accessory bone

An accessory bone is a bone that does not occur regularly in most individuals of a species, but is nevertheless found in a significantly large number of organisms. These are supernumerary bones that develop from additional ossification centers, often representing sesamoid bones or persistent developmental variants.

Accessory bones are most commonly found in the feet and hands, though they can occur throughout the skeleton. The foot is particularly prone to them, with accessory navicular, os trigonum, and accessory ossicles around the ankle being among the most commonly documented.

Accessory bones are typically vestigial or serve minor mechanical roles. Some, like sesamoid bones, may protect tendons from friction or improve mechanical advantage. Many accessory bones have no known functional significance and are considered anatomical variants.

Accessory bones can become painful when subject to repeated trauma, overuse, or when they become secondarily arthritic. Painful accessory navicular syndrome is a common cause of medial foot pain in adolescents and young adults. Accessory bones can also be mistaken for fractures on imaging.

Sesamoid bones are embedded within tendons and have a defined functional role in protecting tendons from friction, while accessory bones are additional bony elements that may or may not be associated with tendons and often have unclear functional significance. Some sesamoid bones are themselves classified as accessory bones when not universally present.