Ankle & Foot Imaging: Systematic Interpretation

Ankle: Standard Views

Read alignment before the fracture lines. Compare talar centring, the medial and superior clear spaces, distal tibiofibular overlap and side-to-side symmetry before calling subtle widening or shift.

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Key Measurements

On the mortise view. Four measurements:

• Medial clear space - less than 4mm, compared with the superior joint space; talar tilt suggests deltoid injury
• Tibiofibular clear space - less than 6mm (syndesmosis)
• Tibiofibular overlap - greater than 6mm on the AP, greater than 1mm on the mortise
• Talar tilt - an asymmetric joint space suggests instability

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On the lateral view. The anterior talofibular distance is a joint effusion sign, the talar dome is checked for osteochondral lesions, and the calcaneus is measured by its Bohler and Gissane angles, covered with the calcaneal fractures below.

Ankle Fracture Classification

Weber/AO. The classification is the level of the fibular fracture relative to the syndesmosis:

• Type A - below the syndesmosis; the syndesmosis is intact and the injury is usually stable
• Type B - at the level of the syndesmosis; the syndesmosis may be injured, so assess the mortise for widening
• Type C - above the syndesmosis; the syndesmosis is disrupted and the injury is unstable, often requiring fixation

Whatever the level, always assess the medial side and the syndesmosis. A medial malleolar fracture alone is rare, so look for the associated lateral injury.

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The Maisonneuve injury. A proximal fibular fracture with a medial ankle injury is a high syndesmotic injury, and the ankle films may show only its distal consequences: medial-sided injury, syndesmotic widening or posterior malleolar involvement. A deceptively small ankle abnormality, or a medial malleolar fracture or deltoid widening with a "normal" fibula at the ankle, should prompt dedicated views of the entire fibula; the fracture is at the fibular neck. Image the whole fibula when medial ankle tenderness, syndesmotic signs or an external-rotation mechanism is present despite an apparently minor ankle film.

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Indications

When to ask for CT. The primary indications:

• Calcaneal fractures (Sanders classification)
• Pilon fractures
• Talus fractures
• Lisfranc injury, if weight-bearing X-rays are equivocal
• Tarsal coalitions
• Complex midfoot fractures

What CT adds. Superior bone detail, 3D reconstruction for surgical planning, fragment size and displacement, the articular surface and subtalar joint involvement. The radiographs establish alignment and fracture pattern; CT resolves the articular morphology and fragment geometry that determine reduction strategy and fixation planning.

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Sanders Classification (Calcaneal Fractures)

Tarsal coalition. Thin-slice multiplanar CT defines the site and extent of a coalition and the degeneration in the adjacent joints when resection or arthrodesis is being considered.

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A suspected talar neck fracture is one of the clearest indications for CT, and the Hawkins classification and the Hawkins sign are the two high-yield imaging points. Both exist for the same anatomical reason: the talus has no muscular attachments and a largely retrograde, intra-osseous blood supply, so displacement tears the vessels entering from the neck and the body is left to survive on what remains. That is why AVN risk rises with each Hawkins type - the classification is really a count of how many blood-supply routes the displacement has destroyed.

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The Hawkins sign. A subchondral lucency, a radiolucent band in the talar dome, on an AP or mortise ankle radiograph at about 6-8 weeks. It reflects subchondral osteopenia from disuse hyperaemia in a talus with an intact blood supply, so a present Hawkins sign predicts a viable talus and AVN is unlikely. Its absence, a persistently sclerotic or dense dome, is concerning for AVN, although MRI is the definitive test for established osteonecrosis.

Sequences

Osteochondral Lesions of Talus (OLT)

Location. The classic teaching is posteromedial (inversion injury) as most common, with anterolateral second, and it is still what most examiners expect you to say. But know that it is contested: mapping operatively treated symptomatic lesions onto a nine-zone grid on MRI put the commonest lesions centrally (central-lateral, then central-medial), not at the two classic corners (Orr, Foot Ankle Int 2012). The likely reason is that the two-corner teaching came from describing lesions loosely by quadrant, whereas a zone grid measures where the lesion actually sits.

