C sign for diagnosis of talocalcaneal coalition.

PURPOSE: To reevaluate the relevance of the C sign for diagnosis of talocalcaneal coalition.

MATERIALS AND METHODS: Weight-bearing lateral radiographs of 55 feet with talocalcaneal coalition (patient group) and 55 feet without coalition (control group) were reviewed retrospectively. In the patient group, 36 cases of talocalcaneal coalition were confirmed histologically, and 19 cases that did not require surgery were diagnosed at computed tomography (CT). At CT, absence of talocalcaneal coalition in control subjects was confirmed, and control subjects were individually matched with patients according to sex, age, and the calendar year of the clinic visit. Two observers who were blinded to personal information regarding the subjects assessed the presence of the C sign on randomly presented radiographs. When judgments differed, a third observer made the final judgment. Affected feet were classified according to location (ie, medial, posterior, or diffuse) of the coalition and age (ie, <12, 13-20, >21 years). Sensitivity, specificity, and likelihood ratios of the C sign were calculated, and a second radiologic sign, posterior joint surface irregularity, was also evaluated. Differences in occurrence of the C sign among groups according to age and type of coalition were evaluated with the Fisher exact probability test.

RESULTS: Sensitivity and specificity of the C sign for diagnosis of talocalcaneal coalition were 49% and 91%. The positive and negative likelihood ratios were 5.44 and 0.56, respectively. Corresponding sensitivity, specificity, and positive and negative likelihood ratios for the irregularity of the posterior talocalcaneal joint were 71%, 93%, 10.14, and 0.31, respectively. C-sign false-negative findings were more frequent in patients younger than 12 years and in those with posterior coalition.

CONCLUSION: C sign sensitivity obtained in our study was low, which indicated that an absence of the C sign does not negate a diagnosis of coalition.