The os trigonum is an accessory, or extra bone, that sometimes develops behind the ankle. It does not always cause symptoms, but when it causes pain, due to irritation and inflammation, a doctor will refer to this discomfort as os trigonum syndrome.
The true ankle joint contains three bones that allow it to support weight and move. One of these bones is known as the talus, or heel bone. Some people may have an extra bone that develops behind the talus called the os trigonum. This extra bone may not cause symptoms. However, in some individuals, it can result in a painful condition known as os trigonum syndrome, particularly after repetitive downward pointing of the toes.
