Ankle & Foot Imaging: Systematic Interpretation

I assess ankle stability primarily on the mortise view, which is obtained with 15-20 degrees of internal rotation to profile the ankle mortise without fibular overlap. Key measurements include: First, the medial clear space - the distance between the medial malleolus and medial talar border should be less than 4mm and equal to the superior joint space. Widening indicates deltoid ligament injury. Second, the tibiofibular clear space - measured 1cm above the tibial plafond between medial fibular cortex and lateral tibial cortex. Greater than 6mm indicates syndesmotic injury. Third, tibiofibular overlap - should be greater than 6mm on AP and greater than 1mm on mortise view. Reduced overlap indicates syndesmotic widening. I also assess talar tilt by comparing the medial and lateral joint space - asymmetry suggests ligamentous instability. The Weber classification describes fibular fracture level relative to the syndesmosis: Type A below (syndesmosis intact), Type B at the level (syndesmosis may be injured), and Type C above (syndesmosis disrupted). A Type C or high Type B with mortise widening indicates unstable injury requiring syndesmotic fixation.

Lisfranc injuries are frequently missed on initial non-weight-bearing X-rays. Key findings to look for include: the 'fleck sign' - a small avulsion fracture at the base of the 2nd metatarsal from the Lisfranc ligament; subtle widening between the 1st and 2nd metatarsal bases greater than 2mm; loss of normal alignment where the medial border of the 2nd metatarsal should align with the medial border of the middle cuneiform on AP view; and on the lateral view, dorsal subluxation of the metatarsals relative to the cuneiforms. If non-weight-bearing X-rays appear normal but clinical suspicion remains, I would request weight-bearing AP, lateral, and oblique views of both feet for comparison - weight-bearing significantly increases detection sensitivity by stressing the injured ligaments. If the patient cannot weight-bear or weight-bearing films are equivocal, I would order CT to assess for subtle fractures and joint incongruity, or MRI which can directly visualize the Lisfranc ligament and surrounding soft tissue injury. MRI is particularly useful for purely ligamentous injuries without obvious bony displacement.

On MRI, I first identify the location of the osteochondral lesion - most commonly posteromedial on the talar dome from inversion injuries, or anterolateral. I assess the lesion size in all three dimensions as larger lesions have worse prognosis. The critical assessment is stability, which determines treatment. Signs of instability on MRI include: fluid signal (T2 hyperintensity) surrounding the fragment indicating loss of bony attachment; a fluid-filled cleft between the fragment and parent bone; cystic change beneath the lesion which can enlarge and destabilize the fragment; and fragment displacement from its bed. A stable lesion shows: no surrounding fluid signal, a low signal rim representing fibrous or bony attachment, and the fragment sitting congruent in its bed. I also assess cartilage integrity - intact overlying cartilage suggests a better prognosis. The degree of bone marrow edema in the surrounding talus indicates activity of the lesion. Unstable lesions and those with significant cartilage damage typically require surgical treatment such as debridement, microfracture, or osteochondral grafting, while stable lesions in children especially may be managed conservatively.