What Is the Os Trigonum Syndrome?

Pain Due to an Extra Bone in the Ankle

Os trigonum syndrome is a rare and painful condition that can affect people who have an os trigonum bone. The os trigonum is an accessory (extra) bone that's present in about 15 to 30% of people in at least one foot. It is a small, round bone that sits just behind the ankle joint. This accessory bone doesn't usually cause symptoms, but it can.

ballet ankle
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What Is the Os Trigonum Bone?

An os trigonum bone is a congenital malformation that develops before birth when one area of the tallus (ankle bone) doesn't fuse with the rest of the bone during growth. When the bone doesn't fuse properly, there's an extra small bone in the ankle.

Even if you have an os trigonum, your ankle bone will still grow to an adequate size. You can have an os trigonum in one or both of your ankles.

If you have an os trigonum, you are unlikely to know it. The bone doesn't interfere with movement, and it doesn't usually cause any pain or discomfort unless it is injured or pinched.

Causes of Os Trigonum Syndrome

Certain injuries or repetitive movements can lead to compression and inflammation of the os trigonum, causing ankle pain. This is often described as os trigonum syndrome.

About 1% of people who have a sprained ankle will develop os trigonum syndrome.

The syndrome most commonly affects ballet dancers or athletes. Prolonged or repetitive positions that plantarflex the ankle (point the toes down), such as the pointe and demipointe positions of ballet dancers, are likely to cause the os trigonum to become pinched in the space behind the ankle.

Os trigonum syndrome is a type of posterior ankle impingement. Sometimes the terms are used interchangeably, but you can have posterior ankle impingement due to compression of tissue at the back of your ankle even if you don't have an os trigonum bone.

Symptoms

Os trigonum syndrome is when the extra os trigonum bone causes problems. The extra bone isn't visible. Pain is the most common symptom, but other symptoms can occur too.

For athletes and dancers who develop os trigonum syndrome due to repetitive movements, the symptoms can worsen gradually. If you develop the condition due to a traumatic injury, your symptoms may be more severe and progress rapidly.

For most people, when the os trigonum becomes pinched in the back of the ankle, it can cause:

  • Pain and tenderness right above the extra bone and in the surrounding area of the ankle
  • Heel pain can develop as well

Rarely, ankle bruising, swelling, or impaired motion can develop. These problems can occur if you have severe trauma of your foot associated with os trigonum compression.

Diagnosis

The os trigonum bone is normally an incidental finding during an evaluation for foot pain or ankle pain. Sometimes, if you have persistent ankle pain, your doctor might order an imaging test to see if you have an os trigonum.

Imaging tests that can identify an os trigonum bone:

  • X-ray: An os trigonum may be seen on an X-ray, which is a test that visualizes the structure of the bones. This test cannot tell whether an os trigonum is the cause of your pain, but it can rule out other causes of pain, such as a bone fracture.
  • Computerized tomography (CT) scan: This test can be used to assess bone structure and swelling, and it can identify an os trigonum. It may support a diagnosis of os trigonum syndrome and rule out some other potential causes of pain, such as an abscess.
  • Magnetic resonance imaging (MRI) scan: An MRI can be helpful for identifying bone structure, as well as fluid accumulation in and around the abnormal bone. It can be used to diagnose os trigonum, as well as to support a diagnosis of os trigonum syndrome, or other problems, like tendonitis.

Os trigonum syndrome is diagnosed based on symptoms, physical examination, and imaging tests. You might have tenderness above the inflamed bone when your doctor examines you. In some cases, you could have swelling at the back of your foot.

Other possible causes of pain in the back of the ankle include arthritis, Achilles tendon problems, or retrocalcaneal bursitis.

Treatment

Treatment of os trigonum syndrome usually begins with rest, activity modification, and ice. If you still have symptoms despite conservative measures, your doctor might recommend that you take a non-steroidal anti-inflammatory (NSAID) to reduce inflammation and discomfort.

Steroid injections are another option to reduce persistent pain and inflammation of os trigonum syndrome.

Sometimes surgery is considered to remove the bone. You would need time to recover after your surgery, and some athletes maintain training throughout the recovery period.

A Word From Verywell

If you have found out that you or your child has an os trigonum bone in one or both of your feet, there's no need to worry. Given the fact that the malformation is identified in athletes, including ballet dancers, you should rest assured that this extra bone won't limit your physical abilities.

However, if you develop pain, make sure you see your doctor so you can get the right treatment. A period of rest might be enough for you, or you may need to take medication or have another intervention before you are back to your regular level of activity.

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