The os trigonum is an extra bone that sometimes develops behind the ankle. It does not always cause symptoms but can cause pain due to irritation and inflammation. Doctors call this os trigonum syndrome.

The true ankle joint contains three bones that allow it to support weight and move. One of these bones is known as the talus, or heel bone. Some people may have an extra bone that develops behind the talus called the os trigonum. This extra bone may not cause symptoms. However, in some individuals, it can result in a painful condition known as os trigonum syndrome, particularly after repetitive downward pointing of the toes.

In this article, we will discuss os trigonum syndrome, including causes and risk factors, symptoms, and more.

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Doctors refer to the os trigonum as an accessory, or an extra bone. It develops in the posterior, or back, of the ankle and connects to the talus by a fibrous band of tissue.

The presence of an os trigonum is congenital, meaning it is present at birth. As many as 15% of people may have this extra bone behind their ankle. However, a person may not realize they have it until adolescence, when one area of the talus does not fuse with the rest of the bone. They may also discover they have it when they begin to experience symptoms.

If someone develops pain from it, doctors refer to it as os trigonum syndrome. Some doctors may refer to it as a nutcracker injury, because when the toes point down, the joint compresses on the bone, similar to a nutcracker on a nut.

A person develops os trigonum at birth. It may occur in one or both ankles. While health experts do not know the underlying causes, they know that overuse often leads to os trigonum syndrome developing. It can also occur due to an acute injury, such as a sprain.

People commonly at risk of developing this condition include:

  • ballet dancers
  • soccer players
  • basketball players
  • other types of dancers and athletes

Pain typically occurs when a person points their toes, causing the bone to become compressed, stretching and tearing the fibrous connection. The motion causes pain and inflammation.

The most common symptom of os trigonum syndrome is pain in the back of the ankle, particularly when pointing the toes and during movements. This typically also results in irritation and inflammation. Discomfort can also occur when a person moves their big toe. This is because the tendon in the big toe runs near the os trigonum. Pain typically worsens with movement and improves with rest.

The os trigonum is also a common cause of posterior ankle impingement. This refers to when someone experiences pain in the back of their ankle. It often occurs when the top of the foot points away from the leg, particularly when a person hyperextends their ankle.

Os trigonum syndrome can often mimic the symptoms of other conditions and injuries. Conditions that can cause similar symptoms include:

  • ankle sprain
  • Achilles tendon injury
  • talus fracture

When a person presents with symptoms, a doctor will often review their medical history and perform a physical examination. They will likely ask questions about when the pain occurs and how it started. This may help rule out some potential causes.

To determine the cause of the pain, a healthcare professional will likely order imaging tests, such as an X-ray. They should be able to see the os trigonum on the imaging test and make an appropriate diagnosis.

Initial treatment typically focuses on nonsurgical options to relieve symptoms. It can include a combination of several different therapies, which can include:

  • Rest: This includes stopping any activities that may cause pain.
  • Ice: Applying cold therapy directly on the back of the ankle can help reduce swelling and inflammation
  • Oral medications: Options, including nonsteroidal anti-inflammatory drugs, such as naproxen or ibuprofen, can help relieve pain and inflammation.
  • Immobilization: Using braces or other methods to stabilize and prevent movement may help reduce discomfort.
  • Injections: In moderate to severe cases, a doctor may recommend steroid injections to help alleviate pain and swelling.

If symptoms become chronic or do not improve with nonsurgical treatments, a doctor may recommend surgical intervention. Surgery typically involves using keyhole techniques to make small incisions on either side of the Achilles tendon to remove the os trigonum and any soft tissue causing discomfort. As this bone is not necessary for typical foot function, it should help relieve symptoms.

A healthcare professional will also likely provide various physical therapy exercises to help gradually increase the mobility of the ankle. A person needs to follow the guidance they receive from their doctor. Exercises that can help strengthen the ankle may include:

  • towel scrunches
  • ankle eversion
  • ankle inversion
  • plantar flexion
  • dorsiflexion
  • tibialis posterior exercises
  • heel raises
  • seated toe taps
  • calf raises

Learn more about ankle strengthening exercises.

It can take about 4–6 weeks to recover from os trigonum syndrome. However, people with higher levels of activity, such as dancers or people who play sports, may experience discomfort for several months.

Following surgery, someone will typically need to wear a surgical boot for about 2–4 weeks. They can then gradually return to activities, such as walking unassisted and driving. They should be able to return to work or school within a few days of surgery with a boot in place.

It can take more than 6 months following surgery for a person to make a full recovery and return to vigorous sporting activities. Some people may find that their foot is now more comfortable than before surgery and that they can do more than they could before the operation.

Os trigonum is an accessory, or extra bone, behind the ankle. Many people with this extra bone do not develop symptoms. However, if the os trigonum causes pain, doctors refer to it as os trigonum syndrome.

Currently, health experts do not know what causes this bone to develop. However, risk factors for triggering discomfort typically include overexertion of the ankle, particularly in those who play sports or dance. Pain typically occurs when pointing the toes down.

Initial treatment options often involve rest, ice, immobilization, and possible pain medications. If they do not work, a person may require surgery. It can take several weeks to months to recover fully from os trigonum syndrome.