Accessory Navicular Syndrome / Os Tibiale Externum
Pain over the bony bump below the inner ankle — common in growing children, and most settle without surgery
The accessory navicular is an extra bone that some people are simply born with, sitting beside the navicular bone on the inner side of the foot. It is found in roughly 10–20% of people in Japan — a normal variation rather than a disease — and in most people it never causes any pain at all.
When that extra bone does become painful, we call it accessory navicular syndrome. The tibialis posterior tendon, which helps hold up the arch of the foot, attaches to this bone. Running, jumping and pressure from a shoe can add up and strain the junction between the two bones, and that is where the pain comes from.
It is most common in active children aged about 10 to 15 and is reported slightly more often in girls. It is often picked up when someone comes in saying "this sprain just won't get better" or "the inside of my ankle is swollen."
There are three types of accessory navicular. Almost all painful cases are the type joined to the navicular by fibrocartilage — a softer connection that does not tolerate shearing forces well.
This tendon lifts the arch and attaches to the accessory navicular. Every sprint, jump and change of direction tugs on that junction again.
Repeated pulling and twisting causes small injuries to the fibrocartilage join. This becomes the source of the pain, which is sharply tender to press on.
Because the bone sticks out, poorly fitting shoes, narrow shoes and the inner side of football boots press directly on it, irritating the skin as well.
By around 15–17, when bone growth is complete, the junction becomes more stable and the pain often fades. This is not true for everyone, and some people first develop pain as adults.
With conservative care — better footwear, insoles, rehabilitation and sensible activity levels — most people find their symptoms ease. By around 15 to 17, when bone growth finishes, the junction tends to become more stable and pain becomes less likely. That said, a painful spell can last several months, and it is quite common for symptoms to flare again when training restarts. Building the load back up gradually, guided by the pain, matters more than rushing.
About flat feet — the extra bone is not proven to cause them
People with a painful accessory navicular often have flat feet as well, but it has not been shown that the extra bone causes the flat foot. A flat foot on its own needs no treatment if it is not causing symptoms. Insoles do not reshape the arch; they are there to ease pain and help the foot work more comfortably.
Pain around the inner ankle can also come from other causes, including a navicular stress fracture, tibialis posterior tendon problems and tarsal coalition. A stiff flat foot whose arch does not move, or one side that is rapidly getting worse, needs these possibilities looked into.