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

Objective. To assess the sensitivity and specificity of the C sign, a C-sha ped line created by the outline of the talar dome and the inferior margin o f the sustentaculum tall on lateral ankle radiographs, for patients with bo th flatfoot deformity and specifically talocalcaneal (TC) coalition. Design and patients. 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 fulfill ed these criteria. Two masked musculoskeletal radiologists determined the p resence 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 clinica l history regarding presence or absence of flatfoot deformity. Results. Ten cases of TC coalition were diagnosed, four of which demonstrat ed a C sign (40%). Eight cases with a C sign were encountered, four of whic h had TC coalition (50%) and four did not. All patients with a positive C s ign had a flatfoot clinically (100%), while only eight of 24 flatfooted pat ients had a C sign (33%). Conclusion. The C sign is specific, but not sensitive, for flatfoot deformi ty, and is neither sensitive nor specific for subtalar coalition.