An accessory bone is an inconsistent bone of the body, meaning that it does not develop in everyone. They occur at an area of ossification (where cartilage becomes bone). This is different than a sesamoid bone.
Some of the more common accessory bones of the foot and ankle are the Os Trigonum, Os Vesalianum, and the Os Tibialae Externum. The Os Trigonum is located at the posterior (behind) and lateral (outside) aspect of the talus. This portion may become inflammed or irritated and labeled Os Trigonum Syndrome. The Os Vesalianum is a rare accessory bone and is located at the outer edge of the base of the 5th metatarsal. It can be hard to distinguish this accessory bone from a fracture or a normal apophyisis (growth zone). The Os Tibiale Externum may also be referred as the accessory navicular bone and is probably the one which has the most clinical significance, or develops symptoms.