What causes C sign in foot on lateral ankle radiograph?
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Answer: Continuous C-shaped arc formed by continuity between the medial cortex of the talar dome and the inferior cortex of the sustentaculum tali, seen in talocalcaneal coalition, a congenital tarsal coalition due to failure of segmentation resulting in bony, cartilaginous, or fibrous bridging between talus and calcaneus, most commonly involving the middle or posterior subtalar facets and presenting with rigid flatfoot deformity or recurrent ankle sprains.
Why is it called so?
Named for the continuous C-shaped arc created by the medial outline of the talar dome and the posteroinferior aspect of the sustentaculum tali on lateral radiograph due to their abnormal continuity from coalition.
Pathophysiology
Talocalcaneal coalition restricts subtalar joint motion, causing secondary structural changes including flattening and continuity of the talar medial cortex with the sustentaculum tali inferior cortex, forming the C sign; more typically seen in middle facet coalitions and deformed in posterior facet coalitions.
Alternative names: Deformed C sign
Other associated named signs: Talar beak sign, duck-face sign, humpback sign
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