Os Trigonum Syndrome (Posterior Ankle Impingement Syndrome)

Os trigonum syndrome, also known as posterior ankle impingement syndrome, is characterized by pain in the back of the ankle. This condition is caused by the impingement of an accessory bone called the os trigonum and the posterior talar process, leading to inflammation in the rear ankle region. It is commonly associated with pain at the back of the ankle and difficulty with toe-pointing movements.

Os trigonum syndrome is more frequently seen in ballet dancers, male ballet dancers, and football players, due to the repetitive ankle movements and hyperplantarflexion involved in their activities.

The first-line treatment for os trigonum syndrome includes rest, physical therapy, and ultrasound-guided corticosteroid injections, which are often effective in relieving symptoms.

In cases where non-surgical treatments fail, arthroscopic (minimally invasive) surgery can be performed successfully to treat os trigonum syndrome by removing the impinging bone fragment.

What Is the Os Trigonum?

The os trigonum is the second most common accessory bone found in the human body. It is present in approximately 20% of the population and is located at the back of the ankle joint.

Where Is the Os Trigonum Bone Located?

The os trigonum is a small, extra bone situated at the rear of the ankle joint, behind the talus bone. It develops when a secondary ossification center of the talus fails to fuse completely during skeletal growth.

Not everyone has this bone. However, when it is present, it is often found symmetrically in both ankles.

In most cases, the presence of an os trigonum does not cause any problems, and many people remain unaware that they even have it.

What Is Os Trigonum Syndrome?

Os trigonum syndrome is a condition characterized by pain at the back of the ankle. Not every person with an os trigonum develops symptoms. The condition typically arises after long periods of repetitive ankle movements, especially in athletes. In some cases, it can be triggered by a single traumatic event, such as landing on the toes from a height.

In athletes like football players who forcefully bend the ankle while kicking the ball, or in ballet dancers who frequently rise en pointe, the os trigonum can become compressed between the bones, leading to pain. Swimmers using the butterfly stroke may also experience symptoms due to repeated forced plantar flexion of the ankle to reduce water resistance.

It’s important to note that not all posterior ankle impingement is caused by the os trigonum. Sometimes, an elongated posterior talar process (a bony projection at the back of the talus) can also lead to impingement and pain.

Common Characteristics of Athletes With Os Trigonum Syndrome

• Repeated forceful plantar flexion of the ankle (pointing the toes downward)

• Weight-bearing while the ankle is in plantar flexion

• Frequent repetition of these movements during training or competition

Symptoms of Os Trigonum Syndrome

Several symptoms may occur, but the two most prominent signs are:

• Sharp pain at the back of the ankle during toe-pointing (plantar flexion)

• Swelling and tenderness in the posterior ankle region

Conditions such as tarsal tunnel syndrome and plantar fasciitis are also common causes of foot and ankle pain. You can click the links to explore more about those topics in our related articles.

How Is Os Trigonum Syndrome Diagnosed?

Diagnosis of os trigonum syndrome is made following a thorough clinical examination of the ankle. Imaging studies such as X-rays, CT scans, and especially MRI are essential to confirm the presence of the os trigonum and assess for soft tissue inflammation or impingement.

How Is Os Trigonum Syndrome Treated?

Initial treatment should always begin with physical therapy. A structured rehabilitation program focusing on reducing inflammation and improving ankle mechanics is often effective.

If symptoms persist, ultrasound-guided corticosteroid injections can help relieve pain and inflammation by directly targeting the area around the os trigonum.

Os Trigonum Surgery

If conservative treatments fail to resolve the symptoms, surgical removal of the os trigonum and/or the prominent posterior talar process may be necessary.

Today, this procedure is most commonly performed using minimally invasive arthroscopic techniques.

Two to three small incisions (approximately 1 cm each) are made to allow the insertion of a camera and surgical instruments. The os trigonum is then carefully removed under direct visualization.

👉 You can watch a video of Dr. Utku Erdem Özer performing arthroscopic os trigonum surgery by clicking [this link].

Pre- and Post-operative X-ray Images of Os Trigonum Surgery

In the surgical example provided, both the os trigonum and the posterior talar process have been successfully removed through an arthroscopic approach.

Os Trigonum Variants

The os trigonum exhibits anatomical variations among individuals. In some people, it exists as a completely separate bone, while in others, it is attached to the talus by a thin cartilaginous or fibrous bridge.

These variations can be seen on radiological images and generally do not cause problems, unless they become symptomatic.

• Completely separate ossicle

• Attached to the talus by a thin fibrous bridge

Conclusion

The os trigonum is a small bone fragment located at the back of the ankle joint that results from incomplete fusion with the talus during development. It is present in some individuals and usually remains asymptomatic.

However, in cases involving repetitive ankle movements, especially among athletes, it may lead to os trigonum syndrome, characterized by pain, swelling, and limited ankle motion.

Treatment typically begins with rest, physical therapy, and corticosteroid injections. In more advanced cases, arthroscopic surgical removal of the os trigonum provides effective relief and helps patients return to their activities.

For more detailed information about diagnosis and treatment options, feel free to contact us through our communication channels.

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