Talocalcaneal Coalition

This 11 year-old girl presented for investigation of suspected tarsal coalition. The image shown is a coronal proton-density weighted MR. There is an abnormal middle facet (click image for arrows) of the talocalcaneal articulation, with irregularity of the bony margins and low signal within the adjacent bone. The bony low signal was bright on water-weighted sequences, consistent with bone oedema. A thin line of intermediate signal can be seen between the bone surfaces, and this followed cartilage on all pulse sequences. These features are typical of talocalcaneal tarsal coalition, probably the cartilaginous type.
Tarsal coalition is an abnormal fibrous, cartilaginous or osseous fusion between tarsal bones. It affects 1-2% of the population. Both feet are involved in 20-50%. The incidence of calcaneonavicular and talocalcaneal coalition are approximately equal (45% each). Other types of coalition are uncommon.
Talocalcaneal coalition typically presents at age 12-16 with painful flat foot. It usually involves the middle facet. Radiographic findings include a prominent talar beak (66%) arising from the dorsal aspect of the talar head, and the “C-sign” - a C-shaped outline of medial talar dome and sustentaculum seen on lateral views. MRI findings include: bone marrow contiguity in osseous coalition; bones in close proximity with irregular surfaces; separation by intermediate signal (cartilage) or low signal (fibrous tissue); and bone marrow oedema surrounding the coalition.
Reference: Dahnert W. Radiology Review Manual 5th edition. Lippincott, Williams & Wilkins 2003
Credit: Dr Laughlin Dawes
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