Accessory ossicles
Also called: Extra bones · Accessory bones
Small extra bones, most often in the foot, that form when a growth centre never fuses with its main bone. They are usually harmless but can be mistaken for fractures on an X-ray.
How common
Do you have this?
How it works
Many bones do not grow as single blocks. They form from several growth centres that spread and fuse together during childhood and the teens1. 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. One of the most common is the os trigonum, a small bone behind the ankle joint2. The accessory navicular sits on the inside of the arch3. 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 usually 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 bone2.
How common it is
One survey of 1,000 adult foot X-rays found at least one accessory ossicle in 40% of them4. Os trigonum alone shows up in 7% to 25% of feet2, and the accessory navicular in 2% to 21%1. Studies disagree because they use different imaging methods, count different bones as accessory ossicles, and sometimes look only at feet already being scanned for pain, which can inflate the numbers.
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
The anatomist Andreas Vesalius first described extra bones in 15435. Most carry Latin names that start with “os”1.
Sources and further material
- Wikipedia Accessory bone
- NCBI Sesamoids and accessory ossicles of the foot: anatomical variability and related pathology Gives os trigonum prevalence of 7% to 25% and describes os trigonum syndrome.
- Wikipedia Accessory navicular bone
- NCBI Accessory Ossicles in the Region of the Foot and Ankle: An Epidemiologic Survey in a Jordanian Population Survey of 1,000 adult foot X-rays. 40.2% had at least one accessory ossicle.
- NCBI Small but troublesome: accessory ossicles with clinical significance States that Vesalius first described accessory ossicles in 1543.
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