C Sign of Talocalcaneal Coalition

Fig. 1: Lateral radiograph of the right ankle shows a C sign (yellow dotted line) that extends from the talar dome through the coalition component of the posterior talocalcaneal joint to the sustentaculum tali.
Fig. 2: Coronal reformatted CT image confirms the presence of talocalcaneal coalition (arrows) between the talus (T) and calcaneus (C).

Facts: Tarsal Coalition

• Abnormal bony, cartilaginous or fibrous articulation between two tarsal bones
• Congenital, caused by lack of bony segmentation
• Most common between calcaneus-talus and calcaneus-navicular
• Bilateral in 20% of cases

Imaging

• C sign seen on lateral view when middle facet talocalcaneal coalition is present (as in our case)
• Talar beak is an indirect sign of abnormal talonavicular motion, it is a bony spur from anterior superior aspect of talus
• CT is an excellent method to identify and characterize tarsal coalition

Reference:

Chew FS, Bui-Mansfield LT, Kline MJ. Musculoskeletal Imaging, 2003.