
In the United Kingdom, two views of the ankle joint are routinely performed:
Mortise view: this is a modified anteroposterior (AP) view of the ankle in 10-20° internal rotation so that the medial and lateral malleoli are in the same horizontal plane and joint visualisation is optimised
In some cases, a weight-bearing or a stress radiograph (gravity stress or manual stress) may also be required.
Consider additional views such as a full-length tibia/fibula radiograph and imaging of the joint above and below (i.e. a knee and a foot radiograph) to rule out additional fractures.
Comparison to previous radiographs, if available, can be especially useful when interpreting radiographs.
Asymptomatic contralateral sides are not routinely radiographed, however,§ if a recent or old image of the contralateral side is available, these may also be compared as the skeletal system is generally symmetrical and could therefore act as a reference point.
Interpreting an ankle X-ray
Use a methodical approach such as ABCs to look at a radiograph.
Ideally, you should be able to see at least the distal third of the tibia and fibula and the talus on the mortise view and in addition to those, you should be able to see the calcaneum and the base of the 5th metatarsal on the lateral view.
Penetration is adequate if you can clearly distinguish between bones and soft tissues.
Look out for artefacts.
In both views, trace the cortical outline of all the bones visible on the radiograph. On the mortise view, trace the mortise and the talar dome surface. On the lateral view, assess the medial, lateral and posterior malleoli, the calcaneum and the base of the fifth metatarsal.
A loss in bone or joint alignment may be a result of a fracture, jointsubluxation (partial dislocation) or dislocation. Note any other abnormalities such as osteophytes or calluses.
Look at the internal architecture of the bone (e.g. a thinner cortex and an increased bone lucency indicate osteopenia or osteoporosis). A fracture in an abnormal bone is termed a pathological fracture.
Beware of normal developmental variants such as accessory ossicles which may be mistaken for fractures.
Cartilages (joint involvement)
Assess the joint space on the mortise view. A loss of joint space may be due to a loss of cartilage and is commonly seen in conditions such as osteoarthritis. Joint widening may be due to fracture or dislocation.
Look at the medial clear space. This is the widest distance between the medial border of the talar bone and the lateral border of the medial malleolus. It should be approximately equal to the superior clear space (the distance between the articular surfaces of the tibia and the talus).
Widening of the medial clear space (i.e. lateral talar shift) suggests syndesmosis disruption and therefore joint instability.
A loss of tibiofibular overlap also suggests syndesmosis injury.
Joint widening with no obvious fracture on the ankle radiograph may also indicate a more proximal fracture such as a Maisonneuve fracture (this is a combination of an unstable ankle due to a ligamentous and/or bony injury together with a proximal fibular fracture). In these situations, consider asking for additional X-rays.
Carefully inspect the soft tissues, as this can provide helpful information. For example, soft tissue swelling or a joint effusion may sometimes indicate the presence of a subtle fracture.
Describing a fracture on an ankle radiograph
When describing an ankle X-ray, use the following structure:
Details of the radiograph and the patient
Site of fracture: which bone, which part of the bone, left or right
Type of fracture (e.g. transverse, oblique, spiral)
Simple or comminuted
Displacement and angulation of the distal fracture fragment in relation to the proximal fracture fragment
Intra-articular or extra-articular (i.e. Does the fracture line extend into a joint?)
Classification of ankle fractures
Anatomical descriptors of ankle fractures include (but are not limited to):