Ankle & Foot Imaging: Systematic Interpretation

Medial Clear Space: Less than 4mm (talar tilt suggests deltoid injury)

Tibiofibular Clear Space: Less than 6mm (syndesmosis)

Tibiofibular Overlap: Greater than 6mm on AP, greater than 1mm on mortise

Talar Tilt: Asymmetric joint space suggests instability

Anterior talofibular distance: Joint effusion sign

Bohler angle: 20-40° (calcaneus)

Gissane angle (crucial): 120-145° (calcaneus)

Talar dome: Osteochondral lesions

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

• Superior bone detail
• 3D reconstruction for surgical planning
• Fragment size and displacement
• Articular surface assessment
• Subtalar joint involvement

The classic teaching of posteromedial and anterolateral talar dome lesions has been questioned. Using a nine-zone MRI grid, operatively treated symptomatic lesions were most often central (central-lateral then central-medial), with medial lesions larger in surface area (Orr, Foot Ankle Int 2012). Describe lesions by zone and size rather than the historical two-location model.

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.

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 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.

• Ankle sprains are among the most common musculoskeletal injuries globally, with lateral ligament sprains predominating
• Ankle fractures are a high-incidence fracture in adults, rising with an ageing, osteoporotic population
• Calcaneal fractures account for the majority of tarsal fractures and typically follow axial load (fall from height, road trauma)
• Lisfranc injuries are frequently missed at first presentation, a recognised source of long-term disability and litigation

• Plain radiography dominates first-line imaging in every health system
• Decision rules (Ottawa) reduce unnecessary radiographs by roughly a third where implemented
• MRI and CT access varies widely between high- and limited-resource settings, shaping pathways

• Medial clear space less than 4mm; tibiofibular clear space less than 6mm
• Tibiofibular overlap over 6mm (AP), over 1mm (mortise)
• Bohler angle 20-40 degrees; Gissane angle 120-145 degrees
• Normal Achilles under 6mm; plantar fascia under 4mm at insertion
• Greater than 2mm diastasis between 1st and 2nd metatarsal bases is abnormal

• "Normal" film with clinical suspicion: request weight-bearing/contralateral views
• High fibular fracture: exclude Maisonneuve and syndesmotic injury
• Fall from height with heel pain: CT for Sanders type and screen the spine
• Lateral talar dome lesion on mortise view: proceed to MRI for stability
• Always state the modality you would request next and why