What has held up. Medial lesions are consistently the larger and deeper ones, which is the part of the classic teaching that survives and the part that matters surgically. The safer formulation is therefore to describe a talar dome lesion by zone and size rather than by one of two fixed locations, which is also what the operative plan depends on: medial lesions may need a medial malleolar osteotomy, whereas lateral lesions are often accessible arthroscopically.

Stability. MRI signs of instability:

• Fluid signal (T2 bright) surrounding the fragment
• A fluid-filled cleft between fragment and parent bone
• A cyst beneath the lesion
• A displaced fragment

A stable lesion shows no surrounding fluid, a low-signal rim of fibrous attachment, and the fragment in situ.

Staging. Examiners also want the staging systems, and the Orr evidence is reported in Hepple stages, so they need defining:

All three describe the same progression from intact cartilage to a detached, displaced or cystic lesion. The higher stages - a detached or displaced fragment (Berndt-Harty III-IV) or a subchondral cyst (Hepple V) - correspond to the instability signs above, and they are the ones that push management toward surgery.

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Tendon Assessment

By compartment. Axial MRI localises tendon pathology by compartment: the anterior extensors, the posteromedial flexors, the posterolateral peroneals and the Achilles. Follow each tendon across sequential slices; fluid, sheath distension and oedema must be distinguished from magic-angle signal and partial-volume artefact.

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Ultrasound for the superficial tendons. It adds dynamic assessment and side-to-side comparison, but the operator must follow the tendon in two planes to avoid anisotropy.

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The Achilles. Normal is low signal and approximately 6mm thick. Tendinopathy thickens it to greater than 8mm with increased T2 signal and fusiform swelling, but the fibres stay intact. Rupture is complete discontinuity, a gap filled with fluid or haemorrhage (T2 bright), and retraction of the tendon ends.

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Plantar Fascia

Normal and inflamed. The normal fascia is less than 4mm thick at the calcaneal insertion and low in signal. Plantar fasciitis thickens it to greater than 4mm with increased T2/STIR signal and periplantar oedema, and there may be calcaneal marrow oedema at the insertion.

Rupture. Discontinuity with surrounding fluid or haemorrhage, associated with prior steroid injection or fluoroquinolone use.

A reproducible interpretation framework is what separates a consultant-level review from a checklist of "looks normal". Pair a fixed search pattern (ABCS) with the modality ladder so the right next test is requested rather than reflexively ordering CT or MRI.

The reporting sequence. Seven steps, in order:

1. Confirm - correct patient, side, projection adequacy, weight-bearing status
2. Alignment - mortise congruity, talar shift or tilt, syndesmosis, Lisfranc and Chopart lines, hindfoot axis
3. Bone - trace every cortex: malleoli, plafond, talus, calcaneus, navicular, cuboid, cuneiforms, all five metatarsals and their bases (especially the 2nd and 5th)
4. Cartilage and joints - tibiotalar, subtalar, talonavicular, calcaneocuboid and TMT joint spaces; osteophytes; erosions
5. Soft tissue - effusion, focal swelling, Achilles contour, Kager fat pad, calcification or foreign body
6. Measure - medial clear space, tibiofibular clear space and overlap, Bohler and Gissane angles, Meary angle for the arch
7. Decide the next test from the modality ladder, not by reflex

Global Epidemiology

Side-by-Side Guideline Comparison

High- vs Limited-Resource Practice Variation

How much imaging for the syndesmosis? Static radiographic measurements (clear space, overlap) miss occult instability, and the fibular incisura is highly variable in shape and depth, so absolute numbers can mislead. Bilateral or post-fixation CT is increasingly used to detect malreduction, with the uninjured side as the reference (Liu, J Foot Ankle Surg 2018). Intra-operative dynamic stress and weight-bearing CT remain active research areas.

Weight-bearing CT. Cone-beam WBCT provides 3D imaging under physiological load and is gaining traction for subtle Lisfranc and syndesmotic instability, hindfoot alignment and arthritis, but availability is limited and validated thresholds are still maturing.

MRI versus arthroscopy for OLT. MRI is the non-invasive standard for staging stability, but it can over- or under-estimate cartilage status versus arthroscopy. CT arthrography or diagnostic arthroscopy may be needed when MRI and symptoms disagree.