Accessory Bones. 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.his 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.