Background: Verifying the exact location of talocalcaneal (TC) coalition is important for surgery, but the complicated anatomy of the subtalar joint makes it difficult to visualize on radiographs. No study has used computed tomography (CT) or magnetic resonance imaging (MRI) to verify the radiological characteristics of TC coalition or those of different facet coalitions. Therefore, this study verified the radiological findings used to identify TC coalitions and those of different facet coalitions using CT and MRI. Materials and Methods: Plain with/without weight bearing anteroposterior and lateral radiographs, CT, and MRI of 43 feet in 39 patients with TC coalitions were reviewed retrospectively. CT or MRI was used to verify the location of the TC coalition. Secondary signs for the presence of a coalition in the anteroposterior and lateral plain radiographs, including talar beak, humpback sign, duck-face sign, and typical or deformed C-sign, were evaluated. Three independent observers evaluated the radiographs twice at 6-week intervals to determine intraobserver reliability. They examined the radiographs for the secondary signs, listed above, and coalition involved facets. Results: The average rates from both assessments were as follows: Middle facet 5%, middle and posterior facets 27%, and posterior facet 68%. The deformed C-sign is more prevalent in posterior facet coalitions. The posterior facet has the highest prevalence of involvement in TC coalitions, and the deformed C-sign and duck-face sign have high correlations with TC coalitions in the posterior subtalar facet. Conclusion: A posterior facet is the most prevalent for TC coalition, and the C-sign is useful for determining all types of TC coalition.
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