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Findings:

1. Pes planus
2. Talar beaking
3. C-sign

Discussion: Subtalar coalitions may be fibrous, cartilaginous, or osseous and can be developmental or acquired after infection, trauma, or surgery. The patient usually has a flat foot and complains of foot pain with limited range of motion.

The most common subtalar coalitions are calcaneonavicular (50%) and talocalcaneal (40%). Radiographic signs of calcaneonavicular are elongation of the anterior superior portion of the calcaneous ("anteater nose" sign) and hypoplasia of the head of the talus. Talar beaking is uncommon.

Radiographic signs of talocalcaneal coalition are:

1. Talar beaking - lifting of the periosteum below the talonavicular ligament creating the bony excrescence.

2. "C" sign - This is a radiographic sign seen on the lateral ankle radiograph which is a C-shaped line formed by the medial outline of talar dome and posteroinferior outline of sustentaculum tali.

3. Pes Planus - Talocalcaneal coalitions occur at the middle facets between the talus and sustentaculum tali and is bilateral in 20% of patients. If the coalition is fibrous or cartilaginous, CT may be helpful in further evaluation.

Lateur LM, VanHoe L. Subtalar Coalition: Diagnosis With the C Sign on Lateral
Radiographs of the Ankle. Radiology. 1194;193:847-851.

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