The C sign: more specific for flatfoot deformity than subtalar coalition

To assess the sensitivity and specificity of the C sign, a C-shaped line created by the outline of the talar dome and the inferior margin of the sustentaculum tali on lateral ankle radiographs, for patients with both flatfoot deformity and specifically talocalcaneal (TC) coalition.

All patients in this retrospective study were under 35 years of age and had a lateral ankle radiograph and follow-up CT scan for a non-traumatic indication. Forty-eight cases over the past 5 years fulfilled these criteria. Two masked musculoskeletal radiologists determined the presence or absence of the C sign for each lateral radiograph by consensus. Each CT study was then assessed by a third musculoskeletal radiologist for the presence of tarsal coalition. Observations were correlated with clinical history regarding presence or absence of flatfoot deformity.

Ten cases of TC coalition were diagnosed, four of which demonstrated a C sign (40%). Eight cases with a C sign were encountered, four of which had TC coalition (50%) and four did not. All patients with a positive C sign had a flatfoot clinically (100%), while only eight of 24 flatfooted patients had a C sign (33%).

The C sign is specific, but not sensitive, for flatfoot deformity, and is neither sensitive nor specific for subtalar coalition.