C sign - talocalcaneal coalition

The C sign is an important radiological sign which may be seen on a lateral radiograph of the ankle in those with the talocalcaneal subtype of tarsal coalition. It can be seen in both osseous and nonosseous coalition.

Radiographic appearance

A continuous C-shaped arc on a lateral ankle radiograph is formed by the medial outline of the talar dome and posteroinferior aspect of the sustentaculum tali due to their bridging.

Differential diagnosis

Considerations include:

  • subtalar syndesmosis or synchondrosis: CT is helpful in differentiating
  • positional artifact