Subtalar coalition: diagnosis with the C sign on lateral radiographs of the ankle.

PURPOSE: To evaluate the usefulness of the C sign, a C-shaped line formed by the medial outline of the talar dome and the inferior outline of the sustentaculum tali, in the diagnosis of subtalar coalition (SC) on lateral radiographs of the ankle. MATERIALS AND METHODS: Lateral ankle radiographs obtained before and after the availability of computed tomography (CT) at the authors' institution were retrospectively and prospectively evaluated for the presence of the C sign. SC was confirmed with surgery for the retrospective study and with CT or magnetic resonance (MR) imaging for the prospective study. RESULTS: The C sign was seen on lateral radiographs in all 18 cases of SC in the retrospective study. Fifteen cases of SC were found in the prospective study, 13 of which had a C sign on lateral radiographs. Two additional findings on the CT or MR images were consistently associated with SC and caused the C sign on the lateral radiographs: a horizontal inferior surface of the sustentaculum tali and pathologic calcaneal valgus in the coronal plane. CONCLUSION: The C sign proved to be the most reliable indicator of SC on the lateral radiograph of the ankle.