The anklebone (talus) sits on the heelbone (calcaneus), forming a secondary joint in the ankle (the subtalar joint). At the back of the anklebone are two small bony lumps (tuberosities), one on the inside and one on the outside. In about 15% of people, the outside lump is naturally separated from the talus, and is called the os trigonum.

The os trigonum can cause problems in people who regularly point their feet downwards sharply – it’s most common in ballet dancers. When the os trigonum gets sandwiched between the bottom edge of the shinbone (tibia) and the top surface of the heelbone, the tissues above and below the os trigonum can become trapped, leading to inflammation and swelling.
The condition is known as posterior impingement.

Pain behind your heel or deep in the back of the ankle. There is usually tenderness by the anklebone on the outside of your leg. Pain is usually worse when your foot is pointed down. You may also feel a painful clicking sensation as your foot is twisted in and out.

The surgeon may inject a numbing medication into the area of the os trigonum – if the pain immediately diminishes, the problem is most likely to be posterior impingement of the os trigonum. An x-ray will confirm the existence of an os trigonum.

The os trigonum is completely removed during open surgery. A combination of insertions and small incisions are made in the back of the ankle and the os trigonum is carefully removed using a scalpel.
After the operation, you will be in a splint for two weeks. Physiotherapy will be needed for the next two to three months.