Accessory ossicles

How it works

Bones do not grow as single blocks. Each one forms from several growth centres that spread and fuse together during childhood¹. Sometimes one centre stays separate. It is then a small, rounded, fully formed bone sitting next to the main bone, joined to it by cartilage or fibrous tissue. That is an accessory ossicle¹.

Dozens of these extra bones have been named¹. Most are in the foot and ankle. The most common is the os trigonum, a pea-sized bone behind the ankle joint, found in 7% to 25% of feet². The accessory navicular sits on the inside of the arch, and between 4% and 21% of people have one³. Others appear in the hand, wrist, and knee¹.

Most cause no symptoms. They matter mainly because of what they look like on X-ray. A small round bone next to a larger bone can look like a chip that has broken off¹. Radiologists know the usual positions and can tell the two apart. An extra bone has smooth, rounded, well-defined edges. A fracture fragment has a sharp, irregular edge that matches a gap in the main bone¹.

Is it a problem?

Usually not. Most people with an extra bone never know. Two exceptions exist. An os trigonum can get pinched between the ankle bones when the foot points hard downward, which happens in ballet and football, and this can cause pain at the back of the ankle². An accessory navicular can rub against a shoe and become sore, and it is sometimes linked to flat feet³. Both are treated with rest, padding, or, rarely, removal. If a doctor tells you an X-ray shows an extra bone, it is a note about your anatomy rather than an injury.

History and culture

Anatomists have been cataloguing extra bones since the nineteenth century, and most carry Latin names of the form “os” plus a description of where they sit¹.