The radiology of the subtalar joint with special reference to talo-calcaneal coalition.

Congenital talo-calcaneal coalition is one of the conditions responsible for spastic flat foot. The common site is across the middle talo-calcaneal joint, and although routine radiographic views of the foot do not show the coalition, important secondary signs may be present. The findings in 17 cases are reviewed, the following secondary signs being recognised: 1. A talar beak. 2. Narrowing of the posterior talo-calcaneal joint space. 3. Rounding of the lateral process of the talus. 4. Failure of visualisation of the middle subtalar joint in the lateral view, and 5. Asymmetry of the anterior subtalar joints in the lateral oblique vies. The differential diagnosis of these signs is discussed and the normal anatomy of the subtalar joints is illustrated. Some diagnostic pitfalls are described.