Os Trigonum Syndrome

The os trigonum is a small, round bone that sits just behind the ankle joint. It is present in about 5-15% of people. An os trigonum occurs when one area of bone does not fuse with the rest of the talus (ankle bone) during growth.

The most common reason people learn they have an os trigonum is that they have an x-ray to help diagnose ankle pain.  An incidental finding is something that happens in medicine very commonly: when looking for one thing, something completely different is discovered.  Often when an x-ray is done, your doctor may notice findings that have nothing to do with the reason for the x-ray.  Often these incidental findings aren't even in the same location, but they happen to show up on the x-ray. The truth is that the os trigonum is seldom the cause of the ankle pain.  Normally, having an os trigonum is of no consequence.

In some people, however, this small bone can get pinched in the back of the ankle, and even prevent normal motion. This is most commonly seen in ballet dancers who assume pointe and demipointe positions. These positions maximally plantarflex the ankle (point the toes down), and can cause the os trigonum to become pinched in the space behind the ankle.

The other common finding in individuals with os trigonum syndrome is tenderness directly in the location of the abnormal bone. There are other causes of pain in the back of the ankle, including cartilage abnormalities of the ankle, Achilles tendon problems, retrocalcaneal bursitis, and other types of tendonitis.

If an os trigonum is seen on an x-ray, and os trigonum syndrome is suspected, an MRI can be helpful to look for fluid accumulation both in and around the abnormal bone. Treatment usually begins with simple steps.  If rest and activity modifications fail to relieve the symptoms, a surgery can be considered to remove the abnormal bone in the back of the ankle.

Also Known As: Posterior Ankle Impingement