Surgical excision of an os talus secundarius: a case report.

Os talus secundarius is a relatively rare developmental abnormality of the talus; it is characterized by its location on the lateral aspect of the talus. This secondarily developed talus dramatically limits the range of motion of the ankle and subtalar joints. Differential diagnosis between talus secundarius and talus accessorius can usually be made radiographically. Talus secundarius secures itself to the lateral body of the talus by either a synchondrosis or synostosis and usually forms a set of articulations with the fibula, the superior lateral surface of the calcaneus, and a portion of the body of the true talus. Talus accessorius, however, is usually on the medial side of the talus and may form in the deltoid ligament. Talus accessorius is similar to the intercalary sesamoid bone between the external malleolus and the talus.