The Os trigonum is an extra bone that forms behind the ankle bone and is present at birth. Most people don’t even know they have an Os trigonum and it is of no consequence until it gets caught in the ankle, sometimes preventing normal movement. Os trigonum syndrome is usually triggered by an injury such as an ankle sprain, or the repeated downward pointing of the toes, which is common among ballet dancers, soccer players and other athletes. The signs and symptoms of Os trigonum syndrome are deep, aching pain in the back of the ankle, when walking or when pointing the toes downward, tenderness in the area when touched, swelling in the back of the ankle and decreased range of motion in severe cases. Conservative treatment includes rest, anti-inflammatory drugs, ice packs or ice baths, and cortisone injections. In some patients, surgery may be required which involves removal of the Os trigonum. This extra bone is not necessary for normal foot function and a full recovery should be expected.