Summary Three anatomic specimens of os trigonum from skeletally immature patients demonstrated anatomic continuity of the cartilage containing the ossicle with the body of the talus, with a synchondrosis being present between the two ossifying regions. The os trigonum may be considered a developmental analogue of a secondary ossification center similar to the posterior Calcaneal apophysis (although there are obvious histologic differences). The chondro-osseous border of the synchondrosis may be injured either as a chronic stress fracture or, less frequently, as an acute fracture, comparable to the injury patterns involving the accessory navicular.