The talocalcaneal articulation has three facets: the large posterior facet, and the combined middle and anterior facets that form part of the talocalcaneonavicular joint complex. The middle facet lies on the sustentaculum tali of the calcaneus and the corresponding talar surface; the anterior facet lies more anteriorly. Together the middle and anterior facets support the talar head and contribute to subtalar inversion and eversion, acting in concert with the spring ligament and the talonavicular joint.
Talocalcaneal coalition most commonly occurs at the middle facet on the sustentaculum tali, producing the classic medial heel bar on an oblique X-ray and the C-sign on lateral view. Middle facet coalition causes a rigid or painful flatfoot by blocking subtalar motion. Computed tomography confirms the coalition type (bony, cartilaginous, or fibrous) and extent. Resection of a coalition confined to less than 50% of the middle facet surface area has acceptable results, while larger coalitions typically require subtalar fusion.
Bony or fibrocartilaginous fusion of the middle facet on the sustentaculum tali produces a rigid painful flatfoot in adolescents or young adults, diagnosed by CT showing osseous bridging or irregular cartilage, managed with coalition resection and fat graft interposition or subtalar arthrodesis for large fusions.
Calcaneal fracture extension into the middle facet on the sustentaculum tali disrupts the subtalar joint support and the flexor hallucis longus groove, contributing to late subtalar arthrosis requiring joint reconstruction.
