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Findings: Ankle x-ray 4/5/93: Tarsal "beaking" and coalition. CT both feet 1/19/96: Chronic osseous coalition of left navicular and tarsal bones; Hypoplastic left sustentaculum tali; Fibro-osseous coalition of left middle subtalar joint.

Diagnosis: Tarsal Coalition

Discussion:The talocalcaneal articulations consist of the subtalar joint between the posterior facets of the talus and calcaneus and the talocalcaneonavicular joint between the talus, anterior and middle facets of the calcaneus, and tarsal navicular. These two synovial-lined cavities are completely separated by the tarsal canal. They function in inversion and eversion movements of the foot.

Three facets are present on the superior surface of the calcaneus which keep contact with the talus. The posterior facet is convex. The middle talar facet is the cartilage - covered upper surface of the sustentaculum tali.

Coalition is the fusion of the two or more tarsal bones, most commonly between the navicular and calcaneus or calcaneus and talus. Symptoms of this condition include foot, ankle or knee pain; valgus foot deformity; and peroneal spastic flat foot. The differential for these clinical findings include chronic juvenile arthritis, osteoarthritis, tuberculosis, and trauma.

Coalition can occur in the fetus and may be an inherited defect in developing mesenchyme although the etiology is not certain. The union may be fibrous (fibroosseous), cartilaginous (fibrocartilaginous) or bony. Cartilaginous or fibrous bridging is sometimes difficult to demonstrate by plain radiograph. Secondary signs are useful in making the diagnosis. Such signs are: proximity of the calcaneus and navicular bones; irregularity, indistinction or increased density of the abutting cortical surfaces; hypertrophic or sclerotic bone reaction; pes planus deformity; planovalgus configuration; talar beaking; hypoplasia of the sustentaculum tali; broadening of the lateral talar process; and narrowing of the posteromedial end of the tarsal canal.

Coalition most commonly occurs posterior to the sustentaculum tali, however, multiple facets may be involved. CT in the coronal and axial plane is useful in identifying all the abnormalities prior to surgical release of the bridges.

References:
1. "Radiology of the Talocalcaneal Articulations" Donald Resnick, M.D.,
Radiology 111:581-586, June 1974.

2. Computed Tomography in Tarsal Coalition. Robert C. Sarno, Barbara L. Carter,
Mark S. Bankoff and M. Christian Semine. Journal of Computer Assisted Tomography,
Vol 8, No. 6, 1984, pp 1155-1160.

3. "Subtalar Joint Coalition in Children: New Observations." Myung Soo Lee,
H. Theodore Harcke, S. Jay Kumar, George S. Bassett. Radiology 1989, 172:635-639.

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