The Os Trigonum
The os trigonum is an extra bone that sometimes develops behind the ankle bone. Present at birth, the os trigonum only becomes evident during adolescence, when one area of the ankle bone (talus) doesn't fuse with the rest of the bone, creating the os trigonum.

What is Os Trigonum Syndrome?
Os trigonum is a painful condition usually triggered by an injury, like an ankle sprain. People who repeatedly point their toes downward, like ballet dancers and soccer players, are also prone to get the syndrome. In these people, the os trigonum is crunched between the ankle and heel bones, like a nut in a nutcracker. As the os trigonum pulls loose, the connective tissues between it and the talus are stretched or torn, leading to inflammation.

Signs and Symptoms

• Deep, aching pain in the back of the ankle, usually occurring when walking or when pointing the toes downward
• Tenderness when the area is touched
• Swelling in the back of the ankle

Diagnosis
As with all foot conditions, your doctor will ask you when the symptoms first started, as well as a thorough examination of the foot in question. Afterwards, you should expect the doctor to order x-rays or other imaging studies to confirm the diagnosis.